Agroforestry Systems 44: 253–265, 1999. 1999 Kluwer Academic Publishers. Printed in the Netherlands.
The establishment and early growth of three leguminous tree species for use in silvopastoral systems of the southeastern USA B. J. ADDLESTONE1, J. P. MUELLER2,* and J-M. LUGINBUHL2,3 1
Department of Forestry; 2Department of Crop Science; 3Department of Animal Science (*Author for correspondence: Box 7620, North Carolina State University, Raleigh NC 27695 USA; E-mail:
[email protected])
Key words: Albizia julibrissin, browse, forage, Gleditsia triacanthos, goats, Robinia pseudoacacia Abstract. Demand for goat meat in the eastern USA is growing as a result of preference by ethnic communities. Meat goat production systems in the southeastern USA should be designed to take advantage of the goats’ natural preference for browse. Trees could contribute to system productivity by supplying required nutrients when demand by growing animals is critical and the quality of forage is limited. A field study was established in Wake County, NC to evaluate the establishment and early growth characteristics of three leguminous tree species, Robinia pseudoacacia, Gleditsia triacanthos, and Albizia julibrissin. The three tree species were planted in single-row plots following a randomized complete block design (3 × 2 × 2, replicated six times) with two planting densities (intra-row spacing of 50 or 100 cm) and two coppice heights (25 or 50 cm). Bare-root seedlings were planted in March 1995, evaluated for browse quality (composited samples) in August 1995, coppiced in February 1996, evaluated for herbage mass and quality in July 1996, and evaluated for goat preference in August 1996. Herbage mass produced during the second season ranged from about 200 (G. tricanthos) to 3,200 kg/ha (R. pseudoacacia). Estimates of herbage quality were high for all species. Crude protein and acid detergent fiber of leaflets ranged from 23 to 28% and 12 to 22%, respectively. Robinia pseudoacacia has a high potential as a browse species for goats due to high herbage production (mean of 2,390 kg/ha) and goat preference. Gleditsia triacanthos was judged to be a low value browse species. Albizia julibrissin, although not highly preferred by goats in the trial holds sufficient potential to warrant further investigation.
Introduction The demand for goat meat in the eastern USA is growing as a result of preference from expanding ethnic communities, primarily European, Hispanic, and West Indian immigrants (Pinkerton et al., 1994). Goat meat may be viewed as an alternative to beef due to potential efficiencies in the production system (Terrill, 1993) and its low levels of total and saturated fat (Pinkerton et al., 1994). Efficient meat goat production systems in the southeastern USA must take advantage of regional pasture ecologies and the goats’ natural preference for browse. Native or naturalized leguminous trees could contribute to system productivity and efficiency by supplying required protein and nutrients during the seasonal production cycle (mid-late summer) when demand
254 by growing or lactating animals is critical and availability of high quality forage is scarce. The objectives of this study were to evaluate the establishment and early growth characteristics and goat browse preference of three leguminous tree species, Black Locust (Robinia pseudoacacia L.), Honey Locust (Gleditsia triacanthos var. inermis Schneid)., and Mimosa (Albizia julibrissin Durazz).
Materials and methods A field study was established at the North Carolina State University Goat and Forage Research Unit, Wake Co., NC at approximately 35.75° N lat. and 78.75° W long. Single-row, 8 m plots of Mimosa, Honey Locust and Black Locust were planted into a pasture composed primarily of endophyte-infested (96%) Tall Fescue (Festuca arundinacea L. Schreb) on a Typic Hapludult soil with 3 m between plots and replications. Border trees were planted at either end of a replication and top and bottom of each plot. Soil fertility at the experimental site was characterized by high base saturation (83%), a pH of 6.1, and no requirement for amendments. The experimental design was a randomized complete block (six replications arranged in a 3 × 2 × 2 factorial) with three tree species, two planting densities (intra-row spacing of 50 or 100 cm) and two coppice heights (25 or 50 cm). Variables measured included total height (TH), root collar diameter (RCD), above ground woody biomass (AGWB), herbage mass (HM; here defined as leaves, leaf petioles, and non-woody stem tips), herbage quality and goat preference. Site preparation activities included mowing, sub-soiling each row to a 30 cm depth, and applying a 30 cm band of 2.2 kg/ha Poast (active ingredient: Sethoxydim; 2-[1-(ethoxyimino)butyl]-5[2-(ethylthio)propyl]-3-hydroxy-2cyclohexene-1-one) to reduce grass competition. After planting, a 1.5 m high temporary electric fence (with 0.75 m high offset wire placed in front of the main fence) was erected to eliminate white-tailed deer (Odocoileus virginianus) herbivory. Trees were planted as bare-root seedlings in March 1995 and TH and RCD were measured before growth began. In August 1995, a foliar sample consisting of one leaf from the south facing side of the upper one-third of each tree was taken from all species (312 trees per species) and composited into three pooled samples representing one sample per species. Each sample was further divided into leaflets and petioles, then dried at 60 °C for 48 h and submitted to the North Carolina Department of Agriculture Forage Testing Laboratory for crude protein (CP) and acid detergent fiber (ADF) analyses. In August 1995, other measurements included TH and RCD. In January 1996, TH and RCD were measured for the second time. In February 1996, all trees were coppiced to the treatment cutting heights and AGWB was estimated by weighing all harvested material from each plot, sub-sampling and drying samples for 72 h at 60 °C. In June 1996, HM was estimated by hand-defoli-
255 ating two randomly selected trees per plot followed by weighing and oven drying at 60 °C for 48 h. These samples were analyzed for neutral detergent fiber (NDF), ADF, cellulose and lignin using methods described by Komarek, 1993. Aliquots of these sample were also submitted to the NCDA Forage Testing Laboratory for analysis of CP, NO3-ion, Ca, Mg, P, K, S, Cu, Fe, Mn, and Zn. At the same date, RCD, TH, ANMB, and SDMB were measured. In September 1996, trees were evaluated for goat preference. The browsing preference of the species was determined by permitting a sequential defoliation of each experimental replication with a group of 20 adult does. Each plot in each replication was given a defoliation score based on the proportion of the foliage consumed by the animals at 0, 4, 8, 22, 26, 30 and 44 h after the start of browsing. Plots were scored using a 10-point system where 0 was no defoliation and nine was complete defoliation.
Results and discussion Tree mortality First-year mortality was extremely low for all species. Mimosa had the highest mortality at 3.5% followed by Black Locust with 1.9% and Honey Locust with 1.6%. Tree height Black Locust grew more rapidly than either Honey Locust or Mimosa (Figure 1). The initial heights at planting on March 16, 1995 revealed that Black Locust (72.1 cm) was slightly more than twice the average height of Honey Locust (33.8 cm) and Mimosa (33.4 cm). Statistical comparisons of initial heights were not appropriate due to the large heterogeneous variation among species heights. The initial height differences among tree species were most likely due to species differences and different nursery practices. Black Locust increased 251% in total height during the first growing season (72.1 to 253 cm or approximately 85 mm/day). This was considerably more than Honey Locust, which increased 112% (approximately 18 mm/day) and Mimosa, which increased 91% (approximately 14 mm/day). The effect of density and the interaction of density and species were nonsignificant. The extent of coppice regrowth from the onset of the second growing season until June 30, 1996 was most dramatic for Mimosa (Figure 1). It increased 115% from the first season’s growth to an average height of 137.7 cm (approximately 49 mm/day). This increase was greater than the increases observed for both Black Locust (18% with a mean of 298.3 cm or approximately 30 mm/day) and Honey Locust (90% with a mean of 136.1 cm or approximately 42 mm/day). The rapid growth rate of Black Locust during the first 16 months after
256
Figure 1. The height of Black Locust, Honey Locust, and Mimosa measured in March, 1995, January, 1996 and June, 1996. June heights represent regrowth after a February coppice.
transplanting may be attributed to its potential for a high net photosynthetic rate (Mebrahtu, 1992). Honey Locust slow growth during the first growing season after transplanting may indicate an intolerance to drought in the juvenile stage. July rainfall was 45 mm (107 mm below normal) during summer of 1995. Mimosa poor growth performance the first growing season can be largely attributed to deer herbivory. Deer were observed to browse selectively upon Mimosa until electric fencing was installed in June. Root collar diameter Black Locust RCD was greater than either Honey Locust or Mimosa at all measurement dates (Figure 2). Initial RCD for Black Locust was 74% greater than that of Honey Locust and 36% greater than that of Mimosa. Black Locust RCD averaged 6.1 mm at planting and increased to an average of 26.3 mm after one season of growth (0.094 mm/day). By January 1996, Black Locust RCD was about 3.5 times that of Honey Locust and 91% greater than Mimosa. Honey Locust increased its RCD to an average of 7.5 mm (0.019 mm/day) and Mimosa to an average of 13.8 mm (0.043 mm/day). The RCD of all three tree species was substantially increased after winter coppicing (Figure 2). Black Locust RCD increased 19% to an average of 31.4 mm until June 30,
257
Figure 2. The root collar diameter (RCD) of Black Locust, Honey Locust and Mimosa measured in March 1995, January, 1996, and June, 1996. June measurement was following a February coppice.
1996 (0.033 mm/day). Honey Locust had the greatest increase in RCD (95%) to an average of 14.6 mm (0.047 mm/day) while Mimosa increased by 46% to an average of 20.2 mm (0.042 mm/day). Black Locust RCD was 115% greater than Honey Locust and 55% greater than Mimosa when measured in late June 1996. Statistical comparisons of RCD in January and June, 1996 were not appropriate due to the large heterogeneous variation among species diameters. The trends in root collar expansion followed the same approximate trends as TH, with a few exceptions (Figure 2). Black Locust exhibited a large increase in RCD during the first growing season. Root collar growth slowed after winter coppicing probably because carbohydrate reserves were utilized to support the growth of multiple branches. Honey Locust showed similar characteristics in RCD and TH during the three measurement periods. It is possible that growth of both RCD and TH were arrested by drought conditions. Mimosa displayed a more dramatic first year growth in RCD (207%) than for TH (115%).
258 Total above ground woody biomass Dry weights of the above ground woody biomass (AGWB) harvested at the coppiced heights of 25 and 50 cm in winter 1996 differed among species. Black Locust yielded the most AGWB with an average of 2,070 kg/ha. This was about 37 times greater than Honey Locust (56 kg/ha) and about 28 times greater than Mimosa (74 kg/ha). The large amount of heterogeneous variation among species, and the significant interaction effects, led to the examination of each species individually. The interaction between density and cutting height for Black Locust was significant and is of most importance because yield at a particular density was found to be dependent upon the height of coppice (Table 1). At the high planting density (50 cm spacing), cutting height had no effect on AGWB whereas at the lower planting density (100 cm spacing) the lowest cutting height (50 cm) produced the most AGWB. Honey Locust trees planted at inter-row spacing of 50 cm produced more (P = 0.06) AGWB (68 kg/ha) than trees planted at inter-row spacing of 100 cm (44 kg/ha). The influence of cutting height had the greatest impact on yield. Trees coppiced at 25 cm produced a greater amount (P = 0.006) of AGWB (75 kg/ha) compared to those coppiced at 50 cm (37 kg/ha). Planting density and cutting height had no effect on AGWB for Mimosa, possibly due to variation in growth caused by deer herbivory.
Herbage mass A large difference in the herbage mass (HM; leaves and non-woody material) harvested on July 16, 1996 was observed among the three tree species. Table 1. The influence of density (intra-row spacing) and coppice height on dry, woody biomass and dry, herbage mass of Black Locust, February and July, 1996. Density (cm)
Coppice Height (cm)
(Intra-row spacing)
25
50
Dry, woody biomass (kg/ha) 050 100
2,695 2,054
2,472 1,058
LSD0.05 = 564 Dry, herbage mass (kg/ha) 050 100
2,443 1,950 LSD0.05 = 498
3,213 1,955
259 Black Locust produced the most HM with an average yield of 2,390 kg/ha, followed by Mimosa (945 kg/h) and Honey Locust (366 kg/ha). Due to large heterogeneous variation, and the presence of significant interactions, data were examined separately for each tree species. With Black Locust the presence of an interaction between density and cutting height indicated that density or cutting height alone couldn’t explain HM production and that at a particular density HM was dependent upon cutting height (Table 1). The highest planting density (50 cm spacing) in combination with high cutting height (50 cm) produced the greatest HM (3,213 kg/ha) followed by the combination of high (50 cm spacing) planting density with low (25 cm) cutting height (2,443 kg/ha). The HM produced at the low planting density (100 cm spacing) was similar at both cutting heights. At the cutting height of 25 cm, planting density had no effect on HM whereas the denser planting yielded more HM at the cutting height of 50 cm. The 50 cm cutting height may have been less stressful to the trees and thus provided more meristematic surface area for bud development and subsequent branching. The high level of herbage productivity by Black Locust was most likely due to its potentially high rate of net photosynthesis (Mebrahtu, 1992). The coppice regrowth in both height and diameter was far greater for Black Locust than for the other two species, and was characterized by profuse branching and leaf production. Contrary to the other two species, branching in Black Locust was not confined to several main branches, but exhibited a high degree of shoot differentiation and branching patterns. Mimosa HM production showed a marked incremental increase from the first year (data not shown). This can be partially attributed to protection against deer herbivory which provided Mimosa the opportunity for increased photosynthetic leaf area and the ability to utilize its carbohydrate reserves for root production and foliar regrowth. Herbage mass yields were affected by both planting density and cutting height, but no interaction was present. Intra-row spacing of 50 cm produced more (P = 0.0006) HM (1,198 kg/ha) than a spacing of 100 cm (691 kg/ha). Similarly, cutting height of 50 cm yielded more (P = 0.013) HM (1,109 kg/ha) than cutting height of 25 cm (780 kg/ha). As observed with Black Locust, planting density played a significant role in determining HM with Mimosa whereby the 50 cm spacing resulted in greatest HM. These results indicated that number of trees per unit area are an important determinant of HM after the first coppice. However, the effects of density may decline gradually over time as root competition becomes stronger and trees continue to ramify and increase in size. Cutting height during the first year also played a very important role in determining HM for Mimosa: the higher the cutting height, the greater the HM yield. Most of the coppice regrowth came from the base of the stem; however, stems cut at 50 cm provided increased surface area for bud and branch development compared with the 25 cm cutting height. Although Mimosa did not produce a great amount of HM, it was judged to hold sufficient potential to warrant further
260 investigation in view of the paucity of available data on its use as a browse species. In the southeast USA, Mimosa is a common naturalized species planted exclusively for ornamental purposes (McArdle and Santamour, 1986). Recently, Bransby et al., 1992, evaluated Mimosa for forage use. They reported a dry matter digestibility of 66% by sheep with no apparent sign of toxicity. Consistent with its growth characteristics and morphology, Honey Locust yielded the least amount of HM. Herbage mass varied among treatments but not dramatically. Planting density was the most important factor in determining HM. Cutting height was not a significant determinant of HM. Planting density of 50 cm resulted in greater (P = 0.002) HM (476 kg/ha) than planting density of 100 cm (256 kg/ha). The low amount of HM produced by Honey Locust is most likely a reflection of a small incremental increase in RCD after the winter coppice. The leaves and leaflets were very small and the differentiation of buds into branches was far less numerous than for the other two species. Although Gold and Hanover (1993) have suggested that Honey Locust may be suitable for foliar forage production, its low rate of HM production precludes its use as a high producing browse species. Its best use is probably as an overstory tree in the production of pods for feed (Le Houérou, 1978).
Foliar analysis Based on analysis of composite foliar samples from August, 1995, apparent herbage quality of leaflets was high (Figure 3). Crude protein (CP) values for the leaflets of all species were greater than 20%. Assuming adequate intake (>3% BW) and no anti-quality factors, this level of CP is above the nutritional requirements of a 20 kg goat kid gaining 150 g/d (NRC, 1981). Leaflet ADF values were low and ranged from 12.5 to 22.0%, an indication of high digestibility. Leaf petioles were of considerably lower nutritive value than the leaflets, indicating that diet quality would be reduced if petioles were ingested by the animal in large quantities. Herbage analysis from July, 1996 revealed numerous significant differences in herbage constituents among the three species (Table 2). Cell wall constituents Mimosa was lower in NDF, ADF and lignin than the other two species and together with Black Locust was higher in cellulose than Honey Locust. The relatively low NDF and ADF values for all species (40.1–49.8 and 24.6 to 32.4, respectively) would suggest herbage that has potential for high digestibility. The lignin values are within ranges observed for other leguminous forage species.
261
Figure 3. Crude protein (CP) and acid detergent fiber (ADF) analyses of composite samples from Black Locust, Honey Locust and Mimosa sampled in August, 1995.
Nitrogen Crude protein values were high for all species, however Black Locust was significantly higher in CP (20.8%) than the other species (Table 2). Mimosa was higher in CP (16.8%) than Honey Locust (14.8%). All species were low in No3 ion. In fact Honey Locust and Mimosa had no detectable NO3 ion. Minerals Calcium and phosphorus percentages of the herbage ranged from 1.34 to 1.17 and 0.15 to 0.42, respectively. These ratios would be considered too wide for optimum performance under most conditions. Ca to P ratio should not be below 1.2:1. No significant differences were noted among species for Mg, Cu, and Fe. Honey Locust had higher levels of P, Mn, Zn than the other two species. With a few minor exceptions, mineral levels of the foliage of these species were similar to that reported for alfalfa (NRC, 1981).
262 Table 2. Leaf (leaflets and petioles) herbage quality estimates from hand plucked samples of Black Locust, Honey Locust and Mimosa, Raleigh, NC, July 1996. Constituent
Tree species Black Locust
Cell wall a
NDF ADFc Cellulose Lignin
Honey Locust
Mimosa
––––––––––––––––––––––––––––––– % ––––––––––––––––––––––––––––––– 049.8 032.4 017.2 015.0
ab a a a
48.5 28.1 15.4 12.5
a b b b
40.1 24.6 17.3 07.0
b c a c
Nitrogen Crude protein Nitrate ion
020.8 a 000.07 a
14.8 b 00.0 b
16.8 c 00.0 b
Ca P Mg S K
001.26 000.21 000.18 000.19 001.95
01.34 00.42 00.22 00.16 01.64
Micro-minerals
––––––––––––––––––––––––––––– ppmd ––––––––––––––––––––––––––––––
Cu Fe Mn Zn
007 170 040 023
Macro-minerals
a a a a
a a a a a
07 95 72 27
a a b b
a b a b b
01.17 00.15 00.27 00.15 01.18
08 83 49 21
b c a b c
a a a a
a
Neutral Detergent Fiber. Values within a row followed by the same letter are not significantly different, P0.05. c Acid Detergent Fiber. d Parts per million. b
Goat browsing preference Observations indicated that Black Locust was the most preferred tree species (Figure 4). High preference for Black Locust was reported by Lambert et al. (1989) and Papachristou and Papanastasis (1994). However, this species has been reported to contain high levels of tannins, phenolics, and robin, a toxic lectin (Cheeke and Schull, 1985). Goats are known to have a high tolerance to secondary compounds, including tannins, due to unique tannin-binding proteins secreted from their salivary glands (Cheeke, 1992). Black Locust plots were completely defoliated and received scores of nine (very high preference). When the goats were released into the experimental blocks, Black Locust was observed to be the first food source consumed. Goats appeared to browse on Black Locust with enthusiasm and would rise on their hind legs
263
Figure 4. Goat preference as indicated by defoliation scores from Black Locust, Honey Locust and Mimosa during a 44 hour browsing period, September, 1996. A score of 1 = light defoliation and a score of 9 = complete defoliation.
to obtain additional browse. Honey Locust was also considered a high preference species as plots were completely defoliated (score of nine). However, due to the small stature of Honey Locust and the lack of HM, little time was spent browsing it. Goats targeted Honey Locust after Black Locust had been mostly consumed. Mimosa was considered a low preference tree by goats in this trial (score of 3–4). Goats would only taste the leaves and then move on to one of the other species. Nevertheless it is possible goats would readily consume Mimosa and perform satisfactorily when it is the only option for browsing.
Conclusions and recommendations Black Locust and Mimosa both seem suitable candidates for further research as a source of high quality browse for silvopastoral systems for meat goats. Although, all three species exhibited a very low mortality rate, Honey Locust did not attain satisfactory height nor root collar diameter and did not produce substantial herbage mass in response to coppicing. The first two years of data
264 indicated that Honey Locust cannot produce enough herbage to justify it as a silvopastoral forage supplement for goats. Both Black Locust and Mimosa showed marked increases in herbage production, height and root-collar diameter, indicating that they have potential to provide adequate summer forage for goats. During the two-year establishment period of this trial, Black Locust produced the most herbage mass (averaging about 2,400 kg/ha) and based on sheer productivity and animal preference, it should be the most suitable silvopastoral species for goats. Nevertheless, the question remains regarding possible anti-quality factors present in this species and the potential performance-retarding characteristics of such factors. This question provides the onus for further research designed to measure animal performance of goats browsing Black Locust. Mimosa produced the second greatest amount of herbage mass and, although its drooping branch growth favors browsing because of easy access, it was not highly preferred by goats under the conditions of our trial. Recognizing that considerable variation exists in animal behavior and in year to year environmental factors, more research is needed to understand the relationship between palatability and goat preference. If Mimosa could be demonstrated to be browsed by goats under severe conditions or when browsed as the only available browse species, then it may be suitable to be integrated in a mixed silvopastoral system as an alternative to Black Locust and herbaceous forages.
Acknowledgements The authors express their thanks to faculty and staff: Dr. Robert C. Kellison, Department of Forestry, for collaborative technical advice, and Tim Hall and Marie Schweinefus, Department of Crop Science for their invaluable assistance with the field work.
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