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a Soil and Crop Science Dep., Texas A&M Univ., Agricultural Research and Extension Center, Overton, TX 75684
b Agronomy Dep., Univ. of Florida, Gainesville, FL 32611-0300
c Dep. De Zootecnia/UFRPE, Av. Dom Manoel de Medeiros, S/N, Dois Irmaos, 52171-900, Recife-PE, Brazil
d Dep. of Animal and Poultry Sciences, Virginia Polytechnic and State Univ., Blacksburg, VA 24061-0306
e Dep. of Animal Sciences, Univ. of Florida Range Cattle Research and Education Center, Ona, FL 33865
* Corresponding author (lesollenberger{at}ifas.ufl.edu)
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ABSTRACT |
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 TOPNOTES ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Abbreviations: ADG, average daily gain • AU, animal unit; BUN, blood urea nitrogen • BW, body weight • CP, crude protein • DM, dry matter • IVDOM, in vitro digestible organic matter • LWG, liveweight gain • OMI, organic matter intake • SR, stocking rate • TDN, total digestible nutrients
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INTRODUCTION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Although the benefits of early weaning, such as improvement of reproduction and reduction of nutrient requirements of the cow, have been recognized for many years, its adoption by producers has been limited because of the lack of information on management of early weaned calves. Mild winters in the southern USA offer an opportunity to raise calves on pasture systems that include high nutritive value winter-annual forages (Arthington and Kalmbacher, 2002). Annual ryegrass, a high-yielding, high quality grass, is the most commonly grown cool-season pasture forage in the southern and southeastern USA (Evers et al., 1997) and is a likely component of pasture systems for calves. Another alternative is small-grain rye, which initiates growth earlier than ryegrass in North Florida and extends the grazing season, making a mixture of rye and ryegrass an attractive option.
Concentrate supplementation is an important component of feeding programs for early weaned calves on pasture because low forage intake and efficiency of cool-season forage utilization may limit calf performance (Arthington and Kalmbacher, 2003). Early weaned calves grazing wheat (Triticum aestivum L.) pastures consumed 27% less forage dry matter (DM) during the first 20 d on pasture than during the subsequent 50 d (Paisley et al., 1998). The authors suggested that initial performance of early weaned calves was limited by low forage intake. Moreover, calves grazing high quality cool-season pasture may have high ruminal ammonia losses if forage CP concentration exceeds 150 g kg–1 (Poppi and McLennan, 1995). Loss of N from the rumen is costly due to significant energetic expenditure associated with urea synthesis and excretion. Supplementation with nonstructural carbohydrates increases utilization of forage N, reduces energy cost of excreting N, and supplies nutrients to the small intestine. Hill (1991) observed that rapidly fermentable energy supplement fed to calves grazing ryegrass provided carbohydrate that was well synchronized with ammonia and peptide production from forage protein degradation. This resulted in greater synthesis of microbial protein than observed for nonsupplemented calves.
These data suggest that an effective supplementation program is necessary to maximize the efficiency of utilization of cool-season grasses by early weaned calves. The objective of this study was to evaluate the effect of different levels of concentrate supplementation on ADG, SR, LWG, forage organic matter intake (OMI), and grazing time of early weaned calves grazing rye–ryegrass pastures in North-Central Florida.
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MATERIALS AND METHODS |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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In late October 2003 and 2004, cool-season forages were planted into a glyphosate (isopropylamine salt [10 g kg–1] of N-phosphonometyl glycine)-controlled bahiagrass (Paspalum notatum Flügge) sod using a no-till drill. The seeding rates were 20 kg ha–1 of ‘Jumbo’ ryegrass and 80 kg ha–1 of a mixed-blend rye (trade name Grazemaster). All pastures received an initial application of 40 kg N ha–1, 17 kg P ha–1, and 66 kg K ha–1 3 wk after planting. Additional applications of 40 kg N ha–1 were made in late December, early February, and early March in both years.
Calves were weaned on 2 Jan. 2003 and 5 Jan. 2004 at 
90 d of age. The average initial BW of the calves was 107 ± 9 kg in 2003 and 94 ± 8 kg in 2004. They were held in dry lot until initiation of the grazing study with access to bermudagrass [Cynodon dactylon L. (Pers.)] hay (ad libitum) and 1 kg d–1 of preconditioning medicated concentrate. Early weaned calves were Angus-sired (crossbred cows sired by Angus bulls), products from primiparous and multiparous cows in 2003 and 2004, respectively. Calves were dewormed with Ivermectin (10 g kg–1 concentration, Ivomec, Merck & Company, Rahway, NJ) on 10 Apr. 2003, and 12 Feb. and 29 Apr. 2004.
Starting dates for the trial were 28 Jan. 2003 and 15 Jan. 2004. Grazing ended on 14 May 2003 and 29 Apr. 2004. Pasture size was 0.2 ha, and each pasture was subdivided into four paddocks. Pastures were rotationally stocked with a 7-d grazing and 21-d resting period. Early weaned calves were paired (1 steer and 1 heifer) such that total BW was nearly equal (± 5kg) across pairs. Pairs were assigned at random to experimental units. Put and take early weaned calves of comparable age and weight to the testers were used to maintain similar herbage allowance across experimental units. The literature does not define specific herbage allowance targets for the species mixture used in this study, thus our primary objectives in managing pasture quantity were to avoid (i) low herbage allowances that would limit calf ADG, and (ii) variation among experimental units in herbage allowance within a year that could be confounded with the responses to supplement treatment. It was also recognized that variable growth conditions, especially due to rainfall, could result in year differences in herbage allowance in these nonirrigated pastures.
Treatments were three levels of a commercial pelleted concentrate, 10, 15, and 20 g kg–1 of calf BW, offered daily in individual tubs and containing 146 g CP kg–1 and 700 g TDN kg–1. Primary energy sources in the concentrate were wheat middlings (400 g kg–1 of concentrate) and soybean [Glycine max (L.) Merr.] hulls (393 g kg–1 of concentrate), and protein sources were soybean meal (25 g kg–1 of concentrate) and cottonseed (Gossypium spp.) meal (50 g kg–1 of concentrate). Each treatment was replicated three times in a completely randomized design, for a total of nine experimental units (pastures) in the study. A salt-based trace mineral mix was supplied free choice throughout the grazing season.
Pasture Sampling
Herbage mass was determined using a double-sampling technique. Indirect estimate of herbage mass was the settling height of a 0.25-m2 aluminum disk meter, and the direct measure was clipping herbage in the same 0.25-m2 area to a 3-cm stubble. The disk was calibrated monthly by taking direct and indirect measures at 20, 0.25-m2 sites that represented the range of herbage mass on the nine experimental units. Clipped herbage was dried at 60°C in a forced-air oven to constant weight. A prediction equation was developed by regressing actual herbage mass on disk height. Pre- and postgraze herbage mass for all pastures were determined biweekly. At each sampling date, 20 disk-height measurements were taken at randomly selected locations in the paddock and the average height entered into the calibration equation to predict herbage mass.
The first and third paddocks were sampled in each grazing cycle. Pregraze herbage mass during a grazing cycle was calculated as the average of the two paddocks sampled per pasture. Herbage accumulation for a given paddock was calculated by subtracting postgraze herbage mass of the previous cycle from pregraze herbage mass of the current cycle. Average herbage allowance was computed as average herbage mass [(pregraze + postgraze)/2] divided by average total calf BW on the pasture during the grazing period (Sollenberger et al., 2005). Stocking rate was a response variable in this study and was expressed in AUs (1 AU = 500 kg of BW0.75) ha–1.
Hand-plucked samples were obtained from the first and third paddocks and used to estimate nutritive value of the grazed portion of the canopy. Samples were taken at 20 locations per paddock on the day before initiation of grazing, and herbage was removed to the stubble height at which the previous paddock was grazed. These samples were dried at 60°C in a forced-air oven to constant weight, ground in a Wiley mill (Model 4, Thomas-Wiley Laboratory Mill, Thomas Scientific, Swedeboro, NJ) to pass a 1-mm stainless steel screen, and taken to the laboratory for analyses. Nitrogen concentration was measured using a modification of the aluminum block digestion technique (Gallaher et al., 1975). Concentration of CP in herbage DM was calculated as %N x 6.25. In vitro digestible organic matter (IVDOM) concentration was determined by the two-stage procedure of Tilley and Terry (1963) modified by Moore and Mott (1974).
Rumen degradable protein and rumen undegradable protein were estimated by the in vitro method proposed by Roe et al. (1991). Hand-plucked samples were taken from the first and third paddock from only the 15 g kg–1 BW supplement treatment pastures and were composited across sampling dates and analyzed. Samples were incubated in a buffer/protease solution for 48 h, the residue recovered through filtering, and it was analyzed for CP concentration. Rumen degradable protein is that which was digested during 48 h, and rumen undegradable protein is the difference between the original CP concentration and the rumen degradable protein.
Animal Response Variables
Total OMI was computed based on fecal output and diet digestibility, viz., total OMI = organic matter output of feces/(1 – [OM digestibility/100]). Fecal output was estimated using a controlled release Cr marker (Captec New Zealand Limited, Auckland, New Zealand). The devices were administered orally to tester calves on 25 Mar. 2003 and 26 Feb. 2004. Fresh fecal samples were obtained from each tester animal on Days 11, 13, 15, and 17 after administration.
In addition, four early weaned calves having the controlled-release Cr marker were used for total fecal collection from Days 11 to 17 after dosing. These four calves were confined in metabolic crates and fed freshly harvested rye–ryegrass ad libitum and were supplemented with 15 g kg–1 BW of concentrate. The total daily fecal output from each calf was collected, weighed, mixed, and a subsample analyzed for DM and Cr concentration. Total Cr output was calculated by multiplying the fecal Cr concentration by total daily fecal weight (OM basis). The total daily Cr output was used to estimate total fecal output of the tester calves by relating total daily Cr output and Cr concentration in the feces OM. Fecal samples were dried at 60°C in a forced-air oven and ground in a Wiley mill to pass a 4-mm stainless steel screen. Chromium concentration in the feces was assayed by atomic absorption spectrophotometry following the procedure described by Williams et al. (1962). The samples were analyzed by day of collection in duplicate and analyses were repeated for samples where the difference in Cr concentration between duplicates exceeded 10%.
Total OMI was calculated based on average fecal output estimations of two testers per experimental unit and IVDOM of the respective pasture herbage. Forage OMI was the difference between total OMI and the known amount of concentrate OM fed. The fecal output estimated with the marker did not equal the fecal output predicted based on estimated forage and supplement digestibility, a response that was affected by associative effects. For this reason, an interactive SAS Institute (1991) program was used to adjust the total OMI based on the equations developed by Moore et al. (1999). This adjustment accounted for the differences in total diet digestibility due to associative effects from concentrate and forage. Thus, to calculate the forage intake, the following assumptions were made: (i) digestibility of hand-plucked forage and supplement were determined by IVDOM; and (ii) digestibility of forage was affected by the level of supplement intake, as determined by the equations described by Moore et al. (1999).
Diurnal grazing time of calves was evaluated every 28 d. Calves were observed and actual grazing time recorded from 0700 to 1800 h. These observations were performed on the first day of the grazing period in Paddock 2 of each grazing cycle.
Animals were weighed every 28 d at 0900 h, before feeding supplement. The change in unshrunk weight of the tester animals was used to calculate ADG. Liveweight gain per hectare in each 28-d period was determined based on the ADG of the testers multiplied by the number of calves within the pasture during that period and adjusted to a hectare basis.
In 2004 only, blood was collected from the jugular vein at each weighing. Samples were collected into 9-mL, Na-heparinized syringes (Luer Monovette, LH, Sarstedt, Inc., Newton, NC) and placed on ice. Blood was centrifuged (2000 x g relative centrifugal force for 30 min) and plasma was separated and frozen at –20°C on the same day. The BUN was determined using a kit (Kit B-7551-120, Pointe Scientific, Inc., Detroit, MI) and read on a plate reader at 620 nm.
Economic Analysis
The objective of the economic analysis was to compare the specific treatments used in this study and not for extrapolation to other production systems. The data and assumptions used in the economic analysis were: concentrate cost = $0.22 kg–1, calf price = $2.2 kg–1, and days of grazing = 100. Where LW = calf liveweight per hectare, concentrate costs were calculated as: Concentrate cost ha–1 = LW x concentrate level x days of grazing x concentrate cost. Income was calculated as: Income ha–1 = gain ha–1 x calf price. The gross return was the difference between concentrate cost ha–1 and income ha–1.
Statistical Analyses
All responses were analyzed by fitting mixed-effects models using the PROC MIXED procedure of SAS (SAS Institute, 1996). Replicate and its interactions were considered random effects. Year was considered fixed because year effects and interactions with year were of interest due to the different rainfall patterns in the 2 yr. Grazing periods within year were analyzed as repeated measures and means were compared using PDIFF (SAS Institute, 1996). Single degree of freedom orthogonal polynomial contrasts were used to test concentrate effects. Treatments were considered different when P 
0.10. Interactions not mentioned in the text were not significant (P > 0.10). 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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There was a significant year x period interaction for herbage accumulation (Table 1). This interaction occurred because herbage accumulation in April 2003 was much greater than in 2004, while in January and February there were no year differences. In April 2003, soils retained sufficient moisture from 243 mm of rainfall in March (30-yr average of 93 mm) to provide high rates of herbage accumulation. In contrast, the decreased herbage accumulation in April 2004 reflects less-than-normal rainfall in March (51- vs. 30-yr average of 93 mm) and April (26- vs. 30-yr average of 75 mm) of that year.
Herbage Nutritive Value
There was no concentrate level effect on herbage CP (279 g kg–1, SE = 4) and IVDOM (838 g kg–1, SE = 3). The CP concentrations were greater in 2004 than 2003 in all periods (Table 1) likely because of the heavy rainfall during parts of the 2003 season. The year x period interaction for CP concentration (Table 1) occurred because the magnitude of the year difference was less in April than in other months. Herbage CP concentration was less in April than February and March due to the greater presence of reproductive tillers in April. The growth period was longer for January herbage (80 d from planting) relative to subsequent forage regrowth periods (21 d), and this may have reduced CP concentration in January. Additionally, the lowest CP observed in this study was in January 2003 following very heavy November and December rains (220 mm vs. 30-yr average of 133 mm).
There was year x period interaction for IVDOM (Table 1). Interaction occurred because there was a greater decline in IVDOM across the 2004 grazing season than in 2003. This decline occurred primarily because of the presence of reproductive tillers after February. The CP and IVDOM concentrations were similar to those reported for ryegrass (285 g kg–1 and 756 g kg–1, respectively) by Arthington and Kalmbacher (2002) and by Redfearn et al. (2002; 232 and 846 g kg–1, respectively).
Ruminal-degradable protein concentration in the samples from the 15 g concentrate kg–1 BW treatment pastures averaged 730 g kg–1 of total CP. Relative to concentrations observed in the current study, the National Research Council (1996) reported lesser values for ruminal degradable protein in ryegrass hay (650 g kg–1) while those reported for ryegrass hay by Michalet-Doureu and Ould-Bah (1992) were similar (740 g kg–1).
Herbage Allowance
Average herbage allowance was similar among treatments (0.62 kg kg–1, SE = 0.02). It was greater in 2004 (0.8) than in 2003 (0.4) primarily due to greater allowance in January 2004. The year difference in HA did not reflect in significant differences in ADG and LWG between years. There was a significant year x period interaction for herbage allowance (Table 2). There was no difference in herbage allowance between January and February 2003. Herbage allowance was less in March than January, and during April, herbage allowance was greater than in the other months. This was a result of the greater herbage accumulation (Table 1) during April. In 2004, herbage allowance was greatest in January and decreased through April. High herbage allowance in January 2004 was due to rainfall and temperature conditions during autumn and early winter 2003 that led to earlier forage growth and greater herbage mass than in the previous year.
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In 2004, two replicates of a no-supplement treatment were included for observational purposes. These calves gained 0.3 kg d–1, well below the ADG of 0.74 kg observed for the 10 g kg–1 BW treatment. Poor performance by these calves supports arguments advanced by Paisley et al. (1998), Poppi and McLennan (1995), and Hill (1991) for including concentrate supplement in the ration of young calves grazing cool-season grass pasture.
The linear increase in LWG with increasing supplement was a function of the linear increase in both SR and ADG as concentrate level increased (Table 3). Horn et al. (1995) observed greater LWG (153 vs. 103 kg ha–1) and a 33% greater SR for steers grazing wheat pastures supplemented with concentrate at 7.5 g kg–1 BW than for unsupplemented calves. In the current study, there was an 18% increase in SR when concentrate level was increased from 10 to 20 g kg–1 BW (Table 3). These results agree with the literature on the effects of supplementation on beef calves grazing cool-season grass pastures, that is, energy supplementation leads to substitution of forage by concentrate, allowing greater SR and LWG.
There was no difference in total OMI among treatments, and total OMI was similar to that reported by Paisley et al. (1998) for early weaned calves grazing wheat pastures (28 g kg–1 BW). There was a linear decrease in forage OMI with increasing level of supplement, supporting the conclusion of Moore (1994) that forage intake is decreased by feeding large amounts of energy concentrate when forage quality is high. Carey et al. (1993) supplemented beef steers grazing tall fescue (Festuca arundinacea Schreb.) pastures with different sources of energy. Forage intake was less for steers supplemented with energy than for control steers, but total intake (forage + supplement) did not differ among treatments. Cravey (1993) reported a substitution ratio of nearly 1:1 when steers grazing wheat pastures were fed energy concentrate at 10 g kg–1 BW. Likewise, the substitution rate of concentrate OM for forage OM in the current study averaged 1:1.
The greater ADG at greater concentrate levels, despite similar total intake, may be a function of the sources of nutrients provided by the concentrate that resulted in better N and energy synchronization in the rumen and increased microbial protein production (Hill, 1991). Specifically, the concentrate may have provided rapidly degraded carbohydrate that increased efficiency of utilization of the large amount of rumen-degraded N provided by the grasses. In addition, concentrate had more rumen-undegradable protein than the forage, potentially leading to more amino acids being absorbed postrumen and decreasing the levels of ammonia absorbed by the rumen epithelium. Absorption and excretion of excess ammonia is a process that spends energy that otherwise could be used to improve animal performance (Van Vuuren et al., 1993).
Another factor that likely contributed to increasing gain with greater supplement is calves that received greater concentrate rates spent less time grazing (Table 3), reducing their energy requirement for maintenance. Macoon (1999) observed that dairy cows supplemented with high levels of concentrate spent less time grazing (135 min) than cows that received low rates of concentrate (180 min), and cows receiving the low rate increased night-time grazing as well. Cowan et al. (1977) observed decreased grazing time of 23 min d–1 for each additional kg of concentrate fed to cows grazing grass–legume pastures. Increased grazing time in the current study suggests that calves receiving the lesser rates of concentrate grazed longer in an attempt to meet their nutritional requirements.
There was no effect of concentrate on calf BUN concentrations (Table 3). Blood urea N concentrations between 11 and 15 mg dL–1 were associated with maximum rates of gain for growing steers (Byers and Moxon, 1980), thus it is likely that the calves had an adequate protein supply during the entire experimental period in 2004. According to the National Research Council (1996), the ruminal-degradable protein requirement for 120-kg calves is 300 g d–1. On the basis of the estimations of forage and concentrate intake for calves receiving the 10 g kg–1 BW supplement and the data for CP fractionation, ryegrass provided 380 and concentrate provided 100 g d–1 of ruminal degradable protein. Therefore, the estimated ruminal degradable protein consumed was 60% greater than the requirement.
Economic Analysis
There was a linear increase in supplement cost and income with increasing supplement levels; however, there was no difference in return among the treatments (Table 4). These data do not justify use of supplement levels above 10 g kg–1 BW.
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CONCLUSIONS |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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0.75 kg d–1 when grazing high quality cool-season grass pasture and receiving concentrate supplement (700 g TDN kg–1) at a rate of at least 10 g kg–1 BW. Rates of concentrate above 10 g kg–1 BW increased daily gain and decreased grazing time and forage intake due to substitution of supplement for forage in calf diets. This allowed for greater SR but did not increase the dollar value of gain minus concentrate cost. Data from 2003, when weather conditions were not ideal and forage accumulation was low during parts of the year, suggest that more than 0.25 ha of cool-season grass pasture should be allocated per AU in this and similar environments and under comparable fertilization management. High levels of rumen-degradable CP in the rye–ryegrass mixture indicate that supplements containing readily available carbohydrates and rumen-undegradable protein should be considered for similar feeding systems. This type of supplement may provide increased synchrony of energy and N release in the rumen. |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Received for publication January 29, 2006.
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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