Published September, 1994
Impact of Grazing Managementon Soil Nitrogen, Phosphorus, Potassium, and Sulfur Distribution B. W. Mathews, L. E. Sollenberger,* ABSTRACT Little informationis available directly comparingsoil nutrient distribution under different defoliation managements.During 1990 (116 d) and1991 (141 d), ’Callie’ bermudagrass (Cynodondactylon var. aridus Harlan et de Wet) pastures grazed by Holstein heifers (Bos taurus) were used to determine the effects of two rotational stocking methods and continuousstocking on lateral andvertical distribution of extractable N, P, K, and S. A hay managementalso was included to compare soil responses under grazing and clipping. Nutrient distribution and concentration in the Apl horizon (0- to 15-cm soil depth) did not differ amonggrazing methods, but N, P, and K accumulated in the third of the pastures closest to shade, water sources, and supplement feeders (lounging areas where cattle tend to congregate or rest). Similar observations were madewith K in the Ap2horizon (15- to 30-cm soil depth). Nutrient concentrations were lower or tended to be lower in the Apl horizon of the hay managementthan in grazed pastures because of nutrient removalin harvested herbage. Across defoliation managements, greater extractable N, P, and K concentrations were observed in the Apl horizon in 1991 than in 1990. For N and K, this was attributed to fertilizer inputs in all managementsand partially to supplementalfeed inputs in grazed pastures. Increases in extractable P appeared to be associated primarily with flooding of the experimental site in late 1991. This study suggests that grazing methodof wellmanagedpastures may have little effect on short-term (2 yr) soil nutrient distribution, especially when grazing occurs during months when temperatures are high.
I
s addition to affecting animal production and plant growth, grazing managementof pastures may influence the redistribution and cycling of nutrients excreted in dung and urine. Unfortunately, the soil component of pasture systems has received little attention in most grazing trials. Animalwaste management is particularly critical in peninsular Florida, because the surface horizons of many sandy flatwoods soils (Spodosols and some Entisols) have high water tables and limited capacity to retain N and P, the nutrients that are associated most frequently with eutrophication of freshwater systems (Allen et al., 1982; Rechcigl et al., 1992). Increasing evidence implicates P rather than N concentration as the primary nutrient controlling algal productivity in Florida’s surface waters (Canfield and Hoyer, 1988). It has been suggested that intensively managed pastures are one possible source of P entering freshwater systems (Allen et al., 1982; Rechcigl et al., 1992). The environmental impact of these P additions and the need for nutrient control programs are currently the subject of much controversy (Canfield and Hoyer, 1988), but B.W.Mathews, College of Agriculture, Univ. of Hawaii at Hilo, Hilo, HI 96720-4091; L.E. Sollenberger, AgronomyDep., Bldg. 477, P.O. Box 110900, Univ. of Florida, Gainesvile, FL 32611-0900; V.D. Nair, Soil and Water Sci. Dept., Univ. of Florida, Gainesville, FL 32611; and C.R. Staples, Dairy Sci. Dep., Univ. of Florida, Gainesville, FL 32611. Florida Agric. Exp. Stn. Journal Ser. no. R-03368. Received 24 Aug. 1993. *Corresponding author (
[email protected]).
V. D. Nair, and C. R. Staples generally is agreed that more data are needed from cattle production systems (Mathews, 1992). Research is needed under a range of environmental conditions to determineif grazing methodaffects distribution of nutrients returned in excreta, thereby impacting the potential for losses to ground and surface waters (Allen et al., 1982; Johnson, 1993). It often is suggested that the magnitude of animal waste accumulation around cattle lounging areas may be smaller under rotational compared with continuous stocking (Williams and Haynes, 1990; Wilkinson and Stuedemann, 1992; Johnson, 1993), but field evaluation of the effect of grazing method on nutrient redistribution has been minimal. Recent recommendation(Allen et al., 1982) and establishment (SFWMD, 1989) of standards for P concentration in surface waters draining from pastures in Florida have highlighted the need to monitor both P and N concentrations in runoff from intensively managedsites. In addition, information is lacking on the composition of soil solution P in pasture soils. The objectives of this research were (i) to quantify Callie bermudagrasspasture the effects of four defoliation managements(rotational stocking with a short grazing period [rotational-short], conventional rotational stocking with a long grazing period [rotational-long], continuous stocking, and a hay system) on concentration and distribution of soil N, P, K, and S; (ii) to measure concentrations of N and P species in surface water draining from an intensively managedsite used to raise replacement dairy heifers; and (iii) to determine the forms and amountsof soil solution P in these soils. MATERIALS AND METHODS This experimentwas conductedat the University of Florida Dairy Research Unit, located 18 kmnorth of Gainesville, FL (29°60’Nlat). The research site was a 3.5-ha Callie bermudagrass pasture that had been established in 1988. The site was cleared in the late 1940sand used until 1988for summer annual silage crop productionand grazingor holdingof highly supplementeddairy cattle. Soils were of the Chipley sand series (thermic, coated Aquic Quartzipsamments).The water table of this soil is frequently40 to 60 cmbelowthe surface, and during the summerrainy season, standing water is quite common.The 1:2 soil/deionized water pH, Walkley-Black organic C (Nelson and Sommers,1982), clay (Miller Miller, 1987), bulk density (Blake and Hartge, 1986), NI-LC1effective cation exchangecapacity corrected for soluble salts (Shumanand Duncan, 1990), Mehlich-1 "double acid" (0.05 MHC1+ 0.0125 MH2SO4)extractable Ca and (Hanlon and DeVore, 1989), acid oxalate (0.1 MH2C204 0.175 M[NI-L,]2C204)extractable A1and Fe (Parfitt, 1989), and Langmuir P-sorption maximum (Yuan and Lucas, 1982) Abbreviations: SFWMD, South Florida Water ManagementDistrict; ICPES, inductively coupled plasma emission spectroscopy; SRP, soluble reactive phosphorus; TKN,total Kjeldahl nitrogen; USEPA,U.S. Environmental Protection Agency.
Published in J. Environ. Qual. 23:1006-1013(1994).
1006
1007
MATHEWS ET AL.: GRAZING& SOIL NUTRIENTDISTRIBUTION
Table1. Selectedchemicalandphysicalpropertiesof the Chipley soil beforeexperimentation (June1990). Horizon Soil property pH -~) Organic C (g kg Clay (g kg-’)~" -3) Bulk density (g cm ECEC(cmol~ kg-~)~: Mehlich-1 extractable Ca Mehlich-1 extractable Mg Oxalate extractable AI Oxalate extractable Fe Langmuir Pm~,§
Apl
Ap2
5.8 16 20 1.60 3.32 576 56 2110 1164 294
C1
6.0 13 23 1.53 2.86 mgkg- ~ 401 38 2382 1088 320
6.1 4 34 1.57 1.31 159 23 1908 756 336
Accordingto data compiledby the Univ. of Florida Soil Characterization Laboratory, clay mineralogyof the Chipleysoil typically ranges from 50 to 60%hydroxy-interlayered vermiculite, 15 to 25%quartz, 10 to 20% kaolinite, and 0 to 20%montmorillonite. Effective cation exchangecapacity(correctedfor soluble salts). Phosphate-Psorption maximum obtained from the slope of the linear Lang-
muirequation.
values for the Apl, Ap2,and C1 horizons (0- to 15-, 15- to 30-, and 30- to 63-cmdepths, respectively) at the start of the experimentare presented in Table 1. Nitrogen was applied to all treatments as NH4NO3 on 20 Mar., 6 July, and 17 Aug. 1990; and on 3 Apr., 22 May, and 10 July 1991(70 kg N ha-] at each application date). In January 1990, Mehlich-1extractable K concentration in the Apl horizon was approximately 30 to 35 mgkg-L Potassium was applied as K2SO4 at a rate of 120 kg K ha-] on 10 Apr. 1990, and as KC1at a rate of 80 kg K ha-] on 3 Apr. 1991. Theapplication of KC1in 1991wasmainlyto ensure adequate K for the hay management. Fertilizer P wasnot applied during the study. Defoliation managements were(i) rotational-short (15 paddocks with cattle movedto a newpaddockevery 1.5 to 2.5 -tgate
water spigot
( ....
0
d; target postgrazestubble height= 15 cm,(ii) rotational-long (three paddockswith cattle movedto a new paddockevery 10 to 14 d; target postgraze stubble height = 15 cm), (iii) continuousstocking(stockedat the samerate as the rotationallong systemas suggested by other studies with bermudagrass [Ball et al., 1991]), and (iv) a hay system(defoliated times each year by mowingto a stubble height of 8 to 10 cm every 36 to 42 d dependingon herbagegrowth). Meancarrying capacity supportedby the rotational-short grazing methodwas 3500kg of cattle liveweight ha-] d-] comparedwith 3000kg ha-] d-~ with rotational-long and continuous.Grazedpastures were 0.3 ha (118 by 25.8 m) and hay plots 0.1 ha (118 8.6 m). Treatmentswerereplicated three times in a randomized completeblock design. Previouscattle loungingareas wereavoidedwhensites were selected for the experimentalunits. Portable shadestructures (3.05 by 3.05 m), waterers, and supplementalfeed tubs were used in the present study. As recommended by Ellington and Wallace(1991), this equipmentwas moveda few meters along the length of the pastures every 2 d in all grazing methods to enhanceexcretal distribution and to prevent localized sod degradation. In addition, this practice allows for evaluation of grazingmethodwithoutthe confounding effect of differential placementof shade structures, waterers, and feed tubs. To facilitate the use of portablewaterers,plastic (polyvinylchloride) pipe with three equally spaced spigots was used along alternate fence lines. A pastureunit is illustrated in Fig. 1. Grazing commencedeach spring when sufficient forage was available to support three (250-kg average bodyweight) Holstein heifers per pasture and endedeach autumnwhenthe pastures could no longer support three animals. Grazingseasons were from 11 June through 4 Oct. 1990 (116 d) and Maythrough 18 Sep. 1991(141 d). Lengthof grazing seasons varied primarilydue to spring moistureconditions,with grazing initiated late in 1990becauseof spring drought. Averagedaily maximum temperature ranged from 29 to 33°C for the months of grazing. -4-+
~
waterer
po,rta.ble snaae ZONE1 = rubbertubsfor supplement)
59 LENGTH (m)
118
Fig. 1. Physical arrangementof soil sampling zones within a grazed pasture (not to scale). The portable shade, waterer, and supplementtubs remained in Zone 1 at ~11 times. This eq~pment w~ moved a few meters ~long the leith of the l~ture~ every 2 d in ~!1 gr~g meth~l~ to enhanceexcretal distribution and to prevent localized sod degradation.
1008
J. ENVIRON.QUAL., VOL. 23, SEPTEMBER-OCTOBER 1994
Table2. ExtractableN (NH~-N + NO3-N), P, K, and S (SO4S) concentrationsin soils underbermudagrass swardsbefore experimentation (June1990). Defoliation management
NutrientS" N
P
K
S
-1 mg kg Apl horizon Rotational-short Rotational-long Continuous Hay SEe Ap2horizon Rotational-short Rotational-long Continuous Hay SE~ C1 horizon Rotational-short Rotational-long Continuous Hay SE~t
11 11 10 10 0.5
95 96 98 95 3
73 70 76 74 5
17 24 21 17 2
4 5 4 4 0.5
78 79 71 80 3
16 19 22 17 2
10 12 16 13 2
3 3 4 3 0.2
64 52 55 61 6
14 17 20 15 3
10 14 17 11 2
Site hadreceivedfertilizer N, K, and S application 2 moprior to sampling. Standard error of a pretreatment mean.
Duringthe grazing seasons, additional animals (put-andtake) were added and removedfrom the pastures according to herbage growthand consumption.Put-and-takeheifers were grazed on reserve bermudagrasspasture until needed. According to standard industry practice, animalson all pastures (including put-and-takes) received 1.5 kg head-1 d-1 of a corn (Zea mays L.)-soybean [Glycine max (L.) Merr.] meal supplementthat included a mineral mix (Harris and Shearer, 1988). This formulation provided 34.4 g N, 13.8 g P, 9.0 K, and 2.4 g S kg-i supplement. Inputs of N, P, K, and S due to supplementaryfeeding (corrected for estimated nutrient removalin animalliveweight gain by the proceduresof Wilkinson and Stuedemann[1992] and Ball et al. [1991]) in the grazed pastures were 62 + 8, 27 + 3, 20 + 2, and 4 + 1 kg ha-1 yr-i, respectively. TheN input value does not account for gaseous (NH3) losses from excreta, whichmaybe substantial under warmtemperatureconditions (30-60%of the input; Williams and Haynes, 1990). Atmospheric deposition of and S was probably less than 10 kg ha-I yr -1 (Brezonik et al., 1980). Liveweightgains were not affected by grazing methodin 1990 (665 -1- 70 kg ha-l) or 1991 (741 + 58 kg ha-i; Mathewset al., 1994b). Estimated N, P, K, and removalin harvestedhay, basedon yield data and plant tissue analyses performedby standard procedures (Jones and Case, 1990), were 208+ 20, 30 + 2, 198 + 12, and 34 -t- -1 2 kg ha yr-l, respectively.A portionof these nutrients, particularly K, mayhave been returned to the soil by leaching from clipped herbage (Ball et al., 1991), because during the wet summer monthsmost hay cuttings experienced somerain damage.Hay yields were 9570 + 1130 kg ha-1 -1. yr Todetermineinitial concentrationsof extractable N, P, K, and S, two 10-core composite samples were collected from the Apl, Ap2, and C1 horizons along zigzag paths in each pasture 1 wkprior to commencement of grazing in June 1990 (Table 2). For subsequentsoil sampling,each grazed pasture was divided into three equal-sized zones, based on distance from shadeand water, runningthe length of the pasture (Fig. 1). Approximately 100 d after the end of each grazing season (first weekof January), a well-mixedcompositeof 15 soil cores (2-cm diam.) was collected along a zigzag line within each zone and from the hay plots to monitor changes in pH
and extractable N, P, K, and S. Areas whereshadestructures, waterers, or feeders had been located were avoided so that small areas with abnormallyhigh concentrations of excreta wouldnot be included in the sample. Samplestaken after the 1990grazing seasonwerecollected only fromthe Ap1 horizon, while samples were collected from the Apl, Ap2, and C1 horizons following the 1991 season. Rainfall for the 7-mo period between initiation of grazing in June 1990and the January 1991 sampling was 905 mm,while rainfall for the succeeding 12-moperiod was 1585 mm. Inorganic N (NIL-N+ NO3-N)was extracted from fieldmoist soil samples(stored for no morethan 1 wkat ° Cprior to extraction) with 1 MKCIusing a soil (dry wt.)/solution ratio of 1:10 and a shaking time of 1 h (Keeneyand Nelson, 1982). Air-dried soils were used for analysis of pH(as described previously) and extractable P, K, and S. Phosphorus and K wereextracted by the Mehlich-1procedureusing a soil/ solution ratio of 1:4 (Hanlon and DeVore,1989), whereas sulfur (SO4-S)was extracted by the acetate methodoutlined by Bonneret al. (1984). Mineral concentrations in the soil extracts were determinedas follows: inorganic N (NI-L-N NO3-N)using the automated phenate and Cd-reduction methods for NIL and NO3,respectively, P and K by inductively coupled plasma emission spectroscopy (ICPES), and SO4-S by ion chromatography. DuringJuly to September1991, surface water draining from the experimental area (not from individual treatments) was monitoredon 10 samplingdates (generally following rainfall events _> 20 mm)for concentrationsof total P, solublereactive P (SRP), total Kjeldahl N (TKN), NIL-N, and NO3-N. swale passed throughthe middleof the experimentalarea and duplicate 25-mLwater sampleswere collected within 2 m of the fence line wheredrainageexited the last pasture. Immediately after collection, the sampleswere filtered through a 0.45-1~mmembrane filter and stored at 4°C until analysis within 1 wk. Only trace amountsof suspended solids were collected by the filter. Total P and TKN wereanalyzedcolorimetrically following heating of a 10-mLsubsamplein the presence of a sulfuric acid, K2SO4,and HgSO4 solution as described by the USEPA (USEPA,1983). The TKNdoes not include NO~-N because a reducing agent was not addedduring digestion. The SRPconcentration also was determinedcolorimetrically as outlined by USEPA (1983). Concentrations NH4-Nand NO3-Nwere determined as described previously for the soil extracts. After the 1991grazing season, soil sampleswerecollected as outlined previously from the three zones and horizons of one of the continuouslystocked pastures to characterize soil solution P in terms of total P and SRP. Samplesalso were collected from the surface 2 cmof the continuously stocked pasture and fromthe Aplhorizonof a rotational-short pasture. The field-moist soil samples were moistenedwith deionized water to saturation (approximately26 %v/w) and equilibrated for 24 h before soil solution collection by vacuumfiltration throughWhatman no. 42 filter paper. Immediatelyafter collection, the solution wasrefiltered througha 0.45-~tmmembrane filter and stored at 4° C until chemicalanalysis. Theconcentrations of total P and SRPwere determined as described previously for the surface water drainage samples. Organic P was considered to be the difference betweentotal P and SRP (USEPA,1983). Datafor extractable N, P, K, and S were analyzedby using PROC GLMof the Statistical Analysis System (SAS Inst., 1985). Becausecarrying capacity (average stocking rate) different for the grazing methodstested, carrying capacity was included as a covariate in analyses of methodeffects on soil nutrient concentrations. Therewas no effect of the covariate
MATHEWS ET AL.: GRAZING& SOIL NUTRIENTDISTRIBUTION Table 3. Effect of defoliation management on concentrations of extractable N (NH~-N + NO3-N), P, K, and S (SO4-S) in the Apl horizon (0- to 15-cm depth) after the 1990 and 1991 grazing seasons. Defoliation management
Nutrient N
P
K
S
1009
Table 4. Effect of defoliation managementon concentrations of extractable N (NH4-N + NO3-N), P, K, and S (SO4-S) in Ap2 (15- to 30-cm depth) and CI (30- to 63-cm depth) horizons after the 1991 grazing season. Defoliation management
Nutrient N
P
-1 nag kg 1990 Rotational-short Rotational-long Continuous Hay SE§ 1991 Rotational-short Rotational-long Continuous Hay SE§
K
S
mg kg- ~
14a? 12ab 13a 9b 1
80a 79a 79a 78a 4
76a 77a 76a 38b 6
15ab~ 20a 17ab 11b 3
18a 16a 16a llb 1
130a 123a 124a 108b 4
107a~: 95a 87a 50b 17
14a~: 16a 13a 8b 2
Defoliation managementmeans within a year not followed by the same letter are different (P _< 0.05) by the SASleast squares meantest (PDIFF) unless otherwise noted. Defoliation managenaent meansnot followedby the sameletter are different (P -< 0.10) by the SASleast squares naean test (PDIFF). Standard error of a treatment mean.
in any of the analyses. Treatmentmeancomparisonswere made using the SASleast squares meantest (PDIFF).Comparisons responses amongzones were madeusing repeated measures analysis of variance procedures(repeated measuresin space) and associatedcontrasts basedon the univariate approach(Littell, 1989).
RESULTS AND DISCUSSION Soil pH and Extractable Nitrogen, Phosphorus, Potassium, and Sulfur Soft pH did not differ amongdefoliation managements in the Apl horizon after the 1990 (5.6) or 1991 (5.6) grazing seasons or in the Ap2(5.4) and C1 (5.8) horizons after the 1991 grazing season. Extractable N, P (1991 data), K, and S in the Apl horizon generally were lower in the hay managementdue to nutrient removal in harvested herbage (Table 3). In 1991, extractable in the Ap2 horizon also was lower (P _< 0.05) in hay fields than in grazed pastures (Table 4). This can attributed to leaching of recycled K in the grazed pastures and to uptake and removal in harvested hay (Wilkinson et al., 1989; Wftliams and Haynes, 1990). There were no differences among managements in nutrient concentration of soft in the C 1 horizon and few roots were observed in soil samples, probably due to frequent water saturation of this horizon. Nitrogen leaching generally is not thought to be a problem with improved grass pastures growingon flatwood softs in Florida (Impithuksa et al., 1984; Sveda et al., 1992). The low concentrations of extractable N in the C 1 horizon are in agreement with the results of a 5-yr plot study (annual N rates up to 896 kg ha- 1) conducted by Robinson (1989) with Coastal bermudagrass on a high water table Olivier sftt loam soil (Aquic Fragiudalf) in Louisiana. In the current study, greater extractable N (P = 0.02) and K (P = 0.04) (Table 3) in the Apl horizon in than in 1990can be attributed to fertftizer inputs for all
Ap2horizon Rotational-short Rotational-long Continuous Hay SE~: C1 horizon Rotational-short Rotational-long Continuous Hay SE~t
lla 9a 9a 7a 2
98a 98a 97a 89a 5
53a~" 5 la 49a 19b 7
13a 15a 13a 9a 4
4a 5a 5a 4a 0.5
77a 63a 61a 75a 13
13a 18a 22a 8a 7
10a 11a 13a 10a 2
Defoliation managementmeanswithin a horizon not followed by the same letter are different (P _< 0.05) by the SASleast squaresmeantest (PDIFF). Standard error of a treatment mean.
managementsand partially to supplementinputs in grazed pastures. In addition, extractable K concentrations in the Ap2 horizon of grazed pastures increased (P < 0.001) two- to threefold in 1991 compared to pre-experimentation values (Tables 2 and 4), presumably due leaching of recycled K. The large increases (P < 0.001) in extractable P concentration across managementsin the Apl horizon in 1991 compared with 1990 (Table 3) may be due in part to wet soil conditions in summer and fall and to prolonged flooding (>10 d) of the site during late summerof 1991. Similarly, extractable P concentrations in the Ap2 and C 1 horizons increased (P < 0.001) in 1991 comparedto pre-experimentation values (Tables 2 and 4). Increases in extractable P under such conditions have beenattributed to a faster rate of solubilization of reprecipitated P minerals, release of Fe, Ca, or Al-bound P, release of occluded P, and possibly enhanced mineralization of organic P (Dalai, 1977; Peaslee and Phillips, 1981). In addition, incubation studies conducted by Mathews(1992) demonstrated that continuously moist conditions favor the release of P from cattle manurecomparedwith alternate wet and dry conditions. Interestingly, P inputs (approximately 27 kg ha-1 yr -1) from supplement in 1990 did not result in greater Ap1 horizon extractable P concentrations in samples taken from grazed pastures following the 1990 season compared to preexperimentation values (Tables and 3). Considerable variation amongmonths within year and in monthly trends across years can be observed with Mehlich-1 extractable P in pastures on flatwoods softs in Florida (Espinoza et al., 1991). In contrast N, P, and K, extractable S slightly decreased (P -- 0.07) across all managements in the Apl horizon in 1991 compared with 1990. There were no zone x grazing method interactions (P > 0.22) or grazing method effects (P > 0.18) extractable N, P, K, or S in either 1990 or 1991. The study mayhave been too short (2 yr) to result in differences among grazing methods (Wftliams and Haynes, 1992; Wilkinson and Stuedemann, 1992). In both 1990 and 1991, there was a zone effect for extractable N and
1010
J. ENVIRON.QUAL., VOL. 23, SEPTEMBER-OCTOBER 1994
Table 5. Effect of sampling zone on concentration of extractable N (NH~-N÷ NO3-N), P, and inthe Apl hori zon (0- to 15-c m depth) after the 1990 and 1991 grazing seasons. Nutrient Year
Zoner
N
P
K
mgkg - i 1990 1 2 3 SE~
16a 12b 12b 1
82a 78a 79a 2
104a 62b 64b 3
1 2 3 SE~
22a 14b 13b 1
138a l19b 120b 3
135a 78b 76b 5
1991
Zone1 = closest, Zone2 = intermediate, and Zone3 = farthest in distance from shade structures, waterers, and supplemental feed tubs. Zonemeans not followedby the sameletter within a year are different (P _< 0.05) repeatedmeasuresanalysis of variance contrasts. Standard error of a zone mean.
K in the Apl horizon(Table5). Zoneaffected extractable P in the Apl horizon in 1991only (Table 5). Extractable Kin the Ap2horizon after the 1991grazing season also was greater (P < 0.05) for Zone 1 (70 mg1) th an for Zone 2 (41 mg kg-1) or 3 (42 mg kg-l). When there was an effect of zone, extractable N, P, and K concentrationsweregreatest in the third of the pastures closest to shade andwater (Zone1), indicating accumulation of excreta in this area. This is in agreementwith other pasture studies based on zone sampling(Wilkinson et al., 1989; Williams and Haynes, 1992) and with contour mapsconstructed using extractable Kdata from individual soil samples(Apl horizon) collected along transects of one block at the experimentalsite (Mathews et al., 1994a). Wilkinsonet al. (1989) also found that K leaching resulted in substantial subsurface (15- to 30-cmdepth) accumulationsof extractable K near shade and waterers in a Cecil sandy clay loam soil (Typic Hapludult). Potassiumredistribution to cattle lounging areas typically is more pronouncedthan N, P, or S becauseof the high K concentrationin forages relative to the amountretained in the animal body (Campkin, 1985). There were no zone effects for extractable S (P > 0.11), regardless of grazing season or horizon. The lack of difference amongzones for extractable S may be attributed in part to slowmineralizationof S in dung and possibly leaching of released SO4(Williams and Haynes, 1990, 1992). Actual amounts (mass balance) of N, P, K, and transfer betweenzonescannotbe calculated in the present study, because total elemental forms were not determined. Recent work in NewZealand (Williams and Haynes,1992) indicates that long-term studies are required to develop massbalance modelsfor P, K, and S redistribution becauseof the variation associated with the large amountspresent in the soil relative to the amountstransferred per year. Short-term balance calculations with extractable nutrients are usually complicated by nutrient fixation andrelease, andthe enhancedcycling of nutrients into soluble forms that occurswhenherbage utilization by grazinganimalsis high (Wilkinsonet al.,
Table 6. Influence of sampling date on total P, soluble reactive P (SRP), total Kjeldahl N (TKN), NHa-N, and NO3-Nconcentration in surface water draining from the experimental site during mid-sununer 1991. Date 8 July~: 12 July~§ 20 July 29 July 3 August 7 August 16 August:~ 26 August 31 August 3 September¶ SD# Avg
Total P
SRP
TKN~" NI-L-N -1 mg L
2.67 1.11 0.95 0.73 0.81 0.89 2.10 0.63 0.93 2.02 0.71 1.28
2.37 1.11 0.93 0.72 0.78 0.89 1.97 0.41 0.49 1.98 0.69 1.17
2.93 8.54 3.58 2.89 3.89 2.47 4.08 2.56 2.98 3.31 1.77 3.72
1.98 8.25 0.88 0.29 2.88 0.23 0.68 0.29 0.17 0.66 2.49 1.63
NO~-N 3.16 8.73 0.05 0.03 0.01 0.01 0.79 0.33 0.32 0.59 2.74 1.40
Does not include NO~-N. Cattle on rotational pastures grazing in or near swale area. Fertilizer N (70 kg N ha-1 as NtLNO~) wasapplied at the experimental site 2 d prior to sampling. Experimentalsite wasflooded. Standard deviation.
1989; Williamsand Haynes,1990). Extractable and total N balancesare further complicatedby NH3volatilization anddenitrification. Regardless of grazing method, visual observations confirmedthat heifers spent the majority of the midmorningto late-afternoon hours under shade structures and aroundwaterers. This resulted in increased deposition of dungand urine in Zone1. In Venezuela,Buschbacher (1987) determinedthat Bos indicus cattle grazing signalgrass (Brachiaria decumbensStapf.) pastures deposited over half their excretain the third of the pasture nearest the lounging areas, while 40%of the pasture receivedless than 15 %of the excreta. It is importantto note that, in the presentstudy, portableshadestructures, waterers, and supplementfeed tubs were movedregularly in all pastures accordingto the best management practice recommendations of Ellington and Wallace(1991). Thus, these results shouldnot be extrapolatedto pastures where these practices are not used. Furthermore,the influence of warmtemperatureson cattle behavior should be considered carefully (Sugimotoet al., 1987). The present and other recent studies (Sugimotoet al., 1987; Hirata et al., 1991)indicate that excretaldistribution andhence, efficiency of use of recyclednutrients are not likely to be improvedby rotational-short stocking in warmseasons or climates becausecattle spend considerable portions of the day near water and shade whenaverage daily maximum temperature exceeds 27° C. Under mild temperatures(12-24° C) at high elevations in Hawaii,excreta has beenreportedto be better distributed underrotationalshort than continuousstocking (Smithet al., 1986). Quality of Surface Water Drainage Total P concentrations(Table6) in surfacewater draining from the study site in 1991 always exceeded and wereoften twoto seventimes the tentative target concentration of _
