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Agronomy Department, P.O. Box 110300, Univ. of Florida, Gainesville, FL 32611-0300
* Corresponding author (les{at}mail.ifas.ufl.edu).
Received for publication October 21, 2002.
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ABSTRACT |
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 TOPNOTES ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Abbreviations: CP, crude protein • DOM, digestible organic matter • DM, dry matter • IVOMD, in vitro organic matter digestibility • LL, lower layer • OM, organic matter • T-5 cm, top 5-cm layer • UL, upper layer
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INTRODUCTION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Vertical heterogeneity is pronounced in rotationally stocked limpograss pastures (Moore et al., 1986; Holderbaum et al., 1992). Herbage bulk density is much less in the upper than lower portion of limpograss canopies (Moore et al., 1986), and canopy bulk density has been reported to have a strong negative correlation with bite weight for tropical grasses setaria (Setaria sphacelata Schumach.) and rhodesgrass (Chloris gayana Kunth) (Stobbs, 1973). Additionally for limpograss, leaf percentage may be as much as three times greater in the upper than lower half of the canopy (Holderbaum et al., 1992). Vertical heterogeneity in plant-part distribution affects chemical composition and digestibility of herbage from different canopy layers. For example, N concentration of limpograss herbage was 25 to 100% greater in the upper than lower half of the canopy (Holderbaum et al., 1992). Because low herbage CP concentration may limit gains of cattle grazing limpograss (Holderbaum et al., 1991), understanding the distribution of N may have a major impact on grazing management recommendations, specifically the height to which canopies should be grazed. Grazing height has been reported to affect canopy structure and nutritive value of C4 grass herbage in other studies (Fisher et al., 1991; Pitman et al., 1994). Thus, there appears to be potential to alter diet composition and animal performance by varying grazing height (Cosgrove, 1997).
Based on these studies, it was hypothesized that different grazing heights of continuously stocked limpograss pastures will affect canopy structure, leaf and stem percentage, and N distribution. Taller, more leniently grazed swards are expected to provide lower bulk density in the grazed portion of the canopy and greater opportunity for leaf selection leading to higher CP concentration in the diet. Shorter, more dense canopies may limit selection for leaf resulting in lower diet CP. The objective of this experiment was to evaluate the effect of grazing height on total and plant part bulk density and nutritive value in various canopy layers of continuously stocked limpograss pastures, with the intent that these data be used to guide choice of grazing height for use in production systems based on limpograss.
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MATERIALS AND METHODS |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Treatments
Treatments were the complete factorial arrangement of three canopy heights (20, 40, and 60 cm) and three layers (top 5 cm, upper 25% by height, and next 50% by height). The design was a split-strip plot replicated four times. Stubble was the whole-plot factor and was allocated to pastures at random. Layers were considered strips within each stubble height, and they are defined in more detail later.
Grazing Management
Two crossbred (75% Angus, 25% Brahman) yearling heifers of similar initial weight (355 ± 34 kg and 335 ± 25 kg for 1998 and 1999, respectively) and medium frame, were assigned to each pasture as testers; they remained on that pasture during the entire experimental period each year. A variable stocking rate was used and animals of the same breed and similar weight as the testers were added or removed as needed to maintain the treatment canopy height for each pasture. Detailed description of the canopy height measurement and grazing management has been provided by Newman et al. (2002).
The sward canopy was characterized on 13 Aug. and 15 Sept. 1998 and 14 July, 9 Aug., and 7 Sept. 1999. There were only two sampling dates in 1998 because of later initiation of grazing and unusually severe levels of insect damage occurred (described in detail in results section) starting in mid-September. Canopy characteristics were affected to some extent by the time of the 15 September sampling but to a much greater degree later, causing a late-season sampling date to be cancelled.
Sampling was conducted at five representative sites in 1998 and four in 1999 in each experimental unit using a 0.5-m2 (0.5 by 1.0 m) quadrat that was vertically adjustable in 5-cm increments. The top 5-cm layer (T-5 cm) was sampled in all pastures to approximate the forage consumed by cattle (Fig. 1) . Then a second layer of 5- and 10-cm depths, respectively, was clipped from the 40- and 60-cm canopies. This allowed comparison among treatments of the top 5 cm of each canopy plus a mathematically composited upper layer (UL) that represented the top 25% by height of each canopy. The UL was the top 5 cm in the 20-cm pastures (UL and T-5 cm were the same for this treatment), 10 cm in the 40-cm pastures, and 15 cm in the 60-cm pastures (Fig. 1). The next lower 50% by height of the canopy was also characterized and identified as lower layer (LL). It corresponded to the subsequent 10 cm (from 5 to 15 cm above soil level) in the 20-cm canopy, 20 cm (from 10 to 30 cm above soil level) in the 40-cm pastures, and 30 cm (from 15 to 45 cm above soil level) in the 60-cm pastures (Fig. 1). Samples were cut using mechanical shears and all the material collected was immediately dried at 55°C. Total limpograss herbage in a layer was hand-separated into leaf blade and stem plus sheath fractions, weighed, ground in a cyclone Udy mill to pass a 1-mm screen, and stored for chemical analyses.
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Response Variables and Calculations
Total herbage, leaf, and stem bulk density of live limpograss herbage were calculated and expressed as kg DM ha-1 cm-1. Variables indicative of the nutritive value of the canopy were also calculated, and they included leaf and stem percentage, CP, and IVOMD. These variables were computed for the T-5 cm layer, perhaps the layer most closely associated with the diet consumed by the animals, the UL, and the LL to characterize the distribution of the different plant components and nutritive value in the canopy profile. Percentage leaf and stem, CP and IVOMD for leaf and stem, and bulk density for total herbage, leaf, and stem are the weighted means across sampling dates within a year.
Statistical Analyses
Data were analyzed using mixed model methodology through PROC MIXED (SAS Inst., 1996). In all models, effects of canopy height, layer, and their interactions were considered fixed effects. Replicates and year were modeled as random effects. The nature of the canopy height effect was assessed using orthogonal polynomial contrasts. All means reported in the text are least squares means.
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RESULTS |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Bulk Density
There were year x canopy height x layer interaction effects for total herbage (P = 0.037) and stem bulk density (P = 0.035), and a year x canopy height interaction effect for leaf bulk density (P = 0.001); therefore, data were analyzed by year for total, stem, and leaf bulk density. Analyses by year showed canopy height x layer interaction in both years for total herbage (P = 0.026 and 0.031, respectively) and stem bulk density (P = 0.011 and 0.009, respectively), so data were analyzed by year and layer.
Total Herbage Bulk Density
Bulk density decreased with increasing canopy height for all layers in 1998 (Fig. 2)
. The response had both linear and quadratic terms because there was a lesser decrease between 40 and 60 cm than between 20 and 40 cm. Canopy height x layer interaction also occurred because the difference between UL and LL density was less pronounced for 60-cm canopies than for 40- or 20-cm canopies, but the difference between T-5 cm and UL was greater for 60-cm canopies than for 40 cm. The general nature of the response was similar in 1999 to that described for 1998, with the exceptions that LL bulk density was consistently greater in 1999 than in 1998, and the difference between T-5 cm and UL was less in 1999 than in 1998.
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Stem Bulk Density
There were canopy height x layer interaction effects on stem bulk density in both years (Fig. 2). Because such a large proportion of limpograss total herbage is stem (86 and 81% across treatments and sampling dates in 1998 and 1999, respectively), the nature of the responses and causes for interaction were very much like those already described for total herbage.
Bulk Density Synopsis
Total bulk density was consistently greater for shorter than taller canopies and for LL compared with UL or T-5 cm. Differences among heights were much greater between 20 and 40 cm than between 40 and 60 cm. Leaf bulk density was quite low and varied to a lesser degree among heights and layers than stem bulk density. Lower density in 1998 than 1999, despite 35% greater precipitation in 1998, was primarily a result of the spittlebug and chinchbug infestations in September 1998 that caused premature tissue senescence and death (Newman et al., 2002). Averaged across canopy heights and layers, total bulk density was 66 kg DM ha-1 cm-1 in September 1998 and 110 and 103 kg DM ha-1 cm-1 in August and September 1999.
Plant-Part Proportion and Nutritive Value
Leaf and Stem Percentage
Leaf percentage was affected by year x canopy height (P = 0.001) and year x layer interaction (P = 0.086). When analyzed by year, there was no canopy height effect in either year, although there was a linear trend (P = 0.105) in 1998. Leaf percentage in 1998 tended to increase as canopy height increased from 20 cm (12%) to 60 cm (16%). In 1999, the trend was opposite, i.e., decreasing leaf percentage with increasing height (20 and 17% for 20- and 60-cm heights, respectively), but did not approach significance (P = 0.341). Thus, canopy height effects on leaf percentage were small or nonsignificant and inconsistent in the 2 yr.
In contrast to the minimal response to height, there was a strong layer effect on leaf in 1998 and 1999 (P 
0.001). In both years, the upper canopy (T-5 cm and UL) had a greater leaf percentage than LL. Leaf percentage in LL in 1999 was only slightly higher than in 1998, but T-5 cm and UL leaf percentages were considerably greater in the second year (Fig. 3)
. Because limpograss was separated into only two fractions, leaf and stem, P values for stem percentage are identical to those for leaf and the pattern of the response is inverse that of leaf (Fig. 3).
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Stem IVOMD was affected by layer (P 0.001) and by year x canopy height interaction (P = 0.031). Stem IVOMD was greater in the upper part of the canopy (558 and 548 g DOM kg-1 OM for T-5 cm and UL, respectively) than in the lower layer (527 g DOM kg-1 OM). When analyzed by year, stem IVOMD tended to be influenced by canopy height in 1999 but not in 1998. Stem IVOMD averaged 523 g DOM kg-1 OM across grazing heights in 1998. In 1999, there was a trend (P = 0.194) toward a linear effect of canopy height, with 60-cm canopies tending to have lower IVOMD (545 g DOM kg –1 OM) than 20- and 40-cm canopies (570 and 580 g DOM kg–1 OM, respectively).
Plant-Part and Nutritive Value Synopsis
Leaf percentage was consistently greater in the top than lower part of the canopy, but the effect of canopy height was not consistent. In both years the T-5 cm layer of 40-cm pastures had as high a leaf percentage as any other layer regardless of height.
Both leaf and stem CP generally decreased from the top to bottom of the canopy and likewise from 20- to 60-cm heights. The most striking aspect of the CP data was that leaf CP was up to three times greater than that of stem. The general pattern of IVOMD response, i.e., greatest at the top of the canopy and for shorter pastures, was the same as for CP, but treatment effects on IVOMD were less pronounced and leaf and stem fractions differed much less proportionately in IVOMD than CP.
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DISCUSSION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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In the current study, total, leaf, and stem bulk densities of all layers of 20-cm pastures were greater than for 40- or 60-cm pastures. Greater stem and total bulk density have been reported for short vs. tall pastures of other C4 grasses including digitgrass (Digitaria eriantha Steud; Ludlow et al., 1982), bermudagrass [Cynodon dactylon (L.) Pers.; Fisher et al., 1991], and Caucasian bluestem [Bothriochloa caucasica (Trin.) C.E. Hubb.; Christiansen and Svejcar, 1987]. Total and stem bulk density were highly negatively correlated with bite weight for the C4 grasses rhodesgrass and setaria (Stobbs, 1973). Even leaf bulk density was negatively correlated with bite weight for some canopies (Stobbs, 1973). Greater bulk density has been associated with high structural strength of herbage, thus requiring greater force to harvest a bite and imposing a constraint on ingestive behavior (Laca et al., 1992; Cosgrove, 1997).
Focusing on the top 25% of the canopy where grazing would most likely occur, total and stem bulk density of 20-cm swards were generally 1.5 to more than two times as great as in 40- and 60-cm swards. High total bulk density of the 20-cm canopies, particular when stem constitutes such a high proportion of the herbage, may affect selection of leaf by grazing livestock. In dallisgrass (Paspalum dilatatum Poir.) pastures, high stem bulk density served as a barrier to selection for leaf (Flores et al., 1993). Because the proportion of stem is relatively high in most C4 grass canopies and because stem often is considerably lower than leaf in nutritive value (Holderbaum et al., 1992), lack of opportunity for selection can have a significant negative effect on animal performance. In fact, access to leaf may have a greater effect on performance than the amount or proportion of leaf in the canopy (Burns et al., 1991; Sollenberger and Burns, 2001a). This is supported by data from Burns et al. (1991), who showed that gains of cattle grazing switchgrass (Panicum virgatum L.; 0.59 kg d-1) were greater than those of cattle grazing bermudagrass (0.22 kg d-1), despite the higher leaf percentage of the latter. Switchgrass leaf grew higher in the canopy and was less closely associated with stem than leaf of bermudagrass, so it was accessible to grazing animals. Although no specific density ceilings, above which selection is limited, have been reported in the literature, it seems likely that the difference in bulk density between 20- and 40-cm swards was large enough to have an important effect on opportunity for selection. This conclusion is supported by the higher average daily gain of animals grazing 40- vs. 20-cm pastures (0.64 and 0.45 kg d-1, respectively; Newman et al., 2002). Differences in density of the upper canopy of 40- and 60-cm swards were very small; thus, there appeared to be little further opportunity for increased canopy manipulation with the 60- compared with the 40-cm height. It should also be noted that very low bulk densities have been associated with low bite weights and poor animal performance (Stobbs, 1973), so some unknown intermediate level is likely optimum.
Compared with a range of forage types (Sollenberger and Burns, 2001a), limpograss leaf bulk density was very low for all treatments. Total canopy leaf bulk density of 30 to 100 kg ha-1 cm-1 is common for C4 grasses like setaria, flaccidgrass (Pennisetum flaccidum Griseb.), bermudagrass, and others (Sollenberger and Burns, 2001a). Limpograss leaf bulk density ranged from 5 kg ha-1 cm-1 to a maximum of nearly 30 kg ha-1 cm-1 in various layers of the canopy, with an average total canopy leaf bulk density of <20 kg ha-1 cm-1. This response was expected for continuously stocked limpograss, particularly in light of reported low leaf percentages (Lima et al., 1999) and low leaf bulk density (Holderbaum et al., 1992) in rotationally stocked swards. In the current study, layer effects on leaf bulk density were not consistent. Because leaf percentage was relatively low across layers, total herbage bulk density had a greater impact than leaf percentage on leaf bulk density.
Plant-Part Proportion and Nutritive Value
There was no consistent effect of canopy height on leaf percentage. Lack of a consistent height effect suggests that any major differences in leaf proportion in the diet of cattle grazing these pastures would primarily be a function of accessibility and selection of leaf, not proportion of leaf in the grazed horizon.
The layer effect on leaf percentage was much more consistent than the height effect, and leaf percentage decreased from top to bottom of the canopy, regardless of canopy height. Leaf percentage of the top layers of the canopy ranged from 13 to 24%, somewhat lower than the range of 24 to 28% reported for 28-d regrowth of rotationally stocked limpograss pastures (Lima et al., 1999). The lower leaf percentage in the current study is likely due in part to the regular removal of leaf blade near the top of the canopy in continuously stocked pastures, at least those of the 40- and 60-cm heights where herbage bulk density did not limit selection for leaf. In addition, it likely was related to the rather erect leaf angle of limpograss that in rotationally stocked pastures results in leaf blade being elevated above stem in upper layer of the pregraze canopy. The difference in leaf percentage between T-5 cm and LL was pronounced, and T-5 cm often had 1.5 to two times greater leaf percentage than LL. For 35-d regrowth of rotationally stocked limpograss pastures, Holderbaum et al. (1992) reported that leaf percentage was 33% in the top half of the canopy and 10% in the lower half.
Leaf CP and IVOMD declined with increasing canopy height in both years, but the response of stem was less consistent than that of leaf. For both leaf and stem, CP and IVOMD were generally greater for herbage from the upper 25% vs. next lower 50% of the canopy. The most striking difference in nutritive value, however, was the difference between leaf and stem CP. Leaf CP of the upper 25% of the canopy was generally above 120 g kg-1, with the exception of the 60-cm treatment in 1999, while stem CP was always <60 g kg-1 and averaged approximately 50 g kg-1. Holderbaum et al. (1992) also reported a two- to threefold difference in leaf and stem CP for rotationally stocked limpograss. Leaf IVOMD was also considerably higher than stem, often by 60 to 80 g kg-1, but stem IVOMD was relatively high compared with other C4 grasses. These differences in IVOMD are significant and would likely translate into effects on animal performance. The difference in CP concentration was particularly large; thus, the proportion of leaf and stem in the diet would have a major impact on the protein status of the grazing animal (considering 87 g CP kg-1 DM as the minimum required for a 350-kg animal gaining 0.5 kg d-1; NRC, 1996).
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CONCLUSIONS |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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NOTES |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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REFERENCES |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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