Tifton, Georgia. CAMILO L. QUARIN. Universidad Nacional del Nordeste. Corrientes, Argentina. CARLOS G.S. PEDREIRA. Universidade de Sao Paulo.
Published 2004
19
Bahiagrass ROGER N. GATES USDA-ARS Tifton, Georgia
CAMILO L. QUARIN Universidad Nacional del Nordeste Corrientes, Argentina
CARLOS G.S. PEDREIRA Universidade de Sao Paulo Piracicaba, Sao Paulo, Brazil
Bahiagrass (Paspalum natatum Flugge), native to South America, has become widely distributed on light-textured soils in warm, humid regions of the western hemisphere. It has been planted extensively as pasture and as ground cover on highway rights-of-way. As a forage, bahiagrass provides adequate nutrition for livestock when it is actively growing and is very persistent when managed under continuous stocking. Bahiagrass tolerates low inputs on marginally fertile soils, Winter temperatures generally limit its range of adaptation to tropical and subtropical areas, TAXONOMY
Bahiagrass is classified as Paspalum natatum Fltigge, and it was described in 1810 using a plant that was collected from St. Thomas Island by Schrader and Ventenat. Synonyms include Paspalum saltense Arechavaleta, Paspalum uruguayense Arechavaleta, and Paspalum natatum var. latijlarum Doell. Common names are bahiagrass (USA), pasto horqueta (Argentina), capii cabayu (Paraguay), and grama batatais (Brazil). A sexual diploid form is classified as Paspalum natatum var, saurae Parodi and has the synonym Paspalum saurae (Parodi) Parodi (Parodi,1948).
Botanical Description Bahiagrass is a perennial grass with strong, shallow, horizontal rhizomes) formed by short, stout internodes usually covered with old, dry leaf sheaths (Fig. IThere is disagreement over the terminology appropriate for this structure. Many suggest stolon is appropriate as the structure often occurs at or above the soil surface. Rhizome has been used most frequently in the literature and is used here for consistency. Copyright © 2004. American Society of Agronomy, Crop Science Society of America, Soil Science Society of America, 677 S. Segoe Rd., Madison, WI 53711, USA. Warm-Season (C4 ! Grasses, Agronomy Monograph no. 45.
651
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19-1). Culms are simple, ascending, geniculate at the node between the first and second elongated internodes, otherwise erect, 10- to 60-cm tall. Leaves are mostly crowded at the base with overlapped keeled sheaths, glabrous or with ciliate margins mainly toward the summit. Blades are usually flat or somewhat folded toward the base, linear-Ianceolate 3- to 30-cm long, 3- to l2-mm wide, usually glabrous or ciliate toward the base, and rarely pubescentthroughout. Inflorescences are subconjugate with a short almost imperceptible common axis, racemes two, rarely three, ascending or recurved-divergent in some races, and 3- to l4-cm long. The rachis is
Fig. 19-1. Illustration of a bahiagrass plant (center; 10-60-cm tall ; leaves, 3-12-mm wide; stem, 1.33.0-mm thick), inflorescence (left; racemes, 3-14-cm long), collar region (lower right; ligule 0.3 mm), spikelet (right center; about 3-4-mm long), and floret (upper right; about 3-mm long).
653
BAHIAGRASS
glabrous, flexuous, and green or purplish. Spikelets are solitary in two rows on one side of the rachis, obovate or ovate, shining, glabrous, 2.5- to 4-mm long, and 2- to 2.8-mm wide. Anthers and stigmata are usually purple, and the fruit are oval, about 1.8-mm long and 1.2-mm wide. Botanical Varieties Common bahiagrass is the typical form of P. notatum in that it is broad leaved, strong-rooted, and spreads slowly by stout rhizomes with short internodes. 'Pensacola' bahiagrass belongs to P. notatum var. saurae, and when compared to common bahiagrass, it is taller, spreads faster, has longer and narrower leaves, smaller spikelets, and can have more racemes per inflorescence.
DISTRIBUTION AND ADAPTATION Center of Origin The genus Paspalum is found in the tropical and warm temperate regions of the New World. It is particularly abundant in Brazil, eastern Bolivia, Paraguay, and northeastern Argentina. The relatively few species not native to the Americas are mostly allies of Paspalum scrobiculatum L. Though bahiagrass has a wide distribution in the Americas, the original distribution of the races of var. saurae was confined to Corrientes, Entre Rios, and the eastern edge of Santa Fe Provinces in Argentina. The area is -300-km wide and 725-km long and extends from the Uruguay River to just west of the Parana River. The common type is one of the most abundant species in native pasturelands of the region, but populations of the var. saurae are infrequent and usually restricted to wet sandy soils along rivers and flat sandy islands of the Parana River. A search for the origin of Pensacola bahiagrass led to the discovery of a large natural population of the var. saurae on Berduc Island in the Parana River (Burton, 1967). The amount of genetic variability on the island was equivalent to that previously observed in the USA. Another popUlation of var. saurae was discovered in Cayasta, 70 km northeast of Santa Fe, Argentina. Because the var. saurae types are native to this region and cytogenetic studies indicate that the common bahiagrass races are autotetraploid and share homologous chromosomes with var. saurae, this region is considered the center of origin of the species. This area of Argentina and the neighboring state of Rio Grande do Sui in Brazil has the greatest diversity of Paspalum species closely related to P. notatum. These are all placed in the informal taxonomic Notata group established by Chase (1929). Several of the species are sympatric with P. notatum var. saurae and they include the following: diploid Paspalum pumilum Nees; diploid and tetraploid races of Pa.spalum cromyorrhizon Trin. ex Doell; tetraploid Paspalum ionanthum Chase; and diploid and tetraploid races of Paspalum maculosum Trin. Geographic Distribution Paspalum notatum var. saurae has spread throughout the Western Hemisphere. Following its introduction into Florida, the grass was brought into cultivation
GATESETAL.
654
throughout the state, the Coastal Plain, and the Gulf Coast regions of the southern USA. It escaped from cultivation and became naturalized throughout the southeastern USA. The cv. Pensacola and Tifton 9 are sold and planted in several South American countries for forage and turf purposes. Even though these plantings are small in comparison with the amount grown in the USA, bahiagrass is an important cultivated forage in southern Brazil, where it also has escaped from cultivated areas (Pozzobon and Valls, 1997). The tetraploid races are distributed in open areas, savannas, and cultivated pastures from sea level to 2000 m from central Mexico to Argentina and throughout the West Indies (Chase, 1929). Common bahiagrass is the primary constituent of many native pasturelands in southern Brazil, Paraguay, northeastern Argentina, and Uruguay. It was introduced into the USA and many other countries around the world. Naturalized populations of both Pensacola and tetraploid races occur in Australia. Bahiagrass is used in Japan (Sugimoto et ai., 1985) and has been evaluated experimentally in Taiwan (Jean and Juang, 1979) and Zimbabwe (Mills and Boultwood, 1978).
Introduction into the USA Scott (1920) reported that bahiagrass was first introduced into the USA by the Bureau of Plant Industry (SPI no. 35067, Office of Foreign Seed and Plant Introductions) and grown by the Florida Agricultural Experiment Station in 1913. The first experimental plantings in Georgia were established with seed introduced from Cuba in 1930. Seed of 'Argentine' was sent to the U.S. Plant Introduction Office in 1944 and 'Paraguay 22' seed arrived in 1947. Both were distributed for testing by the USDA and the Florida Agricultural Experiment Station. Burton (1946) reported four tetraploid bahiagrass types: common, 'Wallace', Paraguay, and 'Wilmington', in addition to diploid Pensacola. Hanson (1972) recognized five tetraploid types: common, Argentine, Paraguay, Paraguay 22, and Wilmington. Pensacola bahiagrass was first discovered growing near the docks in Pensacola, FL, and its seed probably arrived on a ship from Argentina prior to 1926 (Finlayson, 1941; Burton, 1967). The seed may have been discarded in the dock area when the ship was cleaned (Finlayson, 1941). In 1938, Finlayson (1941) began collecting seed from the grass, and shortly thereafter, he started distributing the seed and promoted the grass for pasture and conservation purposes. Hoveland (2000) stated that the introduction of Pensacola bahiagrass was a major achievement in the development of grasslands in the southern USA. It is more cold tolerant than the tetraploids; therefore, it is more widely distributed. The grass grows throughout the southern USA, from Texas to North Carolina, extending into Arkansas and Tennessee (Watson and Burson, 1985). Pensacola was first collected in Virginia in 1950 and has become more common since 1995 (T.P. Wieboldt, personal communication, 2001). The grass recently was discovered growing in southeastern Oklahoma (D.D. Redfearn, personal communication, 2002). It appears Pensacola is gradually spreading northward.
BAHIAGRASS
655
CYTOLOGY AND REPRODUCTIVE BEHAVIOR Chromosome Number Burton (1940b) reported that the chromosome number of bahiagrass was 2n =4x =40; however, Pensacola bahiagrass is a diploid with 20 chromosomes (Burton, 1946). This diploid race also occurs in Entre Rios, Argentina where it is thought to have originated (Saura, 1948). All races with 2n = 2x = 20 chromosomes belong to the botanical variety P notatum var. saurae and are usually referred to as Pensacola bahiagrass. Chromosome numbers were recently determined for 150 naturally occurring bahiagrass accessions; most were from South America. Pozzobon and Valls (1997) examined 127 accessions and 116 (91 %) had 2n = 4x = 40 chromosomes and the remaining 11 were diploids with 20 chromosomes. Tischler and Burson (1995) studied 23 accessions and 17 (74%) were tetraploids, four (17%) were diploids, one was a triploid (2n = 3x = 30), and one was a pentaploid (2n = 5x = 50). Indigenous 20-chromosome plants were found only in northeastern Argentina; whereas, the 20-chromosome accessions collected in southern Brazil were considered escapes from cultivated pastures of Pensacola bahiagrass. Plants with 2n = 5x = 50 and 2n = 6x = 60 chromosomes have been created experimentally by pollinating apomictic 40-chromosome plants with pollen from 20- and 40-chromosome plants, respectively (Burton, 1948; Martinez et aI., 1994). These developed from the fertilization of an unreduced egg (2n + n). Tetraploid plants have been produced by doubling the chromosomes of diploid races with colchicine (Forbes and Burton, 1961a), and occasionally plants with 80 chromosomes also were produced (Quarin, 1999).
Ploidy Levels Most Paspalum species have a base chromosome number of x = 10; however, base numbers of x = 6 and 9 have been proposed for the Paspalum almum Chase complex (Quarin, 1974) and Paspalum contractum Pilger (Davidse and Pohl, 1974), respectively. Bahiagrass has a base number of x = 10, and as mentioned above, known ploidy levels in the wild range from diploid (2n = 2x = 20) to pentaploid (2n = 5x =50). During meiosis I, the chromosomes in pollen mother cells (PMC) of tetraploid plants paired primarily as multivalents and from 2 to 10 quadrivalents were observed (Forbes and Burton, 1961 b; Magoon and Manchanda, 1961; Fernandes et aI., 1973). Chromosome pairing behavior in induced sexual autotetrap10id plants, natural apomictic tetraploid strains, and sexual x apomictic tetraploid hybrids indicate that the apomictic tetraploid races originated by autoploidy (Forbes and Burton, 1961 b). Moreover, triploid hybrids from crosses between diploid and tetraploid plants have as many as 10 trivalents per PMC during meiosis 1, indicating complete homology between the genome in the diploid parent and both genomes of the tetraploid male parent (Forbes and Burton, 1961 b). Quarin et al. (1984) reported similar findings. This supports the hypothesis that P notatum is an agamic
GATESETAL.
656
complex composed of several cytotypes of different ploidy levels, most of which are autopolyploids.
Method of Reproduction The meiotically regular diploid races, including Pensacola bahiagrass, are sexual and highly cross-pollinated because most plants are self-incompatible but crosscompatible (Burton, 1955). The tetraploid races reproduce by obligate apomixis and are pseudogamous (Burton, 1948). Cytoembryological studies revealed that the apomictic mechanism in bahiagrass is apospory (Bashaw et aI., 1970). Pollination is essential for seed formation because the endosperm develops after the polar nuclei are fertilized. Endosperm development appears to occur regardless of the ploidy level of the pollen donor. Moreover, pollen of closely related species, such as P. cromyorrhizon, also can produce fertile seed with hybrid endosperm, although the embryo is of the maternal genotype (Quarin, 1999). Triploid plants occur sporadically in nature (Gould, 1966; Quarin et aI., 1989; Tischler and Burson, 1995) and have been found in experimental plots (Burton and Hanna, 1986). Because triploids reproduce by apomixis, the odd ploidy level is maintained through generations. When these apomictic triploids are pollinated by a diploid race, some of the progeny are new apomictic tetraploids that resulted from the fertilization of unreduced the egg cell (2n + n) [e.g., (2n = 3x = 30) + (n = 10)] (Burton and Hanna, 1986; Quarin et aI., 1989). This indicates the genetic variability in natural tetraploid apomictic races may result from a sexual polyploidization process. Triploids may arise from the fertilization of unreduced gametes in diploid populations, and, as mentioned above, these rare triploids can be pollinated by reduced pollen from naturally occurring diploid plants to produce new apomictic tetraploids. This implies the gene(s) for apomixis exist at the diploid level, although they are not expressed. Quarin et aI. (2001) recovered some facultative apomictic autotetraploid plants after doubling the chromosomes of diploid seedlings. This corroborates that the factor(s) controlling apomixis exists at the diploid level, although its expression requires the polyploid condition.
GENETICS Genetic Control of Apomixis Burton and Forbes (1960) crossed sexual colchicine-induced autotetraploid plants with tetraploid obligate apomicts. The ratio of sexual and apomictic plants in the F2 progeny suggested that apomixis is recessive to sexuality and is controlled by a few recessive genes. Assuming some modifying mechanism, their data were close enough to expected ratios of tetrasomic inheritance to postulate aaaa as the genotype for the apomictic parent. Progeny tests were used to classify the method of reproduction of the offspring (10-15 F2 plants to classify each F, progeny, and five F3 plants to classify the F2 progenies). However, it was not possible to accurately classify the facultative apomicts, especially those with low expression of
BAHIAGRASS
657
apomixis, because their progenies were difficult to differentiate from the progenies of sexual plants. Burton and Forbes (1960) claimed that facultative apomixis did not occur in the progeny. However, several years later one of the hybrids (Q3664) was determined to be a facultative apomict with a high degree of sexuality from cytoembryological studies (Quarin et a!., 1984) and progeny tests using molecular markers (Ortiz et aI., 1997). The genetic system controlling apomictic reproduction in bahiagrass was recently reviewed (Martinez et aI., 2001). An obligate sexual tetraploid plant was crossed with an apomictic tetraploid male parent, and the F I, F2, and backcross progenies were classified for method of reproduction by embryological examinations. The offspring were separated into two categories: aposporous plants, those capable of developing aposporous embryo sacs, and sexual plants, those without any aposporous embryo sac development. All self-pollinated sexual plants, including the female parent, or sexual hybrids from sexual x sexual crosses produced only sexual offspring. This indicates that sexuality is a recessive trait and that plants without aposporous development are homozygous recessive. All crosses between sexual and aposporous plants produced approximately three times more sexual than apomictic offspring. The authors suggested that a single dominant gene with tetraploid inheritance controls apospory. They hypothesized that the excessive number of sexual progeny was the result of some distortion in the segregation patterns may have been due to a pleiotropic lethal effect of the dominant allele with incomplete penetrance, or a partial lethality of factors linked to apospory. As in other apomictic grasses, the genetic control of apomixis in bahiagrass is not completely understood and requires additional research. Genetic Linkage Map
A genetic linkage map of diploid bahiagrass was recently developed, and it is a framework for basic genetic studies and breeding (Ortiz et aI., 2001). A mapping population was derived from crosses between a wild diploid genotype from Cayasta, Argentina, and Tifton 9. The strategy of map construction was based on a comparative approach that used heterologous restriction fragment length polymorphism (RFLP) clones evenly distributed over the maize (Zea mays L.), rice (Oryza sativa L.), and oat (Avena sativa L.) maps to uniformly cover the bahiagrass genome. Random amplified polymorphic DNA (RAPD) and amplified fragment length polymorphism (AFLP) markers were added to consolidate the linkage groups. One hundred twel ve loci were placed on 10 linkage groups, covering a total map distance of 991 Centimorgans. Syntenic regions with maps of maize and rice were observed and several RFLP markers reported to be associated with apomixis locus in both maize- Tripsacum and Brachiaria hybrids also were on the map.
BREEDING HISTORY
Because the naturally occurring tetraploid bahiagrass types reproduce by apomixis, improvement using conventional breeding methods has not been possible. All tetraploid cultivars released in the USA were superior apomictic ecotypes
658
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that were selected from introduced germplasm. Cultivars for which commercial seed is still available are Argentine, Paraguay, and Paraguay 22 (Table 19-1). The cv. Competidor was released in Australia in 1987 (Wilson, 1987). An experimental tetraploid, designated Tifton-7, produced yields superior to Argentine and Paraguay in a 2-yr experiment in Florida (Muchovej and Mullahey, 2000). Sexual diploid germplasm, Pensacola, has been improved using two approaches. Exploiting self-incompatibility in Pensacola bahiagrass, inbred lines were established vegetatively in alternate rows in a field and permitted to cross-pollinate to produce FI hybrid seed (Burton, 1974). 'Tifhi-l' (Rein, 1958) and 'Tifhi2' (G.w. Burton, unpublished data, 1958) were developed using this approach. Both cultivars out yielded Pensacola by 20%, but they were not widely accepted because of inadequate seed supply due to difficulties in vegetatively establishing seed production fields. The longest sustained effort to improve bahiagrass was initiated by G.W. Burton in 1960. Seed collected from 16 farms in Georgia was assembled as the initial population from which selection was started. Burton (1974) used a modified mass selection procedure that was called "restricted recurrent phenotypic selection," (RRPS). This procedure, including subsequent refinements (Burton, 1982), was used to complete 23 cycles of selection for increased forage production. It was the second approach used to improve diploid bahiagrass. From 1000 space-planted individuals, 20% were selected and intermated in a polycross, providing seed for the subsequent selection cycle. Several features of this breeding approach are noteworthy. Selection was based primarily on scoring, which was a reliable indicator of aboveground herbage accumulation of individual plants grown in a noncompetitive environment. A complete selection cycle could be completed every 12 mo by collecting flowering tillers with roots attached at the base from selected plants and placing them in a common vessel filled with tap water. Excellent pollen mixing and adequate seed production were achieved without transplanting. Seeds from this polycross were germinated in the greenhouse during winter and grown to transplant size by spring. An outcome of this selection procedure was the continuity of the range and extent of variation for individual plant herbage accumulation above mower height. Tifton 9 was released as a cultivar from the ninth selection cycle. Seed increases and experimental evaluations also have been made with cycles 14, 18, and 23. Monson and Burton (1974) and subsequently Gates and Burton (1990) noted individual plant variation in in vitro dry matter digestion (IVDMD). Mass selection procedures analogous to RRPS were used to simultaneously select for yield and IVDMD. Evaluation of progeny from this selection method revealed a higher mean IVDMD for the selected plants but the magnitude of increase was small (R.N. Gates, unpublished data, 2002). Selection procedures used in the development of Tifton 9 and subsequent populations involved the selection of seedlings in the greenhouse during the winter, in preparation for spring transplanting. Gates and Burton (1998) found that an increase in early germination and reduction in dormancy in more advanced selection cycles accompanied the selection for increased yield. Efforts to employ early emergence in the greenhouse as a direct selection procedure have resulted in popUlations with
= U.S. Dep. of Agric .. ARS = Agric. Res. Stn .. AES = Agric. Exp. Stn.
Australian release AlabamaAES
1994 1999
Riba AU Sand Mountain
tUSDA
Australian cultivar Japanese cultivar
1986 1991
Competidor Nan-ou
Argentina, accidental introduction Plant Introduction, USDA and Florida AESt Plant Introduction. Florida AES Brazil USDA-ARS and Georgia AES USDA-ARS and Georgia AES Japanese cultivar Japanese cultivar
Origin
1958 1961 1973 1983
1938 1944 1947 1950
Year of release
Paraguay Pensacola Paraguay 22 Argentine Batatais Tifhi I Tifhi 2 Shinmoe Nangoku
Cultivar
Table 19-1. Released bahiagrass cultivars.
Apomictic tetraploid; selected from 27 introductions from the USA Dwarf type, used for turf Cold hardiness, yield
Apomictic tetraploid Sexual Diploid Apomictic tetraploid Apomictic tetraploid Used for turf, propagated vegetatively Sexual Diploid Hybrid-higher yielding than Pensacola Sexual Diploid Hybrid-higher yielding than Pensacola Diploid Diploid; selected for palatability, high yield, and early spring vigor
Characteristics
Loch and Ferguson (1999) Blount et al. (200 I a)
Wilson (1987) Takai and Komatsu (1998
Loch and Ferguson (1999) Takai and Komatsu (1998)
Killinger et al. (1951) Loch and Ferguson (1999) Hein (1958)
Finlayson (1941 )
Reference
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660
GATESETAL.
increased initial germination rates, but the utility of this trait in field plantings has not been determined (R.N. Gates, unpublished data, 2002). An altered response to daylength is an additional trait that apparently accompanied RRPS selection for yield. By examining the response of individual plants to supplemental lighting during the winter, Blount et aI. (2001b) determined that more advanced cycles were less responsive to supplemental light. This may contribute to higher growth rates of advanced cycles during short days of the cool season (Gates et aI., 2001), and efforts are underway to incorporate this trait, as well as greater cold hardiness, into adapted germplasm to improve cool-season forage production. A naturally occurring diploid population (Pensacola) collected in northern Alabama, designated 'AU Sand Mountain' produced more forage than Tifton 9 in that region of Alabama (Blount et aI., 2001a).
IMPORTANCE Characteristics that Make the Species Important Edaphic Tolerance Bahiagrass is best adapted to sandy soils and tolerates low soil fertility and low pH. Sites where bahiagrass flourishes are often have periodic flooding and intermittent wet conditions. Bahiagrass also can survive well on droughty soils, compensating for low moisture availability with a more open stand. Other than N, fertility requirements for moderate growth, sufficient to support livestock grazing, are generally adequate on all but the most infertile sites.
Pest Tolerance With the exception of the recent emergence of introduced mole cricket (Scapteriscus spp.) as a severe pest in Florida, bahiagrass is relatively free of pest problems. Fall armyworm (SpodopteraJrugiperda) may defoliate bahiagrass stands in dry years when more preferred feed is not available, but stands are not affected. Soil and foliar disease organisms can be isolated from bahiagrass. Economic losses often are undetected and fungal diseases may playa more prominent role in loss of yield than previously thought (Blount et aI., 2002).
Persistence Most of the bahiagrass biomass is concentrated at the soil surface in the extensive network of rhizomes. These structures lie below the level where most cutting or grazing occurs and provide substantial storage of organic and inorganic nutrients. This dense sod is extremely competitive, contributing to persistence and minimizing weed invasion. Selection of Pensacola bahiagrass for increased herbage accumulation resulted in a more upright plant (Werner and Burton, 1991; Pedreira and Brown, 1996b). Persistence of advanced populations was not influenced by close, frequent mechanical harvest (Pedreira and Brown 1996a; Gates et aI., 1999). However, when RRPS Cycle 14 was stocked continuously, substantial stand loss occurred, hastened by the
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invasion of common bermudagrass [Cynodon dactylon (L.) Pers.]. Persistence of Tifton 9 is comparable to Pensacola.
Uses Forage-Livestock Production Persistence, even under low fertility, drought, flooding, and particularly, severe continuous stocking, makes bahiagrass a very reliable feed source for low-input beef cattle (Bos taurus) production. Most of the land area planted with bahiagrass in the USA is used for pasture for extensive cow-calf production. In Florida, about 75% of approximately 1.4 million hectares of pasture are dominated by the grass, supporting 1.2 million head of beef cattle. It responds to fertilizer inputs with higher dry matter (DM) production and increased carrying capacity (Table 19-2). With high N inputs, 'Coastal' bermudagrass produced more forage and animal gain than Pensacola bahiagrass at Tifton, GA (Utley et ai., 1974). In most other comparisons at lower fertility levels, more typical of production situations, differences in production and carrying capacity among grass entries at similar fertility levels were small. Energy concentration in bahiagrass (reflected in digestibility) declines substantially as the season progresses, regardless of fertility or defoliation management. Therefore, bahiagrass is not well suited to support demands of livestock with high energy requirements such as growing animals or lactating dairy cows. Utley et al. (1974) reported gains of 0.44 kg d- I for yearling beef steers grazing Pensacola bahiagrass compared with 0.49 when grazing Coastal bermudagrass. Higher individual performance (0.68 kg d- I ) was supported by 'Coastcross-l ' bermudagrass, developed specifically for increased digestibility. Daily gains below 0.5 kg also have been reported for steers grazing bahiagrass (Stephens and Marchant, 1960; Chapman et aI., 1972) and for heifers (Gates et aI., 1999). Daily gains of 0.9 kg were reported for calves grazing Tifton 9 bahiagrass with either continuous or rotational stocking (Hill et aI., 1998). Calf performance was buffered by cow milk even when energy concentration of grazed forage was marginal.
Thrf and Conservation Bahiagrass is widely used for low maintenance turf applications in the same regions where it is used as a forage. Once established, it is very persistent, requires low fertility and pest control inputs, and tolerates frequent close mowing. Because its growth tends to be prostrate, Argentine is preferred to Pensacola for turf (Busey, 1989). Rapid seedhead production during most of the growing season, particularly in early summer, diminishes the attractiveness of bahiagrass turf. Some progress was reported in minimizing this character (Busey, 1985). Highway right-of-ways have been planted or sodded with bahiagrass throughout the southeastern USA. The dense sod effectively limits soil erosion and provides a uniform turf.
Crop Rotation Sod-based rotations have historically been useful to improve crop yields. In the humid southeastern USA, disease incidence reduces the usefulness of forage legumes for this purpose. Establishment from seed makes bahiagrass more attrac-
1968-1971
1959-1964 1949-1951 1952-1954 1960-1964 1966-1970 1943-1947 1948-1950 1952-1954 1953
1968-1971
Tifton
Belle Glade Ona
Tifton
Jay
Gainesville
1953-1956
1973-1974 1944-1946 1948-1949 1960-1965 1966-1967 1966-1970
Jay
Gainesville
34 91 224 112 34 0 0 112 224 448 504
34 63 112 56 34 56 78 56 112 224 128
34 63 112 56 34 56 78 56 112 224 252
874 168 246 426 258 235 381 571 582 650 750 246
14800 10000 12000 11400 5000 2300 10500 7100 6700 9000 11500 4200 5500 8700 15500
258 370 538 459 482 859 370
146 224
8400 9600 12700 3600 4800 7900 22200
6500
16600 12400 18900
526
Avg. animal gain ha- 1, kg
15900
8900 6800 8400 10500
17700
15300
224 101 202 85 34 34 45 34 34 67 134 28 56 112 128
448 101 90 251 34 34 119 134 134 269 538 56 112 224 504
1962-1965 1961-1962
Ft. Pierce Ona
224 101 202 189 34 34 45 101 101 202 403 28 56 112 252
Grass entry Coastal
Coast-Cross I
Pensacola
P 20 S
K20
Fertilization, kg ha- 1
N
Years
Location
459 560 638 683
213
1010 112 246
3800 5100 8200
8800
15200 15000 14100 11700
Argentine
Avg. annual dry forage production, kg ha- 1
Table 19-2. Forage and animal production in response to fertilization and grass entry (from Chambliss and Jones, 1980; Utley et aI., 1974).
269
12300 12800 9900
Paraguay 22
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tive than many vegetatively propagated grasses for use in rotations. Bahiagrass sod provided benefits to peanut (Arachis hypogea L.) production and did not depress maize yields in south Georgia (White et aI., 1962). Subsequently, it was demonstrated that peanut yields benefited when preceded by bahiagrass for up to 5 yr (Norden et aI., 1977). Yields increased for each year the sod remained, but the greatest improvement was after the first year. Reduction in the lance nematode [Hoplolaimus galeatus (Cobb, 1913) Thorne, 1935] has been associated with bahiagrass. In greenhouse studies, root-knot nematode [Meloidogyne arena ria (Neal) Chitwood race 1], one of the most damaging pests of peanut, was controlled as effectively by rotation with bahiagrass as applying a nematicide (Rodriguez-Kabana et aI., 1988). Compared with continuous cropping of peanut, rotating with bahiagrass reduced soil densities of root-knot juveniles near harvest by 96 to 98% and increased peanut yield 27%. Effectiveness of Pensacola, Argentine, and Tifton 9 was equivalent. A study in Florida confirmed the yield benefit of bahiagrass preceding peanut, but nematode reduction was not clearly demonstrated (Dickson and Hewlett, 1989). Yield enhancement of peanut following bahiagrass in rotation has recently been attributed to disease control in addition to nematode suppression. Bahiagrass reduced the incidence of pathogenic fungi on shells of peanuts in a rotation study (Baird et aI., 1995). Stem rot (Sclerotium rolfsii Sacc.) and limb rot (Rhizoctonia solani Kuhn) incidence on peanut were both reduced in rotations following bahiagrass compared to continuous peanut (Johnson et aI., 1999). Yields following 2 yr of bahiagrass were increased about 30%. Similar potential benefits from bahiagrass rotation have been demonstrated with a reduction in root disease incidence from Rhizotonia solani or Pythium spp. in a double crop of snap bean (Phaseolus vulgaris L.) and cucumber (Cucumis sativus L.) (Sumner et aI., 1999). Two years of bahiagrass were required to provide any disease control, and disease severity was only reduced the first year following bahiagrass. Nematode (Meloidogyne spp.) reduction was noted in bahiagrass sod newly planted with peach [Prunus persica (L.) Batsch] trees (Evert et aI., 1992). The sod, however, interfered with tree survival, presumably from competition. Bahiagrass rotation also was effective for controlling root-knot and cyst (Heterodera glycines Ichinohe) nematodes in soybean [Glycine max (L.) Merr.] (Rodriguez-Kabana et aI., 1989a, 1989b). Yield advantage of soybean in rotation with bahiagrass averaged 114% of continuous soybean. Nematode populations increased and yield benefits from bahiagrass diminished in the second and third seasons. Seasonal Growth Patterns Because bahiagrass is a C4 species, it responds to high temperature and moisture, showing a seasonal pattern of herbage accumulation, depending on latitude and altitude. In the humid southeastern USA, for example, bahiagrass can be used for grazing or cutting from April to October. The growing season becomes shorter moving from the Coastal Plain toward the Piedmont and to the north (Ball et aI., 1996). Even in lower latitudes such as peninsular Florida, more than 85% of total annual production occurs during the six warmest months (April-September) of the year (Beaty et aI., 1980; Mis1evy and Everett, 1981; Mislevy and Dunavin, 1993).
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In central Florida, Pensacola bahiagrass shows little growth at temperatures below 15°C, a condition that predominates for about 100 d during the year (Mislevy, 1985). Maraschin (2000) reported a series of experiments that were conducted in the subtropics of southern Brazil (30° S) where only 17 to 18% of the total annual forage production was harvested or grazed during the fall and winter. In southeastern Brazil (21 ° S, 750 m altitude), summer (January-March) herbage accumulation rate of Pensacola bahiagrass, from 20 to 50 d of regrowth following fertilization with 200 kg N ha- i , was 86 kg DM ha- i d- i . The accumulation rate increased to 134 kg DM ha- i d- i over the following 15-d period and then declined to 29 kg DM ha- i d- i over the subsequent 30-d period (Haddad et aI., 1999). In the same region (380 m altitude) and at the same time of the year, Vendramini et ai. (1999) measured a mean warmseason accumulation rate of 130 kg DM ha- i d- i for Tifton 9 bahiagrass from 20 to 48 d of regrowth after clipping and fertilizing with 60 kg N ha- i , but after that, the accumulation rate leveled off. Differences among cultivars are probably related to plant architecture and sward structure. The more erect, taller-growing Tifton 9 had higher accumulation rates over longer regrowth periods, likely because of better use of incident radiation (Mislevy, 1983; Pedreira and Brown, 1996a, 1996b). Except for locations where winters are mild-to-warm, where moisture is available either from rain or irrigation, or where stockpiling is practiced, bahiagrass is not often used for winter grazing or clipping (Gates et aI., 2001). "Winter" forage production is frequently insufficient, even for semi-intensive livestock operations where stocking rates are medium to low. Early research (Stephens and Marchant, 1960) at Tifton, GA(31 ° 27' N) showed average (across five cultivars) mid-October yields (730 kg DM ha- i ) only 48% of those of mid-July (1520 kg DM ha- 1) in plots mowed at 4-wk intervals and fertilized with 112 kg N ha- 1 annually. When Pensacola bahiagrass plots were clipped every 30 d in the southeastern USA (32° N), twice as much forage was harvested in July as in October (Beaty et aI., 1968). In southeastern Brazil (22° 45' S), Pedreira and Mattos (1981) measured mean herbage accumulation rates of common, cv. Batatais, and Pensacola bahiagrasses in a 3-yr study. Herbage accumulation in the spring (September-October) was greater for Pensacola than common (19 vs. 6 kg DM ha- i d- 1). However, rates were almost the same during the summer (November-February; 70 vs. 61 kg DM ha- i d- 1), fall (March-April; 32 vs. 34 kg DM ha- i d- i ), and winter (May-August; above 3 kg DM ha- i d- i ). Higher spring production of Pensacola in that region is due mainly to higher accumulation rates in October (29 vs. 10 kg DM ha- i d- 1 for common bahiagrass), when mean daily maximum and minimum temperatures are about 28 and 15°C, respectively. Seasonal trends of forage production, however, were similar (90% of total annual DM accumulated in the "summer" half of the year and 10% during the "winter" months) for both grasses (Pedreira and Mattos, 1981). The trends at this location in Brazil and two locations in the USA (Gates et aI., 2001) indicate that cultivars interact strongly with the environment in the determination of seasonal yields. Though reasonably cold tolerant (Mis levy and Dunavin, 1993), bahiagrass winter growth is often low. Beaty et ai. (1980) showed that yield distribution of Pensacola bahiagrass can be improved by splitting total annual N rates over two midsummer applications as opposed to applying the total amount early in the growing
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season (April). Gates et aI. (2001) studied fall and winter forage production of Pensacola, Tifton 9, and RRPS Cycle 18 selection at two locations in the southern USA. In September, the two improved populations (Tifton 9 and Cycle 18) were accumulating, on average, three times more herbage per day than Pensacola (around 35 vs. under 10 kg DM ha- 1 d- 1) at Ona, FL (27° 26' N) and about twice as much at Tifton, GA (31 ° 26' N) (20 vs. 10 kg DM ha- 1 d- 1). As winter approached, herbage accumulation rates declined more rapidly in the improved populations, reaching a minimum of 5 kg DM ha- J d- J at Ona and essentially none at Tifton. Herbage accumulation curves were associated with the minimum temperature curves at both locations, and the improved populations (Tifton 9 and RRPS Cycle 18) were twice as productive as Pensacola during the cool season. In southeastern Brazil, Pedreira and Mattos (1981) measured average mid-winter herbage accumulation rates of 1.8 and 1.4 kg DM ha- 1 d- 1 for Pensacola and common bahiagrasses, respectively. Despite their higher productivity during both the warm and cool seasons, Pedreira and Brown (1996b) suggested that some of the advantageous winter hardiness of Pensacola (Burson and Watson, 1995) in the selection Tifton 9 and Cycle 14 had been lost. In a 3-yr field evaluation of spaced plants of Pensacola, Tifton 9, and RRPS Cycle 14 at Athens, GA (34 ° N), survival of Pensacola and Tifton 9 was 92% but only 64% for Cycle 14. Plants of Cycle 14 that were collected from field plots and had survived the two previous winters, survived better in the spaced-plant experiment, probably because of natural selection for winter hardiness. Both among populations and within Cycle 14, winter survival seemed to be related to morphology and growth habit. The more rhizomatous, prostrate, low-growing plants better withstood low temperatures (Pedreira and Brown, 1996b). This suggests that winter stand survival and ultimately persistence, may be critical in areas of marginal adaptation (higher altitudes and higher latitudes) of bahiagrass, such as the upper South in the USA.
Light Response Evidence exists that variation in temperature alone often cannot fully account for variation in forage production on a seasonal basis, even when moisture is not limiting. As a result, growth models based on thermal sum (e.g., degree days, DD) are sometimes inadequate for predicting yield or yield-related responses. For example, a 500-DD period in mid-fall to early winter will result in less than half the forage of a 1000-DD period in late spring or early summer. Bahiagrass DM yield and phenology respond to daylength (Marousky et aI., 1991; Blount et aI., 200 I b). Loch (1980) included Pensacola and Argentine bahiagrasses in the long-day species group, although sensitivity of the species to daylength can be modified by temperature. Mislevy et aI. (200 I) reported that despite adequate temperature and water availability, there is a general decrease in warm-season grass growth from early fall to mid-winter (October-February) in the southeastern USA. By increasing daylength from 10.4 to 15 h during this period, the authors recorded as much as a 167% increase in total cool-season herbage accumulation of Pensacola bahiagrass. Blount et aI. (2001 b) pointed out, however, that Tifton 9 and other RRPS Cycles are apparently less sensitive to day length than the original Pensacola population from
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which they originated. This suggests that fall/winter dormancy may be related to plant morphology and growth habit. Presence of rhizomes may be associated with DM partitioning and carbon balance and, ultimately, to herbage accumulation (top growth), in a manner that is more controlled by daylength than previously thought. When the night periods were interrupted by red, far-red, or red + far-red light, Tifton 9 plants grown in short days had less starch accumulation but greater leaf growth and dry weight than plants that did not experience night interruptions (Marousky and Blondon, 1995). Although carbon metabolism in bahiagrass responds to day length, interactions exist involving cultivars and the environment.
Compatibility with Legumes Because of its dense sod, bahiagrass does not associate well with legumes. However, some cool-season species, such as white clover (Trifolium repens L.), crimson clover (T. incarnatum L.), and arrowleaf clover (T. vesiculosum Savi), may persist in mixtures with bahiagrass ifP and K are not limiting and the grass is managed to limit shading ofthe legume (Burson and Watson, 1995). Bogdan (1977) suggested that due to the difficulty in maintaining a stable association between bahiagrass and legumes in swards, N fertilizer may be more suitable and profitable, especially when the demand for N is not high. He mentioned, however, cases where the association ofbahiagrass with white clover, lotononis (Lotononis bainesii Baker), alfalfa (Medicago sativa L.), and siratro [Macroptilium atropurpureum (DC.) Urb.] were successful in increasing forage yields and raising crude protein concentration. Dzowela et ai. (1986) found that heavy grazing pressure and short periods of deferment were critical to white clover productivity in bahiagrass pastures. White clover depended on annual re-establishment of seedlings from the soil seed bank. Extensive research in Florida has investigated the establishment and management of annual warm-season legumes in bahiagrass pastures. Overseeding aeschynomene (Aeschynomene americana L.) on bahiagrass improved summer gains of yearling cattle (Hodges et aI., 1976). When overseeded in combination with hairy indigo (Indigofera hirsuta L.), aeschynomene reduced N fertilization needs (Hodges et aI., 1977). Kalmbacher et aI. (1978) demonstrated that herbicidal suppression of bahiagrass sod was beneficial to the establishment of aeschynomene, alyceclover [Alysicarpus vaginalis (L.) DC.], or hairy indigo. Additional research revealed the importance of light penetration through the grass canopy for legume seedling establishment (Kalmbacher and Martin, 1983). Properly timed grazing was as effective as any herbicide tested in promoting seedling establishment. Carpon desmodium [Desmodium heterocarpon (L.) DC.], a perennial warmseason legume, has been successfully managed in bahiagrass pastures (Aiken et aI., 1991; Adjei et aI., 1999). Early work by Prine (1964) showed the forage potential ofbahiagrass grown in mixtures with rhizoma perennial peanut (Arachis glabrata Benth.) in the southeastern USA. Donnelly and Hoveland (1966) reported the advantages of growing a hybrid vetch [Vicia sativa L. x V. sativa subsp. cordata (Wulfen ex Hoppe) Asch. & Graebn.] with bahiagrass in areas of the USA where the grass is dormant during the cool months of the year.
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Williams (1994) proposed that a number of factors might interact to determine the relative competitiveness of grass and legumes in some mixed pastures. When grown with rhizoma peanut in the southeastern USA, Pensacola bahiagrass may be selectively grazed during dry periods in the spring because it withstands water deficits better than the legume and, therefore, accumulates more DM. If spring moisture is not limiting, N shortage to the grass and competition for light will confer advantage to the legume, compromising bahiagrass persistence in the mixture. When grown in mixtures with rhizoma peanut, less than optimal N supply may also affect bahiagrass persistence, as suggested by Valentim et al. (1987). Botanical composition dynamics ofbahiagrass-legume swards may also impact animal responses. Williams et al. (1989) studied the relative preference of beef steers (Bos indicus x B. taurus) for specific pasture components, by characterizing the diet of animals grazing a mixture of grasses (Cynodon spp. and Paspalum spp.) with 'Florigraze' rhizoma peanut. Steers did not graze either sward component preferentially during the summer, although during the dry spring months, water deficit caused a reduction in the peanut's contribution to the total DM because its leaves were shed to minimize drought stress. Associative Nitrogen Fixation Gains of nitrogen (N) in tropical soils in the absence of legumes have stimulated the examination of relationships between grasses and Nrfixing organisms. Dobereiner (1966) described an association between an Azotobacter species and the rooting zone of bahiagrass. Further research indicated the association occurred between broad-leaved, pubescent bahiagrass forms found on infertile soils of southern and central Brazil (Dobereiner, 1970). The association occurred only rarely with other bahiagrass types. Dobereiner et al. (1973) calculated fixation amounts from 15 to 90 kg N ha- I . Fixation is sensitive to oxygen tension and dependent upon photosynthetic activity of the grass, displaying a diphasic diurnal pattern (Dobereiner, 1976). Assays using 15 N confirmed that fixed N was incorporated into plant tissues. Addition of sucrose stimulated higher levels of fixation but did not enhance incorporation in plant tissues (De PoUi et aI., 1977). When Azotobacter pa~pali was successfully established in the rhizosphere of bahiagrass grown in sand culture, it was with a tetraploid form (PI 310176 from Brazil) and with a diploid. Use of acetylene reduction to assess soil-plant systems confirmed the fixation of N, but there was no evidence of N gains in the aboveground plant parts (Kass et al., 1971).
MANAGEMENT CONSIDERATIONS Establishment Establishment of bahiagrass, like many warm-season perennial grasses, is complicated by germination that occurs slowly over an extended period of time (West and Marousky, 1989) and by small, rather weak seedlings (Williams and Webb, 1958; Stephens and Marchant, 1960; Beaty and Powell, 1978). Burton (1939, 1940a) reported difficulties in establishing bahiagrass, particularly without
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the preparation of a good seedbed and careful seed placement. Acid scarification was proposed as a means to accelerate germination by overcoming dormancy (Burton, 1939, 1940a). Seed coat factors have been implicated as a cause of protracted germination and dormancy (Marousky and West, 1988; West and Marousky, 1989). Combinations of Tifton 9 seed lots, planting dates, seeding rates, and dormancy-reducing seed treatments were evaluated in field plantings in south Georgia and peninsular Florida (Gates and Mullahey, 1997). Even though seed lots were selected with high and low germination, as reported by state seed testing laboratories, differences in establishment were not encountered in field plantings. Likewise, seed treatments, although dramatically improving germination measured in the laboratory, provided no advantage in field establishment. Planting date effects were only important when fall-planted seed emerged and was damaged by early freezing temperatures. Increased seeding rate provided the only reliable predictor of increased rate of establishment, but the advantage was short-lived because adequate stands were obtained within 2 yr for all seeding rates. Stanley (1990) also examined seeding rate effects and concluded that rates as low as 2.2 kg ha- i eventually provided full stands. Removing the recalcitrant seed coat using a pneumatic device provided essentially naked caryopses which germinated very rapidly in greenhouse flats compared to intact seed (18.3 vs. 0.7% after 7 d) (Gates and Dewald, 1998). However, after 28 d there were no differences in total germination. When planted at equivalent rates in the field, no advantage was realized from seed coat removal. No longterm differences in stand resulted from planting dates between March and June. Slow and erratic establishment frequently produces an incomplete stand, even after several years. Tillage and seeding were evaluated as strategies to improve coverage in open stands (Gates, 2000). Only very open stands benefited from renovation. Stands with only a few plants per m 2 did not benefit from tillage or seeding and were best managed to limit weed encroachment and encourage the spread of existing plants.
Fertilization Nitrogen is generally the most limiting nutrient for grass growth and this is true for bahiagrass. Increasing annual N rate from 0 to 270 kg ha- i increased bahiagrass DM yield from 3.34 to 10.3 Mg ha- i but did not alter the seasonal distribution of herbage production (Beaty et aI., 1960). When cutting intervals were varied from 1 to 4 wk with increasing N rates, forage yield increased with increasing N, but neither N nor cutting interval influenced the ratio of seed stalk to leaf (Beaty et aI., 1963). Greenhouse and field studies were used to document that increased yield in response to N rate resulted from frequency and number of new tillers initiated (Sampaio and Beaty, 1976; Beaty et aI., 1977). Bahiagrass can accumulate a large pool ofN in rhizomes and roots (Impithuksa and Blue, 1978). Nitrogen supply is strongly buffered, minimizing any potential advantage of multiple applications during the growing season. Evidence that yield response to phosphorus (P) and potassium (K) fertilization was not economical for established bahiagrass (McCaleb et aI., 1966) prompted an extensive examination of fertilization of established pastures. Findings from the
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study revealed that with annual N application of 67 kg ha- 1, appropriate for typical cow-calf production, no PorK fertilization was needed for established pastures unless soil test levels were low (Sumner et aI., 1991). A comparison of response to fertilization by Pensacola, Tifton 9, and RRPS Cycle 18 showed no differential response among entries (Burton et aI., 1997). Herbage yield increased as N rate increased from 56 to 448 kg ha- 1, but no response to P (25 or 50 kg ha- 1) or K (38-232 kg ha- 1) fertilization was evident. Fertilization treatments had no influence on IVDMD.
Forage Quality/Antiquality Factors Moore et al. (1971) showed a decline in concentration of in vitro digestible organic matter (IVDOM) from 599 to 432 g kg- 1 in bahiagrass hay when age at cutting increased from 6 to 14 wk. Nitrogen concentration also declined between the two ages from 12.2 to 10.1 g kg-I; whereas, acid detergent fiber (ADF) and lignin concentrations increased from 409 to 453 g kg-I and from 37 to 54 g kg-I, respectively. In the southern USA, Cuomo et al. (1996) found a slight advantage of Argentine bahiagrass over Pensacola and Tifton 9. Argentine had a higher leaf percentage (87%) than the latter two cultivars (80 and 81 %, respectively), and this was associated with fewer inflorescences per unit area in Argentine, regardless of plant maturity (20, 30, or 40 d). The authors also reported lower lignin concentrations in Tifton 9 (40 g kg-I) forage than in the other two cultivars (44 g kg-I). This probably was associated with the trend for higher in vitro true digestibility in Tifton 9 (598 vs. 588 and 589 g kg-I for Pensacola and Argentine, respectively). Concentration of ADF was similar (320 g kg-I) across cultivars and maturities. Neutral detergent fiber (NDF) concentrations were 640, 657, and 642 g kg-I in Tifton 9, Pensacola and Argentine, respectively. Forage crude protein (CP) concentration also was similar (avg. 115 g kg-I) across cultivars and maturities in this study, where plots were fertilized with 224 kg N ha- I annually. In southeastern Brazil, Haddad et al. (1999) reported a decline in the nutritive value of Pensacola bahiagrass forage from the 20th to the 50th day of regrowth after cutting (IVDMD, 678 to 448 g kg-I; NDF, 600 to 635 g kg-I; CP, 145 to 97 g kg-I). In Tifton 9 bahiagrass fertilized with 60 kg N ha- I after a staging cut, Vendramini et al. (1999) found no effect of maturity on IVDDM (mean = 60 I g kg-I) or NDF (mean = 725 g kg-I) concentrations from the 20th to the 55th day of growth; although CP declined linearly from 117 to 71 g kg-I. Gates et al. (1999) compared 2-,4- and 8-wk-old regrowth of Pensacola, Tifton 9, and RRPS Cycle 14. They found a seasonal (April-September) decline in digestibility that was of greater magnitude than the effects of age of regrowth, cultivar, or cutting height. When animal nutritional requirements are high, such as for high-producing dairy cows, bahiagrass pasture or hay alone often are not enough to meet the energy and protein demand (Rollins and Hoveland, 1960). In the beef cattle industry of the southeastern USA, where grazed bahiagrass is the major nutrient supply for cow-calf operations and for raising yearling animals (Chambliss and Sollenberger, 1991), total-season average daily gains seldom exceed 0.5 kg (Sollenberger et aI., 1988).
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Mott and Moore (1977) reported that common bahiagrass and carpetgrass [Axonopus fissifolius (Raddi) Kuh1m.] accounted for much of the planted pasture in Florida until the introduction of 'Pangola' digitgrass (Digitaria eriantha Steud.) and Pensacola bahiagrass. Those authors suggested that the main reason for low animal outputs (gain) was the low carrying capacity of the former two grasses. The introduction of the two improved grasses, associated with increased use of N fertilization or white clover, had a dramatic effect upon carrying capacity and animal daily gain. While unfertilized carpetgrass and bahiagrass produced daily gains of 0.23 and 0.24 kg, respectively, fertilized Pensacola and unfertilized Pangola gave gains of 0.5 and 0.6 kg d- i , respectively (Mott and Moore, 1977). Despite the advances brought about by the introduction of improved cultivars and management technology, beef production from bahiagrass pastures in the southeastern USA are modest when compared with intensive systems based on higher-producing forages such as elephantgrass [Pennisetum purpureum (L.) Schumach.] (Sollenberger et aI., 1986; Sollenberger and Jones, 1989). Typical carrying capacity rarely exceeds four to five animal units ha- i from mid-March to mid-November. Forage quality is usually too low for weaned calves or stocker yearlings, especially in July and August, supporting low animal growth during that period, known as "summer slump". Decreased animal performance in mid-summer has been attributed to decreased nutrient digestibility, N concentration (Sollenberger et aI., 1988), and/or intake (Prates et aI., 1975). Low forage quality can be viewed from the perspective of plant characteristics, referred to as anti-quality factors. These may include chemicals such as tannins, alkaloids, saponins, and phenolic compounds or they can be structural components of cells and tissues. Wilson et al. (1989) reported the existence of poorly degradable tissues, mainly sclerenchyma fibers, linking the leaf epidermis with the vascular tissue and forming a "girder structure" that protects the epidermis from physical breakdown during ruminal digestion. This anatomical feature was discovered by Flores et al. (1993) in Pensacola bahiagrass and leaf cross-sectional area of sclerenchyma was proportionally more than three times (5.4 vs. 1.6%) that of 'Mott' elephantgrass. Although Mott had a higher percentage of cross sectional area represented by epidermis than did Pensacola (32.8 vs. 25.9%), the epidermis was more digestible in Mott, which probably explains why, when Mott hay was fed to sheep (Ovis aries), higher digestible organic matter intake and less ruminating time resulted. In addition to structural constraints, hydrocyanic acid (HCN) may accumulate in bahiagrass forage, although at relatively low levels «30 g kg-i) (Bogdan, 1977). There have been no reports of deleterious effects of HCN on animals grazing bahiagrass or fed bahiagrass hay. Defoliation Management Optimum defoliation ofbahiagrass by grazing or clipping depends on the specific goals of the pasture manager or hay producer. Producers whose objective is to raise yearling beef animals, for example, will adopt a different harvest strategy than a producer maintaining dry dairy cows and both will differ from the management chosen by a bahiagrass hay producer. Harvest methods may vary from the harsh
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conditions imposed by close defoliation under continuous stocking of cattle or horses (Equus caballus) to an intensively managed hay field clipped intermittently under a well planned schedule. In either case and under all other possible combinations among frequency, intensity, and time of the year, success depends ultimately upon knowing how bahiagrass responds to defoliation. Bahiagrass is known to tolerate intense defoliation. This was demonstrated by Beaty et aI. (1970) who determined that frequent defoliation at the soil level for 2 yr had little effect on total herbage accumulation of Pensacola bahiagrass. Even in sods clipped weekly to the soil level, tiller number was not reduced. Vegetative growing points, mostly on the underside of the rhizomes (which are often partially embedded in the soil), were practically impossible to remove. In addition, close defoliation may improve forage nutritive value by reducing the amount of dead material that accumulates under lax defoliation methods (Beaty et aI., 1968, 1970). Besides the morphological features, its tolerance to intense clipping and grazing can be partially explained by physiology of growth and carbon (C) budgeting. Beaty et aI. (1974a) used 14C02 to establish photosynthate assimilation and translocation patterns in Pensacola bahiagrass. Young leaves (6- and 13-d-old) assimilated more 14C than older (24- and 39-d-old) leaves, and they translocated a much smaller proportion (-10%) of the 14C assimilated than the older leaves (as much as 70%). The authors concluded that because of the rapid 14C translocation to new sites of tissue synthesis, dependence on reserves for regrowth is relatively brief. Sampaio et aI. (1976) demonstrated that during vegetative growth of Pensacola bahiagrass a new phytomer is added on average every 10 d. During the first 2 or 3 d, young leaves are sinks of photosynthate, retaining assimilates for the following 9 or 10 d. From that point on, leaves begin operating as sources, exporting to other sinks in the sheath, rhizome, roots, and new tillers. The authors also determined that 13 wk were required to kill the rhizomes by daily removal of allleaftissue and concluded that the survival mechanism ofbahiagrass seems to be minimal exposure of storage organs to defoliation by grazing and intermittent generation of new leaves over a long period of time. With temperate grasses such as orchardgrass (Dactylis glamerata L.) and ryegrass (Lalium spp.), synthetic events (tillering and leaf expansion) occur simultaneously and a decrease in the level of organic reserves results (Leafe et aI., 1974). In contrast, the herbage accumulation pattern of bahiagrass follows the leaf production cadence of one new leaf per tiller on the average every 10 d. This lack of a period of vigorous growth (or regrowth) suggests that the organic reserve pool is seldom jeopardized and can easily be replenished by photosynthesis. Sampaio et aI. (1976) concluded that bahiagrass is not likely to be eliminated by defoliation, regardless of severity, and that management practices should emphasize production of young, digestible forage. Morphology and growth habit affect the relative proportion of forage produced that is actually harvested or grazed to a given stubble height. Early clipping experiments (Beaty et aI., 1968) indicated that as much as 58% of the forage accumulated over monthly intervals was between soil level and 50 mm. Because of this, bahiagrass is not regarded as a good hay crop (Johnson, 1990); although, over the growing season, more than 80% of the total herbage accumulated is leaves (Beaty et aI., 1968).
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The amount and plant part composition of bahiagrass herbage harvested by cutting or grazing is affected both by environment and genotype. Nitrogen fertilization alters the vertical distribution of herbage as it promotes vegetative growth. Whereas, without N fertilization, as much as 67% of the total Pensacola bahiagrass herbage mass is found in the bottom layers (0-2.5 cm) ofthe canopy, often
