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a Agronomy Dep., Ona Range Cattle Research and Education Center, Ona, FL 33865
b Dep. of Animal Sci., Ona Range Cattle Research and Education Center, Ona, FL 33865
c Agronomy Dep., Univ. of Florida, Gainesville, FL 32611-0300
d Dep. of Animal Sci., Univ. of Florida, Gainesville, FL 32611
e Dep. de Zootecnia/UFRPE, Av. Dom Manoel de Medeiros, S/N, Dois Irmaos, 52171-900, Recife-PE, Brazil
f Alltech, Lexington, KY 40546-0215. This research was sponsored in part by a USDA-CSREES Tropical and Subtropical Agricultural Research Program Grant
* Corresponding author (jv{at}ufl.edu).
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ABSTRACT |
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 TOPNOTES ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Abbreviations: BW, body weight • CP, crude protein • DM, dry matter • NPN, nonprotein nitrogen
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NOTES |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Received for publication April 17, 2007.
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INTRODUCTION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Warm-season perennial grasses are the basis for livestock production in the southeastern United States, but climatic conditions also allow use of cool-season annual forages. Annual ryegrass is the most commonly grown pasture forage in this region from November to May (Evers et al., 1997). In some areas, small grains are mixed with annual ryegrass to improve early season forage production. Small grains are higher yielding than annual ryegrass from late December to mid-February in Florida, and annual ryegrass is characterized by rapid forage growth during March to May. In northern Florida, annual ryegrass–small grain mixtures can provide 5 mo of grazing from December to May, and mixtures were found to provide better seasonal distribution of yield and nutritive value than cool-season monocultures (Fontaneli et al., 2000). In spite of very high nutritive value, concentrate supplement is an important component of the nutritional management program for early weaned calves grazing cool-season forages (Vendramini et al., 2006). For example, calves weaned at 90 d of age and grazing a rye–annual ryegrass mixture gained 0.30 kg d–1 when fed no energy or protein supplement, while those fed supplement (CP and total digestible nutrient concentrations of 146 and 700 g kg–1) at 10 g kg–1 of body weight d–1 gained 0.74 kg d–1 (Vendramini et al., 2006).
Bermudagrass [C. dactylon (L.) Pers.] is an important forage for beef and dairy cattle in the southern United States (Hill et al., 1998). Tifton 85 bermudagrass has been widely grown in the United States, Central and South America, and southern Africa (Mandebvu et al., 1999). When compared with ‘Coastal’ and ‘Tifton 78’ bermudagrasses, Tifton 85 is higher yielding and more digestible (Hill et al., 1993). Early weaned calves grazing Tifton 85 gained 0.33 kg d–1 with no supplementation and 0.65 kg d–1 when fed supplement (CP and total digestible nutrient concentrations of 146 and 700 g kg–1) at 15 g kg–1 BW (Vendramini et al., 2007).
The most appropriate quantity and composition of supplement to be fed to livestock grazing cool- and warm-season forages are a function of management practices and their impact on quantity and nutritive value of the grasses (Macoon, 1999; Vendramini et al., 2006). To better understand the need and potential response to supplement by early weaned calves, more information is needed regarding the composition of the base forage diet. The objective of these experiments was to quantify the effects of regrowth interval and N fertilization on concentration of CP fractions in rye–annual ryegrass and Tifton 85 bermudagrass swards.
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MATERIALS AND METHODS |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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In early October, the area was sprayed with 3.0 L ha–1 of glyphosate (Monsanto Co., St. Louis, MO; isopropylamine salt [10 g kg–1] of N-phosphonomethyl glycine) to kill bahiagrass (Paspalum notatum Flügge). Three weeks later, the pastures were overseeded with a rye–ryegrass mixture using a no-till drill. Seeding rates were 20 kg ha–1 of ‘Jumbo’ ryegrass and 80 kg ha–1 of a mixed-blend rye (trade name Grazemaster). Three weeks after planting, all pastures received 40 kg N, 17 kg P, and 66 kg K ha–1.
Treatments were the factorial combinations of two regrowth intervals (3 and 6 wk) and three N fertilizer levels (0, 40, and 80 kg N ha–1 per period), and treatment effects were evaluated in two periods each year. Thus total N fertilizer applied was 0, 80, and 160 kg ha–1 yr–1 for the cool-season experiment. The first period was the 6-wk interval from 10 Jan. to 21 Feb. 2003 and 16 Jan. to 27 Feb. 2004, and Period 2 was the 6-wk interval from 21 Feb. to 4 Apr. 2003 and 27 Feb. to 10 Apr. 2004. Treatments were arranged in three randomized complete blocks. Plot size was 2 by 4 m, with a 1-m alley between plots. Plots were staged on 10 Jan. 2003 and 16 Jan. 2004 to begin the first period. The second 3-wk harvest and the first 6-wk harvest of the first period served as the staging cut for the second period. The 6-wk cutting-interval plots received the entire N application for each period at the beginning of the 6-wk interval, while the 3-wk plots received half of the total-period rate at the beginning of each of the two, 3-wk growth intervals per period. At harvest dates, herbage was clipped to a 5-cm stubble height from two representative 0.25-m2 quadrats per plot and dried at 60°C for 48 h. Remaining herbage was clipped to the same stubble height using a sickle bar mower and removed.
Experiment 2: Warm Season
This study was conducted at the Beef Research Unit, but in this case the soils were Adamsville fine sand (hyperthermic, uncoated Aquic Quartzipsamments) and Sparr fine sand (siliceous, subactive, hyperthermic Grossarenic Paleudult). These soils are moderately well-drained with rapid permeability. Before initiation of the experiment, mean soil pH was 5.5, and Mehlich-1 extractable P, K, Mg, and Ca concentrations in the Ap1 horizon (0- to 15-cm depth) were 44, 17, 40, and 328 mg kg–1, respectively.
Treatments were the factorial combinations of two regrowth intervals (2 and 4 wk) and three N fertilization levels (0, 40, and 80 kg ha–1 in each 4-wk period) evaluated in two periods of 4 wk in each of 2 yr. Thus the total N fertilizer applied was 0, 80, and 160 kg ha–1 yr–1. Treatments were replicated three times in a completely randomized design. Plot size was 2 by 4 m with a 1-m border between plots.
In early April 2003 and 2004, plots received an initial application of 40 kg N, 17 kg P, and 66 kg K ha–1 to stimulate growth and provide maintenance P and K. Plots were staged to a 15-cm stubble on 20 June 2003 and 21 May 2004 to begin the first 4-wk period. The later date was chosen in 2003 due to a dry spring. The first experimental period was from 20 June to 18 July 2003 and 21 May to 18 June 2004. The second 4-wk period followed immediately after the harvests on 18 July 2003 and 18 June 2004 and ended on 15 Aug. 2003 and 16 July 2004. The 2-wk treatment plots were harvested twice in each 4-wk period and the 4-wk treatment plots were harvested once. The 4-wk cutting-interval plots received the entire N application for each period at the beginning of the 4-wk interval, while the 2-wk plots received half of the total-period rate at the beginning of each of the two, 2-wk growth intervals per period. Forage samples were harvested at a 15-cm stubble height from a 1 by 2 m area using a sickle bar mower and dried at 60°C for 48 h.
Sample Preparation and Nitrogen Analyses
Forage samples were ground in a Udy cyclone mill (Udy Corp., Fort Collins, CO.) to pass a 4-mm screen. Nitrogen concentration of harvested forage samples and those from the in situ disappearance procedures were determined using a micro-Kjeldahl method, a modification of the aluminum block digestion technique described by Gallaher et al. (1975). Crude protein was determined by multiplying N concentration by 6.25.
In Situ Disappearance Procedure
Within an experiment, forage samples from each experimental unit were composited across the two periods within a year, so that all samples within an incubation time could be accommodated in one cow at the same time. Four grams of dried and ground herbage of each of these samples was weighed and placed into N-free polyester bags (pore size, 50–60 µm). Bags were 20 by 10 cm and heat sealed using an impulse sealer (model MP-8; Midwest Pacific Co., Baltimore, MD). The ratio of weight to surface area was 20 mg cm–2. Duplicate samples were incubated for 0, 3, 6, 9, 12, 24, 48, and 72 h. The bags were soaked in water, attached to a rope, and placed into a ruminally-fistulated Holstein cow. The cow was housed in a free-air circulating barn in an individual stall and fed a diet of Coastal bermudagrass hay ad libitum supplemented with 0.4 kg of soybean [Glycine max (L.) Merr.] meal d–1.
For a given incubation time, 72 bags representing all experimental units (three N levels, two regrowth intervals, 2 yr, and three field replications, with all run in duplicate) within an experiment were incubated and withdrawn at the same time. This procedure guaranteed identical rumen conditions among treatments at each incubation time. The 0-h bags were not incubated in the rumen but were subjected to the same rinsing procedure used for the ruminally incubated bags. After removal of bags from the rumen, bags were placed in a plastic bucket and rinsed with water repeatedly until the rinse water was colorless. Bags were frozen (–20°C), and after all incubations were completed they were washed together in a typical washing machine used for clothes. It was set at a low water level and run for one cycle. Bags were dried at 60°C in a forced-air oven for 48 h and weighed. No correction was made for microbial N contamination because techniques to predict microbial contamination have proven unsatisfactory for universal application (Vanzant et al., 1998). The N concentration in the samples post-incubation was determined using the micro-Kjeldahl procedure described earlier.
Dry Matter and Crude Protein Degradation Kinetics
Crude protein fractions were estimated using an in situ ruminal degradation method where CP is partitioned into Fractions A, B, and C (Krishnamoorthy et al., 1983). Fraction A represents the soluble portion and was assumed to be degraded rapidly and completely. It was measured as the CP that washed out of the bag when rinsed with water. Fraction C was considered to be ruminally unavailable and was the portion remaining in the bags following incubation for 72 h. Fraction B was calculated by difference (B = A – C) and represents that which is potentially degradable.
Kinetic parameters of DM disappearance were estimated using the nonlinear model proposed by (McDonald, 1981);
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For the rye–ryegrass CP disappearance kinetics, the nonlinear model proposed by Ørskov and McDonald (1979) was used. This model is similar to that described above, however, it does not include lag time (Van Vuuren et al., 1991). Different models were used for two grass species because unlike cool-season grasses, warm-season grasses are likely to have CP disappearance lag time because of their cell wall structure.
Effective DM and CP degradability were calculated by fixing the particle turnover at 0.06 h–1 for rye–ryegrass (Cerneau and Michalet-Doureu, 1991; cited by Michalet-Doureu and Ould-Bah, 1992) and 0.04 h–1 for Tifton 85 (Ellis et al., 1994). The model used was proposed by Ørskov and McDonald (1979) [ED = a + (bc)/(c + particle turnover)].
Statistical Analysis
All responses were analyzed by fitting mixed models using the PROC MIXED procedure of SAS (SAS Institute, 1996). Block (Exp. 1) and replicate (Exp. 2) and their interactions were the random effects, and N fertilizer, regrowth interval, year, and their interactions were considered fixed effects. Single degree of freedom orthogonal polynomial contrasts were used to test N fertilization effects. Regrowth interval means were considered different when F test P values were <0.05. Interactions not mentioned in the text were not significant (P > 0.05). The means reported are least squares means.
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RESULTS AND DISCUSSION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Crude protein concentrations of the herbage harvested at 6 wk was less than at a 3-wk regrowth interval (161 vs. 208 g kg–1). The increase in stem/leaf proportion associated with the appearance of reproductive tillers for the 6-wk-old herbage decreased the CP concentration.
Crude Protein Fraction Concentration
The CP fractions in rye–ryegrass herbage are presented as a proportion of the total CP. Fraction A increased linearly and Fraction B decreased linearly as level of N fertilization increased from 0 to 80 kg N ha–1 (Fig. 1
). In addition, there was a linear decrease in Fraction C from 74 to 48 g kg–1 as N fertilization level increased from 0 to 80 kg ha–1. The concentrations of Fraction A in rye–ryegrass swards in this study are in the range reported by Hoffman et al. (1993) for five species of cool-season grasses (from 370–600 g kg–1). Similar values of Fractions A, B, and C (430, 490, and 80 g kg–1, respectively) were observed by Michalet-Doureu and Ould-Bah (1992) with perennial ryegrass (L. perenne L.).
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There was a regrowth interval effect on Fraction C concentration only. Forage harvested at 3 wk (48 g kg–1 of total CP) had less Fraction C than that harvested at 6 wk (71 g kg–1).
Dry Matter and Crude Protein Degradation Parameters
There was no effect of N fertilization (Table 1
) or regrowth interval on in situ DM degradation rates, lag time, and potential DM degradability. Cool-season grasses generally have lesser DM disappearance lag time than warm-season grasses. For the most highly digestible warm-season forages like elephantgrass (Pennisetum purpureum Schum.), Vieira et al. (1997) found lag times of 4 h. With cool-season grasses, Waghorn and Burke (2001) reported values ranging from 0 to 4.5 h. The DM disappearance lag time in this study averaged 3 h.
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Experiment 2: Warm Season
Herbage Crude Protein Concentration
There was N fertilization x regrowth interval interaction for CP concentration of Tifton 85 (Table 2
). At both regrowth intervals, there was a linear increase in CP concentration with increasing N fertilization, but interaction occurred because the rate of increase was greater for the 2- than the 4-wk interval. The CP concentrations were greater at 2- than 4-wk regrowth intervals at all N fertilization levels tested. Johnson et al. (2001) reported a linear increase in bermudagrass CP concentration with increasing levels of N fertilization. In that study, herbage CP concentrations were 98, 146, and 180 g kg–1 for fertilization levels of 0, 78, and 156 kg N ha–1 harvest–1. Similar results were found by Lima et al. (1999a) who reported that increasing N fertilization from 50 to 150 kg ha–1 yr–1 increased limpograss (Hemarthria altissima [Poir] Stapf. & Hubbard) CP concentrations from 97 to 115 g kg–1.
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Dry Matter and Crude Protein Degradation Parameters
There was no effect of N fertilization on rate of DM disappearance and lag time, however, a linear increase in effectively degradable DM was observed with increasing N fertilization levels (Table 3
). Assis et al. (1999) tested the effect of N fertilization levels of 0 and 400 kg ha–1 yr –1 on DM digestibility parameters of Tifton 85. They found no significant effect of N fertilization level on lag time (3.5 h) or effectively degradable DM (400 g kg–1). The DM disappearance lag time found in this study (5.9 h) was greater than those reported by Vieira et al. (1997) for elephantgrass (3 h) and shorter than the values reported by Brown and Pitman (1991) for bahiagrass (14.1 h). There was an effect of regrowth interval on rates of DM disappearance and effectively degradable DM (Table 4
). In contrast to these results, Mandebvu et al. (1999) did not find differences in rate of DM disappearance (0.03 h–1) or effective degradability (650 g kg–1) of Tifton 85 at regrowth intervals from 3 to 7 wk. Newman et al. (2002) did not find differences in lag time (9 h) or rate of disappearance (0.06) for limpograss herbage from continuously stocked swards grazed at different canopy heights, however, a linear decrease in effectively degradable DM was observed as canopy height increased from 20 to 60 cm. The rate of CP disappearance and lag time were not affected by the N fertilization level with averages of 0.06 h–1 and 4.2 h, respectively (Table 3). The CP lag time observed in the current study was similar to those reported by Assis et al. (1999) (3.5–6.0 h) for Tifton 85 and shorter than the average lag time found by Newman et al. (2002) for limpograss (18 h). Assis et al. (1999) did not find significant effects of N fertilization on rate of degradation, 0.02 h–1 for N fertilization levels of 0 and 400 kg ha–1, corroborating the results of this study.
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Although there was no regrowth interval effect on DM disappearance lag time, rate of DM disappearance and effectively degradable DM were less at the 4- than the 2-wk regrowth interval (Table 4). There was no effect of regrowth interval on rate of CP disappearance and effectively degradable CP (Table 4). However, lag time was shorter for Tifton 85 at a 2- (1.1 h) than a 4-wk (7.3 h) regrowth. The lag time observed in this study for 2-wk-old Tifton 85 is comparable to the lag time in cool-season grasses (0–4.5 h) reported by Waghorn and Burke (2001), possibly because of the greater concentration of soluble protein in younger plants (Minson, 1990).
The CP disappearance of Tifton 85 herbage from the three N fertilization levels was similar during the first 12 h of incubation (Fig. 4 ). A difference between the zero N treatment and the others was observed after 24 h when the zero N treatment started to show a greater proportion of undegraded CP. This continued through 72 h. A similar pattern of CP disappearance was observed for the two regrowth intervals (Fig. 4). After 12 h of incubation, the forage harvested at a 4-wk regrowth interval was less degraded and that difference was maintained through 72 h.
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CONCLUSIONS |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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