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a Agron. Dep., P.O. Box 110300, Univ. of Florida, Gainesville, FL 32611-0300
b W.E. Kunkle (deceased)
c Dep. of Anim. Sci., P.O. Box 110910, Univ. of Florida, Gainesville, FL 32611-0910
* Corresponding author (les{at}mail.ifas.ufl.edu)
Received for publication October 27, 2001.
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ABSTRACT |
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 TOPNOTES ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
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Abbreviations: ADF, acid detergent fiber • CP, crude protein • DM, dry matter • IVOMD, in vitro organic matter digestibility • NDF, neutral detergent fiber
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INTRODUCTION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
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Although the low CP concentration of limpograss is well documented, the composition and degradation patterns of limpograss CP are not well understood. Preliminary work assessed the effect of different N fertilization rates on N concentrations in limpograss neutral detergent fiber (NDF) and acid detergent fiber (ADF) (Lima et al., 2001). Neutral detergent–insoluble N concentrations were 427 and 378 g kg-1 of total N for fertilization rates of 17 and 50 kg N ha-1 per harvest while acid detergent–insoluble N was not affected by fertilizer rates and averaged 47 g kg-1 of total N. This work demonstrated that more than one-third of limpograss N was associated with the NDF fraction and would require activity of rumen microbes to be released.
Brown and Pitman (1991), using an in situ technique, characterized the rate and extent of ruminal protein degradation for warm-season grasses. They found that concentrations of soluble and degradable N in limpograss DM were relatively small. Additionally, they showed a relatively long lag (11 h) before N degradation began, and using in vitro procedures, they also showed that there was little or no NH3 released in the first 8 h of incubation. In a complementary fiber digestion study, Brown et al. (1991) suggested that rumen fluid from animals fed only limpograss was N deficient and limited fiber digestion.
Because limpograss N concentration is often marginal for grazing livestock and because it varies widely from the top to the bottom of the canopy (Holderbaum et al., 1992; Sollenberger et al., 1997), a clearer understanding of CP fractions and degradation patterns may aid management decision-making. Knowing how these characteristics vary with canopy height and canopy layers may support the design of more effective grazing programs. Thus, the objective of this experiment was to evaluate the effect of limpograss canopy height, canopy layer, and their interaction on limpograss N fractionation and degradation characteristics using in situ methodology.
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MATERIALS AND METHODS |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
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Treatments were three pasture canopy heights (20, 40, and 60 cm from soil level) that were chosen because they bracket the heights previously reported under grazing. Treatments were replicated twice in a completely randomized design, and the experimental units were 0.5-ha pastures. The swards were continuously stocked, and a variable stocking rate was used. Crossbred (three-fourths Angus, one-fourth Brahman) yearling heifers were added or removed from pastures as needed to achieve the target treatment canopy height. Pasture canopy height was monitored weekly at 50 random locations per pasture using a metric ruler. Stocking rates averaged 7.8, 6.3, and 5.9 heifers ha-1 for 20, 40, and 60-cm pastures, respectively.
The experimental grazing period was from 15 July to 7 Oct. 1998 and 24 June to 16 Sept. 1999, for a total of 84 d each season. Detailed canopy characterization occurred twice in 1998 (13 August and 15 September) and three times in 1999 (14 July, 9 August, and 7 September). Five representative sites in 1998 and four in 1999 were sampled using a 0.5-m2 quadrat that was vertically adjustable in 5-cm increments. Within each canopy height, herbage was sampled in two layers (upper, top 25% of the canopy by height and lower, next lower 50% of the canopy by height). The upper layer (top 25%) was 5 cm in the 20-cm pastures, 10 cm in the 40-cm pastures, and 15 cm in the 60-cm pastures. The lower layers (next 50%) were 10 cm (from 5–15 cm above soil level) in the 20-cm pastures, 20 cm (from 10–30 cm) in the 40-cm pastures, and 30 cm (from 15–45 cm) in the 60-cm pastures. Samples were cut using mechanical shears, and all the material collected was immediately dried for 48 h at 55°C. Total limpograss herbage in a layer was ground in a cyclone Udy mill (Udy Corp., Fort Collins, CO) to pass a 1-mm screen and stored for chemical analyses.
In Situ Degradation Procedure
A ruminally fistulated crossbred (Angus x Brahman x Limousin) steer (body weight of 750 kg) was used for determining in situ degradation characteristics of limpograss samples. The steer was housed in a free-air circulating barn in an individual stall. The steer's diet consisted of ad libitum bermudagrass [Cynodon dactylon (L.) Pers.] hay (116 g CP kg-1 DM, 769 g NDF kg-1 DM, and 537 g digestible organic matter kg-1 organic matter) with free access to water and a mineral mix. Hay was offered at 0800 and 2000 h each day during the experimental period and for 21 d before the start of the trial to ensure adaptation and stabilization of the rumen microbial population. Forage samples were incubated in the rumen in five sequential runs, with 2 to 3 d between runs.
Forage samples were weighed into N-free polyester bags (pore size = 50 ± 15 µm; Ankom, Fairport, NY) that were cut to 7.6 by 10 cm and heat-sealed using the impulse sealer (Model MP-8, Midwest Pacific Co., Baltimore, MD). Four grams of sample were weighed into each bag, resulting in a sample weight to surface area ratio of 26.3 mg cm-2. One bag per replicate–treatment combination was prepared for each incubation time (0, 2, 4, 6, 12, 24, 48, and 72 h). Separate polyester-mesh zippered bags (30 by 45 cm), each with a 50-cm nylon cord for retrieval, were used to hold all sample bags for a given incubation time. Mass et al. (1999) discussed the mesh size and the functionality and advantage of heat-sealed bags to accommodate samples in the rumen with an adequate volume to guarantee contact with rumen fluid. The 96 sample bags per run included two field replicates of the six treatments within a sampling date and a separate bag for each of the eight incubation times.
Before ruminal incubation, the mesh bag containing the sample bags was soaked for 20 min in water (Nocek, 1985). Immediately after water soaking, mesh bags were inserted in the rumen to initiate incubation; bags were secured by tying the nylon cord to the cannula plug. The sample-insertion sequence started with those corresponding to the longest incubation time, and sequentially decreasing times followed. All mesh bags for one run were removed together at the end of 72 h, including the 0-h incubation sample group (only wetted in rumen liquor and immediately removed and rinsed). This procedure guaranteed identical rinsing conditions for all bags (Vanzant et al., 1996). After withdrawal from the rumen, bags were rinsed in cold water at the barn to remove rumen forage residue. Sample bags were placed in 19-L white plastic buckets, and repeated cycles of filling the bucket, manually and gently agitating, and dumping were performed until the rinse water was colorless, as demonstrated in the videotape referenced by Wilkerson et al. (1995). The rinsing time was extended to 90 min to achieve minimum bag-attached microbe contamination. At the final rinse, care was taken to handle sample bags in a vertical position to maintain residue at the bottom of the bag. Bags were then dried for 24 h at 55°C and weighed to determine DM disappearance.
Protein Degradation Kinetics
Crude-protein fractionation was based on ruminal degradation described by Krishnamoorthy et al. (1983) where N is partitioned into Fractions A, B, and C. Fraction A is defined as the instantly soluble protein fraction plus nonprotein N; Fraction B corresponds to the slowly degradable portion for which the degradation rate is measurable; and Fraction C corresponds to the rumen undegradable portion. Fraction A was determined as the difference between the initial N in the sample and that remaining after incubation at 0 h, basically, that N solubilized in water. Fraction C was determined as the undegradable portion after 72 h, and extent of degradation was calculated as the total degradation at that time. Fraction B was obtained indirectly through the difference between Fraction A and C. Kinetic parameters, describing N degradation at a point in time, were estimated using the Mertens and Loften (1980) nonlinear model:
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, which measures rate of adjustment or degradation over time. The optimum was determined by maximizing the likelihood function or, equivalently, minimizing the error sums of squares. The output from this function was used to develop the initial values for the iterations in the Marquardt method.
Laboratory Analyses
Pasture samples were analyzed for DM, in vitro organic matter digestibility (IVOMD), and N concentration. The incubated samples were oven-dried for 24 h at 55°C after incubation and analyzed only for DM and N concentration. In both cases, a 1-g subsample was used to determine absolute DM (drying at 105°C for 15 h; AOAC, 1990). Organic matter was determined by ashing for 15 h at 550°C (AOAC, 1990). Nitrogen concentration was determined using a micro-Kjeldahl method, a modification of the aluminum block digestion technique described by Gallaher et al. (1975). The two-stage technique of Moore and Mott (1974) was used to determine IVOMD of preincubation samples.
Corrections for microbial N contamination of incubated samples were not made. Although microbial contamination is possible (Vanzant et al., 1998), the use of an intense rinsing technique, as demonstrated by Coblentz et al. (1997)(1999), provides concentrations of microbial N in residue samples after incubation that are negligible. Procedures for quantifying microbial N contamination are regarded as labor intensive and imprecise relative to the small error due to contamination from microbial N (Mullahey et al., 1992; Redfern et al., 1995; Vanzant et al., 1996; Mass et al., 1999; Coblentz et al., 2001).
Statistical Analyses
Data were analyzed using mixed-model methodology through PROC MIXED (SAS Inst., 1996). In all models, effects of canopy height and layer were considered fixed effects; replication, harvest, and their interactions were considered random effects. The nature of the canopy height effect was assessed by using orthogonal polynomial contrasts. All means reported are least squares means.
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RESULTS AND DISCUSSION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
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0.056), and there was a layer effect (P = 0.023) only for the undegradable C fraction. There were no interactions of canopy height with layer for Fraction A (P = 0.649), Fraction B (P = 0.421), or Fraction C (P = 0.104); however, data are shown by layer within height because for Fraction C, the interaction approached significance. Fraction A concentration was greatest and Fraction C least in the 20-cm canopy (Fig. 1). Fraction A concentration decreased as height increased from 20 to 40 cm, but it changed relatively little thereafter (linear, P = 0.034; quadratic, P = 0.023 effects; Fig. 1). Greater concentration of the soluble N fraction was observed with increasing N fertilization rates of several tropical forage grasses (Johnson et al., 2001). With a soluble Fraction A >600 g kg-1 CP on 20-cm pastures and as reported previously by Johnson et al. (2001), animals are provided mostly with rumen-degradable protein that can be used for microbial protein synthesis. When diet N concentration of cattle grazing limpograss is low or marginal, then a greater proportion of Fraction A is desirable because it is released in the rumen and reduces the likelihood that N deficiency will limit the growth of rumen microorganisms. Fraction C concentration increased linearly (P = 0.011) with increasing canopy height (Fig. 1). Because this fraction is considered undegradable, the lower concentrations observed for 20-cm pastures are preferred.
The response of Fraction B to height was quadratic (P = 0.023) with greater concentrations for the 40-cm canopy compared with the 20- and 60-cm canopies (Fig. 1). In situations where ruminal N concentration is not limiting, greater concentration of the B fraction may have some benefit to livestock. The portion of the potentially degradable fraction that is not degraded in the rumen may be absorbed in the lower tract, supplying the ruminant with additional metabolizable protein that may enhance performance of growing cattle. If N in the B fraction is associated with NDF or ADF, however, it may pass out of the lower tract and be of no use to the animal.
When fractions are compared on a DM basis (Fig. 2), the interpretation changes considerably due to the much greater CP concentration in herbage from 20-cm canopies. There were canopy height effects only for concentration of Fraction A and layer effects only for the B fraction in the DM. There were no height x layer interactions for CP of the fractions in the DM, but the interaction approached significance for Fraction B (P = 0.129). Thus, data are shown by layer within heights. For Fraction A, concentrations decreased as height increased from 20 to 40 cm but remained relatively constant thereafter (linear, P = 0.029; quadratic effects, P = 0.102). The greater concentration in the DM of Fraction A for 20-cm canopies is the result of a much greater overall CP concentration compared with that of the 40- or 60-cm canopies. The layer effect (P = 0.043) on Fraction B concentration occurred because the upper layer had approximately twice the concentration of the lower layer for the 40- and 60-cm heights. In the upper layer of the 40-cm canopy, the concentration of potentially degradable protein was greatest, and quantities of this fraction may escape degradation in the rumen and contribute significantly to the metabolizable protein needs of grazing animals (Klopfenstein, 1996).
Degradation Parameters of Dry Matter and Crude Protein
In situ degradation parameters are presented for both DM and CP (Table 2). There were no canopy height x layer interactions for any of the degradation parameters (P > 0.111). Neither DM degradation lag time nor rate (Kd) was affected by canopy height (P = 0.613 and P = 0.218, respectively) or layer of the grazed canopy (P = 0.688 and 0.313, respectively). Mean degradation lag time of DM across canopy heights and layers was 9.3 h (10.4 h, upper layer; 8.2 h, lower layer), similar to the lag times (13.8 and 14.1 h) reported by Brown and Pitman (1991) for limpograss and bahiagrass (Paspalum notatum Fluegge), respectively. However, the lags observed in the current study are greater than those reported for other C4 grasses, including elephantgrass [Pennisetum purpureum Schumach] (3.0 h; Vieira et al., 1997) and bermudagrass cv. Tifton 78 (2.1 h; Emanuele and Staples, 1988). Mean degradation rate (Kd) across canopy heights and layers was 0.06 h-1. Values for the upper layer of the canopy were 0.06, 0.06, and 0.07 h-1 for 20-, 40-, and 60-cm heights, respectively, and for the lower layer of the canopy, they were 0.07, 0.06, and 0.05 h-1 for the same heights, respectively. Extent of DM degradation was affected (P = 0.103) by canopy height but not by layer (P = 0.60); it was least for the taller 60-cm canopy (690 g kg-1) and increased linearly with decreasing canopy height (Table 2). Degradation extent for upper and lower layers was 715 and 717 g kg-1 DM, respectively.
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In reviewing several studies with forage species that range widely in DM digestibility (Brown and Pitman, 1991; Coblentz et al., 2001; Waghorn and Burke, 2001), there appears to be a link between lag time of CP degradation and in vitro DM digestibility. Grouping these forages from most digestible to least digestible species, e.g., more digestible legumes like alfalfa (Medicago sativa L.) (Waghorn and Burke, 2001), more digestible temperate cereal grain forages (Coblentz et al., 2001), less digestible temperate grasses, and the least digestible warm-season grasses (Brown and Pitman, 1991; Waghorn and Burke, 2001), shows that lag times vary from zero in the first group to more than 10 h in the warm-season grasses. Alfalfa CP degradation begins immediately; however, for temperate grain forages, reported lag time ranged from 0.8 to 2.2 h, 4.5 h for other temperate grasses, and 11.5 to 13.6 h for the warm-season forages associated with decreased digestibility. Again, the bahiagrass data from Brown and Pitman (1991) vary from this pattern.
Long lag time likely plays an important role in the utilization of the limpograss B fraction. If the supply of N to the rumen is limiting (Brown et al., 1991) and the lag phase for CP degradation is long, as may often be the case for cattle grazing limpograss, then higher concentrations of Fraction B may not alleviate the N limitation. In this situation, much of Fraction B may pass to the lower tract before it can be degraded, and it will not be available for microbial growth and forage digestion in the rumen. In contrast, if rumen N is adequate for optimum microbial growth, then there may be an advantage to greater concentrations of Fraction B as described earlier.
Crude-protein degradation rates were not affected by canopy height or layer of the canopy. The mean value across heights was 0.08 h-1. Crude-protein degradation extent was affected by canopy height and layer (P < 0.025). It decreased linearly and quadratically with increasing canopy height (Table 2), and it was 790 and 730 g kg-1 for upper and lower layers, respectively.
Dry matter degradation curves for the different canopy heights across layers (Fig. 3A) were similar in terms of the amount of instantly soluble material disappearing at 0 h (Fraction A) and the slowly degradable fraction. Likewise, their kinetic parameters, lag time and rate of degradation, did not differ. Despite these similarities in DM degradation, CP degradation curves (Fig. 3B) were different among heights in terms of CP fractionation and degradation parameters. Dry matter curves show that approximately 250 to 300 g kg-1 DM was soluble and immediately degraded or lost from bags within the range of canopy heights evaluated (Fig. 3A). In contrast, the soluble CP fraction was 450 to 500 g kg-1 of total CP for the taller canopies, almost twice the concentration of soluble DM in total DM, and soluble CP was even greater (650 g kg-1 CP) for the 20-cm height (Fig. 3B). Soluble CP represented 300, 120, and 130 g kg-1 soluble DM in herbage from the 20-, 40-, and 60-cm canopies, respectively, while total CP was only 115, 67, and 66 g kg-1 total DM for the same three heights, respectively.
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SUMMARY AND CONCLUSIONS |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
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NOTES |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
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REFERENCES |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
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