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a Agronomy Dep., Range Cattle Research and Education Center, Ona, FL 33865
b Agronomy Dep., Univ. of Florida, Gainesville, FL, 32611
c Depto. de Zootecnia/UFRPE, Av. Dom Manoel de Medeiros, S/N, Dois Irmaos, 52171-900, Recife-PE, Brazil
d Dep. of Animal and Poultry Sciences, Blacksburg, VA 24061
e Dep. of Animal Sciences, Range Cattle Research and Education Center, Ona, FL 33865
* Corresponding author (lesollen{at}ufl.edu)
Received for publication December 26, 2005.
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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 N • BW, body weight • CP, crude protein • DM, dry matter • IVDOM, in vitro digestible organic matter • LWG, liveweight gain • 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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Bermudagrass [C. dactylon (L.) Pers.] is one of the most important forage grasses in the southeastern USA with 10 to 12 million ha used for livestock grazing and hay (Taliaferro et al., 2004). Tifton 85, a hybrid between a South African bermudagrass and ‘Tifton 68’ stargrass (C. nlemfuensis Vanderyst), was released in 1993, and Burton et al. (1993) described it as taller, with larger culms and broader leaves than other bermudagrass hybrids. Compared with ‘Coastal’, Tifton 85 yielded 26% more dry matter (DM) and was 110 g kg–1 more digestible. Herbage NDF concentration of Tifton 85 was greater than Coastal (by an average of 30 g kg–1 DM) over a range of regrowth intervals, but NDF digestibility of Tifton 85 exceeded that of Coastal by nearly 80 g kg–1 DM (Mandebvu et al., 1999). Greater NDF digestibility was due in part to lesser concentrations of lignin and ether-linked ferulic acid in Tifton 85 (Mandebvu et al., 1999). The relatively high nutritive value of Tifton 85 suggests that it may have a role in pasture systems for early weaned calves.
Vendramini et al. (2003) reported gains of 0.59 and 0.44 kg d–1 for early weaned calves fed concentrate at 10 g kg–1 BW and grazing stargrass and atrapaspalum (Paspalum atratum Swallen), respectively. Thus, to achieve target ADG for replacement animals of approximately 0.7 kg d–1, greater levels of supplement feeding than 10 g kg–1 BW may be needed. Gains up to 0.92 kg d–1 have been reported for early weaned calves grazing pearl millet [Pennisetum glaucum (L.) R. Br.] and receiving ground ear maize (Zea mays L.) ad libitum (Harvey and Burns, 1988).
The complex relationships that exist between the source and quantity of nutrients, growth, and digestive development of early weaned calves at pasture are not clear. Galloway et al. (1992) indicated that moderate levels of supplement (200–300 g kg–1 of diet DM) can improve nutrient intake and performance of 12-mo-old cattle consuming bermudagrass. At greater amounts, forage nutrient digestion, intake, or both, can be affected negatively. Among several theories used to explain the associative effect of concentrate on forage intake, Horn and McCollum (1987) suggested that greater consumption of readily fermentable carbohydrates decreased rumen pH and quantity of cellulolytic bacteria, and reduced forage digestibility and passage rates. Thus, associative effects make it difficult to predict the impact of feeding greater amounts of concentrate supplement. This is especially the case for early weaned calves, which have received scant attention in previous research.
To support the increasing industry practice of early weaning in the southeastern USA, additional information is needed regarding the effect of concentrate supplement level on performance of early weaned calves grazing C4 grass pastures. Thus, the objectives of this study were to evaluate the effect of different levels of concentrate on ADG, SR, LWG, and grazing time of early weaned calves grazing Tifton 85 bermudagrass pastures in north–central Florida and to assess the economics of these treatments.
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MATERIALS AND METHODS |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Bermudagrass pastures were planted in August 2002 and were fully established before frost. In April 2003 and 2004, the pastures received 40 kg N ha–1, 17 kg P ha–1, and 66 kg K ha–1. An additional 80 kg N ha–1 was split in two applications of 40 kg ha–1 in early June and early July of both years. Phosphorus and K were applied based on soil-test recommendations for grazed bermudagrass. Nitrogen was applied at rates comparable with those used in other grazing studies with Tifton 85 in this region (Fike et al., 2003). During the 2-yr trial, pastures were grazed from 14 May through 13 Aug. 2003 (86 d) and 18 May through 10 Aug. 2004 (86 d).
Treatments consisted of three levels of a commercial pelleted concentrate (Beef Max, Lakeland Animal Nutrition, Lakeland, FL), 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. These levels were chosen based on previous work that showed maximum ADG of 0.59 kg for calves grazing C4 grasses in Florida and fed concentrate at 10 g kg–1 of BW (Vendramini et al., 2003). Target ADG for replacement animals is approximately 0.7 kg, thus concentrate levels lower than 10 g kg–1 of BW were not considered to be of practical value to producers. Additionally, there was concern that levels above 20 g kg–1 BW would be cost prohibitive or might interfere with normal rumen function (Galloway et al., 1992). Primary energy sources in the concentrate were wheat (Triticum aestivum L.) middling (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, resulting in nine experimental units (pastures) in the study. A salt-based trace mineral mix (P.D.Q. Pasture Supplement, Lakeland Animal Nutrition, Lakeland, FL) was supplied free choice throughout the grazing season.
Early weaned calves were Angus-sired from crossbred cows. Calves were approximately 200-d old with an average BW of 190 kg at the start of summer grazing. Calves had been weaned in January at 90 d of age and grazed rye (Secale cereale L.)–annual ryegrass (Lolium multiflorum Lam.) pastures from January to April (Vendramini et al., 2006). Calves received the same supplement treatment in winter and summer. They were vaccinated with Ultrabac 8 and Bovashield 4 (Pfizer Animal Health, New York, NY) and dewormed with Ivermectin (Ivomec, Merck & Company, Rahway, NJ) at a 10 g kg–1 concentration on 23 July 2003 and 14 July 2004.
Pasture size was 0.15 ha subdivided into three paddocks for rotational stocking. The grazing period was 7 d and the rest period was 14 d. The short rest period was chosen because of rapid growth of Tifton 85 during the time of the trial and to provide forage of high nutritive value. Two early weaned calves (one steer and one heifer) were assigned as testers to each pasture and "put and take" early weaned calves of comparable age and weight to the testers were used to maintain similar herbage allowance across experimental units. Our goals for managing pasture quantity were to avoid forcing the calves to graze low into the canopy, reducing diet nutritive value, and to avoid variation among experimental units in herbage allowance within a year that could be confounded with the responses to supplement treatment.
Pasture Sampling
In each 21-d grazing cycle, herbage mass was determined on Paddock 2 using a double sampling technique. Indirect estimates of herbage mass consisted of dropping a 0.25-m2 aluminum disk plate meter to measure settling height, and direct measures were done by clipping herbage in the same area to a 3-cm stubble. Herbage was dried at 60°C in a forced-air oven to constant weight. A calibration equation was developed using these double sampling data by regressing actual herbage mass on disk height. According to Sollenberger and Burns (2001), C4 grasses usually have greater heterogeneity of herbage bulk density in canopy strata, thus, separate calibration equations were used for pre- and postgraze samples. Disk settling height was measured at 20 randomly selected locations in the paddock the day before the paddock was grazed and again at the end of the grazing period. To predict herbage mass for any given pre- or postgraze sampling date, the average of the 20 disk height measurements was calculated and entered into the calibration equation.
Herbage accumulation rate was calculated by subtracting postgraze herbage mass of the previous grazing cycle from pregraze herbage mass of the current cycle and dividing by days in the rest period (i.e., 14 d). There were no herbage accumulation data reported for May because a postgraze measurement was required to start the regrowth period. Average herbage allowance was computed as average herbage mass [(pregraze + postgraze)/2] divided by calf BW on the experimental unit during the grazing period (Sollenberger et al., 2005). Stocking rate was a response variable and was expressed in AUs (1 AU = 500 kg of BW0.75) ha–1.
Hand-plucked samples were used to estimate nutritive value of the grazed portion of the canopy. Hand plucking was chosen as the sampling technique because it is most applicable when selective grazing is at a minimum, i.e., in single-species swards that are rotationally stocked and moderately to heavily grazed (Sollenberger and Cherney, 1995). 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 and ground in a Wiley mill (Model 4, Thomas-Wiley Laboratory Mill, Thomas Scientific, Swedeboro, NJ) to pass a 1-mm stainless steel screen. 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 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 using the in vitro method of Roe et al. (1991). Hand-plucked samples from only the 15 g kg–1 BW supplement treatment pastures were analyzed because there was no supplement effect on other measures of herbage nutritive value and the analysis is expensive and time consuming. Samples were incubated in a buffer/protease solution for 48 h, the residue recovered through filtering, and the residue analyzed for CP concentration. Rumen-degradable protein is that which was digested during 48 h, and rumen undegradable protein was the difference between the original CP concentration and the rumen-degradable protein.
Animal Responses
The animals were cared for by acceptable practices approved by University of Florida Institutional Animal Care and Use Committee.
Grazing time of early weaned calves during daylight hours was evaluated every 21 d. Calves were observed, and time spent grazing was recorded from 0700 to 1900 h. These observations were performed on the 1st day of the grazing period in Paddock 2 in each grazing cycle.
Animals were weighed every 21 d at 0900 h before feeding supplement. The change in unshrunk weight of the tester animals was used to calculate ADG. The LWG in each 21-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, sodium-heparinized syringes (Luer Monovette, LH, Sarstedt, 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. Blood urea N (BUN) was determined using a kit (Kit B-7551-120, Pointe Scientific, Detroit, MI) and read on a plate reader at 620 nm.
Economic Analysis
The data and assumptions used in the economic analysis (done on a plot basis) were: purchase cost of concentrate = $0.22 kg–1, calf price = $2.2 kg–1, and days of grazing = 86. Where LW = calf liveweight ha–1, concentrate costs were calculated as: purchase cost of concentrate ($ ha–1) = LWG (kg ha–1) x concentrate level (kg kg–1 d–1) x days of grazing (d) x purchase cost of concentrate ($ kg–1). Income ($ ha–1) was calculated as LWG (kg ha–1) x calf price ($ kg–1). The return was the difference between purchase cost of concentrate per hectare and income per hectare. This approach was used because each producer has different labor, equipment, and supplement transport costs depending on conditions.
Statistical Analysis
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 and month were considered fixed because the effects of year, month, and treatment interactions with year and month were of interest. Single degree of freedom orthogonal polynomial contrasts were used to test linear and quadratic effects of the concentrate treatment. Treatments were considered different when P 
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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There was no year x month interaction effect on herbage IVDOM, but there were year and month effects. Mean herbage IVDOM was greater in 2004 (640 g kg–1) than in 2003 (580 g kg–1). The herbage IVDOM response to month was similar to that of CP concentration, with the lowest values occurring during the first month of grazing (573 g kg–1) because of the longer regrowth period of the forage. Herbage IVDOM was greater in June (681 g kg–1) than in July and August (603 and 640 g kg–1, respectively).
Rumen-degradable and rumen-undegradable protein concentrations were 550 and 450 g kg–1 CP, respectively. Similar values for bermudagrass rumen-degradable protein (570 g kg–1) were found by Basurto et al. (2001). Mathis et al. (2001) measured bermudagrass ruminal-degradable protein from seven different locations using the enzymatic methodology of Roe et al. (1991). The average ruminal-degradable protein across locations was 610 g kg–1 CP.
Animal Responses
There were linear increases in ADG and LWG as level of supplement fed increased (Table 3). The performance of early weaned calves grazing Tifton 85 and receiving 10 g kg–1 BW in concentrate was comparable with that of early weaned calves grazing stargrass (0.59 kg d–1) and receiving the same level of concentrate (Vendramini et al., 2003). There was a year x month interaction for calf ADG (P < 0.01, Table 2). The major difference between years was the markedly greater ADG in May 2003 than in 2004. Reasons for lesser gain in May 2004 are not known. Herbage allowance and nutritive value data do not explain the response, but lesser herbage mass in May 2004 may have affected ADG.
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In 2004, two replicates of a zero supplement treatment were included in the study for observational purposes. The ADG of calves that did not receive concentrate was 0.42 kg compared to 0.52 kg for calves fed 10 g kg–1 BW of concentrate (Table 3). These data support the conclusion that supplement plays an important role if gains of early weaned calves on bermudagrass pasture are to approach a target ADG of 0.7 kg.
Calf liveweight gain increased linearly up to 43% as supplementation level increased from 10 to 20 g kg–1 BW (Table 3). Increasing LWG was associated with linear increases in ADG and SR for calves receiving greater levels of supplement (Table 3). Rouquette et al. (2003) reported that supplementation with 0.9 kg d–1 of a maize–soybean meal mixture (28% CP and 85% TDN, respectively) increased LWG of yearling heifers on Tifton 85 pastures from 521 (unsupplemented control) to 615 kg ha–1. The increase in LWG was associated with ADG of 0.76 and 0.92 kg d–1 for nonsupplemented and supplemented heifers, respectively. The overall greater ADG for heifers in Rouquette et al. (2003) was likely due in part to greater size and ruminal capacity of yearlings compared with the calves in the current study, allowing greater forage consumption.
Goetsch et al. (1991) reported that beef steers supplemented with maize at 10 g kg–1 BW reduced bermudagrass hay intake 0.46 kg for each kg of maize fed. Thus, on pastures managed using a variable SR, greater SR is expected when more supplement is fed. This was the case in the current study (Table 3), and grazing time data provide evidence to support substitution of supplement for forage. Grazing time during daylight hours decreased linearly with increasing supplementation level (Table 3). Macoon (1999) also found that dairy cows supplemented with greater levels of concentrate spent less time grazing (135 min) than cows that received lesser rates of concentrate (180 min).
There was no effect of increasing supplementation rate on calf BUN (Table 3). Based on the CP concentration of the forage (180 g kg–1) and concentrate (160 g kg–1), it was expected that calves consuming more forage would have a greater BUN concentration (Hammond, 1983); however, the forage rumen-degradable protein (550 g kg–1) was less than concentrate rumen-degradable protein (700 g kg–1) (NRC, 1996). As a result, calves consumed similar total amounts of rumen-degradable protein, resulting in similar BUN among the treatments. According to Hammond et al. (1993), cattle BUN concentrations from 9 to 12 mg dL–1 represent a transition range below which daily response to protein supplementation is positive. The BUN concentrations in this study suggest that protein supplementation would be unlikely to improve calf performance.
Herbage Allowance
There was a linear decrease in average herbage allowance with increased supplementation level, although the magnitude of the change was small (Table 1). Herbage mass was not different among treatments, but greater SR for the 20 g kg–1 BW treatment (Table 3) caused herbage allowance to be slightly reduced. There was no month effect or month x year interaction on herbage allowance, however, a year effect was observed with averages across treatments of 1.0 and 0.8 kg DM kg–1 BW for 2003 and 2004, respectively. An important question to consider is whether the slightly lower herbage allowance on the 20 g kg–1 BW supplement level limited ADG. Burns et al. (1989) suggested that above some forage mass threshold, perhaps 2 Mg ha–1 for temperate and 4 Mg ha–1 for tropical swards, animals select a diet of their choice in a sustainable daily grazing time and forage mass has little causative influence on animal response. In the current study, pregraze herbage mass averaged nearly 5 Mg ha–1 (Table 1). Thus, the small difference observed in herbage allowance was not likely the reason why ADG did not increase when supplement level was increased from 15 to 20 g kg–1 BW supplement level.
Economic Analysis
Cost and income increased linearly with increasing supplement rate from 10 to 20 g kg–1 (Table 4). The greater concentrate cost decreased income at the greatest supplementation rate. There was a quadratic effect on economic return indicating that the 15 g kg–1 BW treatment provided greater return than the average of the 10 and 20 g kg–1 treatments. Return was comparable for unsupplemented calves and the 15 g kg–1 BW treatment, however, the level of performance of unsupplemented calves does not meet industry standards for replacement animals.
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CONCLUSIONS |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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NOTES |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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REFERENCES |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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