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ANIMAL NUTRITION |
* Department of Animal Sciences, University of Illinois, Urbana 61801; and
and
Department of Animal Science, Michigan State University, East Lansing 48823
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Abstract |
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 Top Abstract INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONIMPLICATIONSLITERATURE CITED |
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Key Words: body composition • calf • diet • energy • protein
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INTRODUCTION |
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONIMPLICATIONSLITERATURE CITED |
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established a new approach for specification of nutrient requirements for dairy calves, largely based on an earlier compilation of data (Davis and Drackley, 1998). The system is based on limited older data for body composition of young dairy calves that may not reflect composition of Holstein calves of current North American genotypes. Furthermore, calves in those older studies were fed milk replacers based on skim milk protein, rather than on whey proteins used currently.
Our research group (Blome et al., 2003) and others (Diaz et al., 2001; Tikofsky et al., 2001) have sought to define relationships between protein and energy intakes and accretion of body tissue in Holstein calves fed whey protein-based milk replacers. Diaz et al. (2001) determined that calf growth was directly related to intake of milk replacer (i.e., energy) when dietary protein presumably was not limiting growth. Blome et al. (2003) demonstrated that increasing the amount of CP in iso-caloric milk replacers fed at 1.5% of BW daily (DM basis) linearly increased ADG, G:F, and accretion of lean tissue in viscera-free carcass. Donnelly and Hutton (1976a,b) found that manipulating protein to energy ratios in milk replacers based on skim milk affected growth rates and body composition in preruminant dairy calves. The effects of dietary protein concentration (Donnelly and Hutton, 1976a,b; Blome et al., 2003) and the interactions of protein and energy (Donnelly and Hutton, 1976a,b) are not modeled by the current NRC (2001) system. Our objective was to quantify the effects of increasing CP concentrations in whey-protein-based milk replacers fed at 2 rates on growth and body composition of preruminant Holstein calves. Our hypothesis was that the effects of dietary protein to energy ratios on growth observed previously (Blome et al., 2003) would vary depending on energy intake.
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MATERIALS AND METHODS |
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONIMPLICATIONSLITERATURE CITED |
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Upon arrival in the afternoon, calves were weighed, fed 1.8 kg of nonsaleable whole milk, and supplemented with an oral electrolyte solution if deemed necessary because of visible dehydration. The following day, calves were fed nonsaleable whole milk at 10% of BW daily (as-fed basis) and again supplemented with oral electrolyte solution as needed. To allow them to adjust, calves remained on the milk diet for 14 d. Water was available at all times, but no dry feed was offered. Within 24 h after arrival, calves were vaccinated with TSV-2 (Pfizer, Exton, PA) and were given 2 mL of BO-SE (Schering-Plough Animal Health, Union, NJ). Approximately 2 wk after arrival, calves were vaccinated with 2 mL of Bovishield 4 (Pfizer).
Health problems were common during the adjustment period, including pneumonia, heat stress, scours, and cryptosporosis. A total of 85 calves survived the 14-d adjustment period (24, 31, and 30 in groups 1, 2, and 3, respectively) and were allocated to experimental treatments. Two calves in group 2 did not complete the 5-wk treatment period; 1 of the 2 died during wk 4 of the treatment period and the other was removed during wk 2 due to an apparent birth defect. These 2 calves were subsequently replaced with 2 calves in group 3.
After the 14-d adjustment period, calves were stratified from greatest to lowest BW and then were randomly assigned to either an initial slaughter (baseline) group or to 1 of 12 experimental treatments. Eleven calves were slaughtered for baseline measurements (2, 5, and 4 in groups 1, 2, and 3, respectively), and 74 remained on their respective diets for 5 wk before slaughter. Data from all 11 baseline calves were pooled and used to calculate beginning body composition in treatment calves. Eight of the 12 treatments were designed to address the current objectives, and only data for those treatments are reported here. A complete description of the other 4 treatments (26% CP milk replacer fed at 2.25% of BW daily, 2 whole milk treatments, and a high-fat milk replacer) and the resulting data can be found in Bartlett (2001).
For the experiment reported here, 48 Holstein calves (n = 6 per treatment) were assigned to treatments in a 2 x 4 factorial arrangement consisting of 2 feeding rates (1.25 or 1.75% of BW as DM daily, adjusted weekly) and 4 dietary CP concentrations (14, 18, 22, or 26% CP). Milk replacers were reconstituted to 12.5% DM with warm tap water; the DM concentration of the dry milk replacer (ca. 97%) was accounted for. Equivalent as-fed feeding rates were 10 and 14% of BW daily. All calves were fed twice daily, and milk replacer intake was measured daily. Milk replacers were formulated to be isocaloric; consequently, the protein to energy ratio increased as dietary CP increased at the expense of lactose and fat. The milk replacers (formulated and manufactured by Milk Specialties Co., Dundee, IL) were based on whey protein concentrate, dried whey, lard, and tallow. Milk replacers did not contain antibiotics. Milk replacers were sampled weekly and composited by group of calves.
Water was offered for ad libitum consumption, and intake was recorded daily. Calves were not offered dry feed and were housed on crushed rock to minimize ingestion of bedding material. Calves were weighed weekly, and the amount of DM offered was adjusted weekly to maintain the desired feeding rates. Calf height, length, and heart girth were measured weekly. Calves were monitored several times daily, and all observations concerning health were recorded. Fecal scores were assigned and recorded daily using the following system: 1 = dry, hard; 2 = soft, formed; 3 = pudding-like; 4 = mix of liquid and some solids; and 5 = liquid. The individual assigning fecal scores was not blinded to treatment.
Blood samples were collected once weekly via jugular venipuncture at approximately 0700, which was before the morning feeding. Blood was drawn into two 10-mL tubes containing sodium heparin and one 5-mL tube containing EDTA (Vacutainer, Becton Dickinson Vacutainer Systems USA, Rutherford, NJ). Tubes were placed on ice and were centrifuged within 1 h at 1,200 x g for 8 min. Plasma was frozen (–20°C) until analyzed.
Comparative Slaughter Procedures
Eleven calves were slaughtered for initial body composition data after the 14-d adjustment period. The remaining 48 calves were slaughtered 5 wk later. Calves were weighed on the day before slaughter, which represented the final live BW. On the morning of the slaughter day, calves were transported (0.6 km) by livestock trailer to the University of Illinois Meat Science Laboratory. Calves were weighed at the Meat Science Laboratory immediately before slaughter, which was approximately 17 h after the last feeding; this weight was considered to represent shrunk BW (SBW).
Calves were slaughtered using captive bolt stunning followed by exsanguination. Blood was collected and the weight recorded. The hide and viscera were removed. The body was separated into 3 fractions: head, hide, feet, and tail (HHFT); viscera; and carcass. The gastrointestinal tract (GIT) was removed and weighed. Digesta was removed from the GIT by rinsing the stomach and intestines thoroughly with water. The empty GIT then was weighed, and the amount of digesta in the GIT at the time of slaughter was determined as the difference in weight between the full GIT and the empty GIT. Individual weights were recorded for the kidneys, liver, and heart; all internal organs were then pooled to form the visceral fraction. The fractions were refrigerated overnight and processed the following day.
The HHFT and carcass were ground twice through a whole-carcass grinder (model 801 GP15, Autio Co. Inc., Astoria, OR) fitted with a 1.3-cm plate and then were subsampled. The visceral fraction was ground twice through a Butcher Boy grinder (model 52HF, Laser Manufacturing, Los Angeles, CA) and then subsampled. The subsamples were frozen and then reground twice through the Butcher Boy grinder, using a smaller die (3 mm). Subsamples were frozen (–20°C) for later analyses, and another subsample was lyophilized.
We assumed that the variation in HHFT composition between calves would be insignificant and that diet would have little influence on HHFT composition. The HHFT fraction was extremely difficult to grind; therefore, HHFT composites were made. A HHFT composite was created for each day that calves were slaughtered. All HHFT from calves used to determine initial body composition were composited, and 5 HHFT from the treatment calves were composited, thereby creating 3 initial and 3 final HHFT samples. No other fractions were derived from the composites.
Analytical Procedures
Milk replacers were analyzed for GE concentration using an adiabatic bomb calorimeter (Parr Instrument Co., Moline, IL). Metabolizable energy in milk replacers was calculated as 0.9 x measured GE, based on a balance trial with similar calves and similar milk replacers fed at 1.5% of BW daily (Blome et al., 2003). Nitrogen was determined using the Kjeldahl digestion procedure according to AOAC (1990); N was converted to protein by multiplying by 6.25 for both milk replacers and body tissues. Blood collected during the slaughter procedure was not combined with any fraction, and samples inadvertently were not retained for analysis. Consequently, concentrations of DM, protein, and energy were determined after the completion of the experiment in whole blood samples obtained from calves fed and managed similarly at the University of Illinois Dairy Unit. Concentrations of fat (Novakofski et al., 1989), ash (AOAC, 1990), and water (AOAC, 1990) were determined in frozen tissue samples from each body fraction. Energy was measured in freeze-dried tissue fractions by bomb calorimetry. The fatty acid concentration and profile of the milk replacers was determined by gas chromatography of methyl esters formed by acid-catalyzed transes-terification (Sukhija and Palmquist, 1988) using a Supelco SP-2380, 100-m, fused silica capillary column (Supelco, Bellefonte, PA) in a Shimadzu GC-17A gas chromatograph (Shimadzu Scientific Instruments Inc., Columbia, MD).
Concentrations of glucose (kit 315-500, Sigma Chemical Co., St. Louis, MO), urea N (kit 535B; Sigma), total protein (kit 541-2, Sigma), and NEFA (kit 994-75409, Wako Chemical, Richmond, VA) in plasma were determined by standard enzymatic-colorimetric procedures. Concentrations of IGF-I in plasma were determined by RIA after removal of binding proteins using acid/ ethanol extraction (Sharma et al., 1994). Recombinant human IGF-I and the primary antibody were obtained from GroPep (Adelaide, Australia). Concentrations of insulin in plasma were determined by RIA using a kit (Coat-a-Count Insulin kit, Diagnostic Products Inc., Los Angeles, CA) as modified by Studer et al. (1993). Sensitivity was 0.3 µIU/mL; between and within assay CV were 3.1 and 2.7%, respectively.
Calculation of Body Composition and Composition of Gain
Empty body weight (EBW) was the sum of weights of carcass, digesta-free viscera, HHFT, and blood. The composition of gain for each calf was calculated as the amounts of water, protein, fat, and ash at slaughter minus the calculated amounts of those components present in each calf at the beginning of the experiment, as estimated from the average composition of the baseline calves applied to the beginning BW of each calf.
Statistical Analysis
Data were analyzed using the Mixed procedure in SAS (version 8.1, SAS Inst. Inc., Cary, NC). Block (i.e., group) was designated as a random effect, whereas feeding rate, dietary CP concentration, and the interaction of feeding rate and CP concentration were fixed effects. For variables with repeated measures, the model also contained calf (as a random effect) and the fixed effects of time (as a repeated factor) and treatment x time interactions. Covariance structures considered were compound symmetry, autoregressive order 1, and unstructured; the autoregressive order 1 structure was found to be the most appropriate for all variables based on Akaike’s Information Criteria. Initial measurements of BW, length, height, heart girth, and blood samples during the adaptation period before treatments were assigned were used as covariates when analyzing the respective stature measurements and blood variables. Orthogonal polynomial contrasts were used to estimate the linear, quadratic, and cubic effects of increasing CP concentration and the interactions of feeding rate with the linear, quadratic, and cubic effects of increasing CP. For variables with repeated measurements (e.g., compounds in blood), polynomial contrasts were used to test linear, quadratic, and cubic effects of week. Contrasts also were constructed to determine interactions of the treatment contrasts with effects of week. Least squares means and standard errors are reported. Significance was declared at P < 0.05.
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RESULTS AND DISCUSSION |
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONIMPLICATIONSLITERATURE CITED |
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. Diets were isocaloric on a GE basis by design. Increasing CP concentration of isocaloric milk replacers necessitates decreases in fat and lactose concentrations; consequently, increasing CP increased the dietary protein to energy ratio. As discussed elsewhere (Blome et al., 2003), treatment designations and results are presented in the context of increasing CP concentration for simplicity. Variation during manufacturing resulted in analyzed content of CP being slightly greater than target values for the 14, 18, and 22% CP milk replacers and less than formulated for the 26% CP diet.
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). Intake of CP increased, whereas intakes of lactose and total fatty acids decreased as dietary CP concentration increased. Intakes of DM and nutrient fractions increased by week as calves grew (data not shown). The interaction of feeding rate and the linear effect of CP was significant for CP intake, reflecting differences in milk replacer composition and in the rate of feeding due to differences in growth. Consumption of water was highly variable among calves and was not affected by dietary treatments (Table 2).
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4) was 4 d greater for calves fed at 1.75% of BW daily than for those fed at 1.25% of BW daily (12 vs. 8) but was not affected by CP concentration (data not shown). Calves did not have elevated body temperatures during days of soft feces and remained alert and hungry, indicating that soft feces did not constitute diarrhea and was not of infectious origin.
Growth and Feed Efficiency
Initial BW did not differ among treatments (Table 3). Final BW, ADG, and final heart girth were greater for calves fed at 1.75% of BW daily and increased linearly as CP increased. The ADG ranged from 0.251 kg/ d (14% CP at 1.25% of BW daily) to 0.703 kg/d (26% CP at 1.75% of BW daily). Length of calves increased linearly, and withers height tended (P = 0.12) to increase linearly, as dietary CP increased.
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). Stimulation of growth by increasing dietary CP has been reported previously in calves (Donnelly and Hutton, 1976a,b; Gerrits et al., 1996), as well as in lambs (Jagusch et al., 1970), pigs (Noblet et al., 1987), and rats (Zhao et al., 1996). Increased growth of calves fed at 1.75% of BW daily compared with those fed at 1.25% of BW daily was expected and is consistent with previous studies demonstrating greater growth for calves fed larger amounts of whole milk (Khouri and Pickering, 1968; Johnson and Elliott, 1972; Jasper and Weary, 2002) or milk replacers based on skim milk (Donnelly and Hutton, 1976a; Gerrits et al., 1996) or whey proteins (Diaz et al., 2001; Brown et al., 2005).
Gain:feed was greater for calves fed at 1.75% of BW daily than for calves fed at 1.25% of BW daily and increased in a quadratic manner as CP increased, with greatest efficiencies observed for calves fed milk replacer with 22% CP. Greater efficiency from the greater rate of feeding resulted primarily from providing more energy and nutrients above maintenance requirements, similar to results from others (Khouri and Pickering, 1968; Gerrits et al., 1996; Diaz et al., 2001). On the other hand, the improved G:F as dietary CP increased was attributable to increased accretion of body protein and the associated water in lean tissue. The marked improvements in efficiency of gain measured both in this study and our previous study (Blome et al., 2003) as a result of simply increasing dietary CP concentration in isocaloric milk replacers fed in limited amounts are noteworthy. The largest G:F in this study (0.72 for calves fed 22% CP milk replacer at 1.75% of BW daily) compares favorably with results reported for the rapidly growing young of other species (Davis and Drackley, 1998).
Body Fractions and Organ Weights
The SBW of baseline calves was 96.9% of live BW, and SBW averaged 97.7% of final live BW for calves at the end of the growth period. Both SBW and final EBW were greater for calves fed at 1.75% of BW daily than for calves fed at 1.25% of BW daily, and increased linearly as dietary CP increased (Table 4). Final EBW averaged 97.4% of SBW, which was similar to that of baseline calves (97.5%). The amount of digesta in the GIT at slaughter tended (P < 0.10) to be greater as a percentage of SBW for calves fed at 1.25% of BW daily. Although calves were housed on crushed rock to minimize ingestion of potentially fermentable material, variable amounts of crushed rock were found in the stomach of some calves. Digesta averaged 1.4% of SBW for calves at the end of the study, compared with 1.9% for baseline calves. The sum of EBW plus digesta averaged 98.8 and 99.4% of SBW for treatment and baseline calves, respectively.
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). Percentages of final SBW as EBW, carcass, and HHFT were similar to data presented by others (Diaz et al., 2001; Tikofsky et al., 2001; Blome et al., 2003). Weights of body fractions generally increased in proportion to changes in EBW caused by differences in feeding. As a percentage of final EBW, carcass was slightly greater, and HHFT slightly less, for the calves that were larger at slaughter. Similarly, carcass as a percentage of EBW was greater, and HHFT as a percentage of EBW smaller, than for baseline calves. These changes likely were driven by the decrease in surface to mass ratio as calves grew. Weight of recovered blood increased linearly as dietary CP increased but was not different between feeding rates (Table 4). Recovered blood as a percentage of BW tended to increase as dietary CP increased. As estimated by dye dilution techniques in live calves, plasma volume is about 9% of BW (Quigley et al., 1998). Smaller values in our study reflect the amount of blood retained within tissues and not recovered.
Total visceral weight was greater for calves at the greater feeding rate but was not affected by dietary CP concentration (Table 4). As a percentage of EBW, total visceral weight was greater for calves fed at the greater rate but decreased as dietary CP increased. Final weights of the GIT, liver, heart, and kidneys were greater for calves fed at 1.75% of BW daily; liver weight increased quadratically and kidney weight tended (P <0.10) to increase linearly as dietary CP increased. Weights of organs expressed as percentages of BW were similar to data reported by others (Diaz et al., 2001; Tikofsky et al., 2001; Blome et al., 2003).
Empty Body Chemical Composition
Amounts of water, protein, and fat in the final empty body were greater for calves fed at 1.75% of BW daily (Table 5). Amounts of water and protein increased linearly, whereas fat decreased linearly, as dietary CP increased (Table 5). The amount of ash was greater for calves at the greater feeding rate and tended (P < 0.10) to increase quadratically as dietary CP concentration increased. The sum of water, protein, fat, and ash was 99.8% of EBW for baseline calves and averaged 100.2% of EBW for treatment calves. The final amount of body tissue energy was greater for calves fed at the greater rate but was not affected by dietary CP.
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). The percentages of water and protein increased linearly and the percentage of fat decreased (linear, P < 0.001; cubic, P < 0.01) as CP concentration increased. The interaction of feeding rate with the quadratic effect of CP approached significance (P < 0.10) for the percentage protein in final EBW. The percentage of ash was not affected by feeding rate or dietary CP. For ash, the interaction of feeding rate and the linear effect of CP approached significance (P < 0.10), indicating that percentage ash tended to decrease as dietary CP increased for calves fed at 1.25% of BW daily but tended to increase with dietary CP for calves fed at 1.75% of BW daily. Although ash content should increase in proportion to lean tissue gain if skeletal growth is increased, analytical variability may have precluded our ability to detect such an effect. Final tissue energy density was greater for calves fed at 1.75% of BW daily and decreased with increasing dietary CP. The concentration of water at the end of the experimental period was less than that for baseline calves, whereas concentrations of fat and energy were greater.
Our body composition data are generally similar to those reported by Tikofsky et al. (2001) and Donnelly and Hutton (1976b), with the exception that calves in the latter study were somewhat fatter. Donnelly and Hutton (1976b) found that increases in dietary CP were associated with significant increases in amounts of water, protein, and ash and decreases in fat in the body. Furthermore, they reported that increasing the feeding rate significantly increased fat content in the body. Average DMI for the feeding rates studied by Donnelly and Hutton (1976a,b) was 0.962 and 1.191 kg/d, compared with 0.647 and 0.964 kg/d for calves in our experiment fed at 1.25 and 1.75% of BW daily. The respective ADG were 0.512 and 0.716 kg for Donnelly and Hutton (1976a) compared with 0.332 and 0.616 kg in our study.
Empty Body Component Gains
Gains of EBW and empty-body water, protein, fat, and ash were greater for calves fed at 1.75% of BW daily than for calves fed at 1.25% of BW daily (Table 6). Gains of EBW, water, and protein increased linearly, whereas fat gain decreased linearly, as CP concentration increased (Table 6). The interaction of feeding rate and the linear effect of dietary CP concentration approached significance (P < 0.10) for EBW gain and were significant for water and protein gains; the interaction of feeding rate and quadratic effect of dietary CP was significant for ash. These interactions demonstrated that component gains decreased slightly as CP was increased from 22 to 26% for calves fed at 1.25% of BW daily, but component gains increased in response to the greater CP for calves fed at 1.75% of BW daily. The sum of component gains agreed closely with EBW gain, as expected.
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), but feeding rate did not significantly affect percentages of water, protein, or ash in EBW gain. Increasing dietary CP concentration increased the percentages of water and protein in gain, and decreased the percentage of fat (Table 6). As a result of the increased water and protein and decreased fat in EBW gain, the energy density of EBW gain decreased with increasing dietary CP.
Although the amount of final tissue energy was unaffected by milk replacer composition, increasing dietary CP had marked effects on the components of EBW gain as observed previously (Blome et al., 2003). Lean tissue (primarily protein and water) deposition increased as dietary CP concentration increased and as feeding rate increased, implying that calves fed at lesser CP concentrations and smaller intakes did not consume adequate amounts of energy or protein to maximize lean tissue deposition (Donnelly and Hutton, 1976a,b; Gerrits et al., 1996). The interactions of feeding rate with the linear effect of CP for empty-body gains of water and protein indicate that rates of increase in lean tissue gain as CP increased were less for calves fed at 1.25% of BW daily than for calves fed at 1.75% of BW daily. Calves fed at 1.25% of BW daily probably consumed insufficient ME to utilize greater amounts of dietary CP for growth.
Fat deposition decreased linearly as dietary CP concentration increased, consistent with previous results (Donnelly and Hutton, 1976a,b; Blome et al., 2003). As a proportion of total EBW gain, fat decreased in a nonlinear manner as dietary CP increased. The concentration of fat in empty-body gain varied inversely with that of water and protein as dietary CP concentration increased, as expected. Our results indicate that, when protein intake is less than required for deposition of lean tissue at a particular ME-allowable growth, excess energy intake is stored as fat. As a result of these differences in composition of gain, calves fed milk replacers containing 14 or 18% CP contained a greater percentage of fat in the whole body at slaughter. The decreasing proportion of fat in EBW gain as dietary CP increased seemed to be approaching an asymptotic minimum, as noted earlier by Donnelly and Hutton (1976a,b), that would represent the fat concentration of EBW gain at maximal growth rates. However, the range of dietary CP concentrations in our experiment was too small to verify this supposition. Calves fed at 1.75% of BW daily deposited greater amounts of fat and had slightly greater fat concentration in EBW gain at each dietary CP concentration than did calves fed at 1.25% of BW daily. Diaz et al. (2001) reported similar results in calves fed increasing amounts of milk replacer containing 30% CP for target rates of gain of 0.50, 0.95, and 1.40 kg/d.
A comparison of our data for calves fed milk replacer containing 26% CP at 1.75% of BW daily with data from Donnelly and Hutton (1976b) for Holstein calves fed a similar dietary CP concentration (25.4%) and reaching similar final body weights (70.6 vs. 70.0 kg) is informative. Gain of EBW for those calves slaughtered in our study averaged 19.3% protein, 65.3% water, 11.5% fat, and 3.9% ash. Donnelly and Hutton (1976b) reported that the composition of gain was 19.0% protein, 62.3% water, 15.3% fat, and 3.3% ash for the comparable calves. The final empty-body composition of our calves was 71.1% water, 18.6% protein, 6.5% fat, and 3.8% ash. Donnelly and Hutton (1976b) reported final empty-body composition of 67.3% water, 19.6% protein, 9.1% fat, and 4.0% ash. Greater fatness in calves studied by Donnelly and Hutton (1976b) might be attributable to the greater intakes and ADG in their study, to differences in genetics, or to changes in the baseline composition of preruminant Holstein calves that have may occurred in the last 25 yr.
Our data reported here, together with those from our previous study (Blome et al., 2003), data from Donnelly and Hutton (1976a,b), and data from older calves (80 to 240 kg of BW; Gerrits et al., 1996), substantiate that biological interrelationships exist between dietary CP concentration and feeding rate. Moreover, these data clearly demonstrate that body composition and the composition of EBW gain in preruminant calves can be altered markedly by manipulating the dietary protein to energy ratio. This fact is particularly evident when calves are fed to less than requirements for maximal gain, as is typical in the North American dairy industry. Current NRC (2001) models do not account for this effect of dietary protein:energy.
IGF-I, Insulin, and Metabolites in Plasma
A secondary objective of our study was to determine the impact of changes in feeding rate and dietary composition on IGF-I and insulin concentrations. The concentration of IGF-I in plasma was greater for calves fed at 1.75% of BW daily and increased linearly as dietary CP increased (Table 7). The interaction of feeding rate and the linear effect of dietary CP concentration was significant, such that IGF-I concentration decreased slightly as CP was increased from 22 to 26% for calves fed at 1.25% of BW daily, but continued to increase with dietary CP up to 26% for calves fed at 1.75% of BW daily.
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); the 3-way interaction among feeding rate, the linear effect of dietary CP, and the linear effect of week (Figure 1) showed that IGF-I concentrations increased more with time for calves fed at the greater feeding rate and that differences among dietary CP concentrations were more pronounced for calves fed at 1.75% of BW daily than for those fed at 1.25% of BW daily. Others have reported increased IGF-I concentrations in response to increased protein intakes and increased feeding rates (Gerrits et al., 1998; Smith et al., 2002; Brown et al., 2005) in young calves. Smith et al. (2002) found that the somatropic axis was functional in young calves, provided that plane of nutrition was adequate to allow IGF-I response to growth hormone. Across all treatments in our study, plasma IGF-I concentrations were strongly correlated (P < 0.01) with EBW gain (r = 0.73) and with empty-body gains of protein (r = 0.72), fat (r = 0.39), and energy (r = 0.59). Although the significance of the circulating pool of IGF-I in regulating growth remains uncertain (Smith et al., 2002), the correlation between IGF-I concentrations and growth rates in this study and others (Gerrits et al., 1998; Smith et al., 2002; Brown et al., 2005) suggests that IGF-I is at least indirectly related to stimulation of growth by greater nutrient supply. Moreover, IGF-I may impact other systems of the body in addition to growth. For example, in other species, IGF-I plays a direct role in integrating the growth, maintenance, repair, and function of the immune system (Clark, 1997).
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found that plasma IGF-I concentrations were greater in calves fed for medium (0.94 to 1.04 kg/d) and high (0.93 to 1.21 kg/d) ADG than for calves fed for low rates of gain (0.52 to 0.60 kg/d). Correlations between ADG and IGF-I increased with greater growth rates (Smith et al., 2002). Gerrits et al. (1998) concluded that protein intake, but not protein-free energy intake, influenced plasma IGF-I in calves between 80 and 240 kg of BW. Data from our study lend support to the conclusion that IGF-I is more strongly related to protein intake than energy intake (Figure 2). Plasma IGF-I increased as dietary CP intake increased for both feeding rates, although the rate of increase was less for calves fed at 1.25% of BW daily (Figure 2, top panel). With the exception of IGF-I for calves fed the 26% CP milk replacer at 1.25% of BW, however, all values are well represented by 1 regression line with slope and intercept similar to those of calves fed 1.75% of BW daily. This relationship further supports the conclusion that energy was more limiting for calves fed the 26% CP milk replacer at 1.25% of BW daily. In contrast, a comparison of the effects of ME intake on plasma IGF-I shows that ME intakes for groups of calves with the same mean IGF-I concentrations could differ by more than 1.3 Mcal daily (Figure 2, bottom panel). Thus, plasma IGF-I in preruminant calves of this BW range seems to be more strongly related to protein intake, or dietary protein:energy, than to total ME intake.
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) were greater for calves fed at 1.75% of BW daily and increased linearly as dietary CP increased. The interaction of feeding rate and the linear effect of dietary CP also was significant, which occurred because increases in insulin with dietary CP were greater at the greater feeding rate. Concentrations of insulin were correlated (P < 0.05) with gains of EBW (r = 0.33), protein (r = 0.37), and energy (r = 0.34) but not with fat gain (r = 0.19; P = 0.20). Similar to our study, Smith et al. (2002) reported that plasma insulin concentrations in male Holstein calves increased in proportion to intake of milk replacer when the increased intake promoted greater ADG. Concentrations of insulin were smaller in our study than those reported by Smith et al. (2002), which is not surprising because samples were collected before the morning feeding after approximately 14 h without feed, whereas Smith et al. (2002) sampled their calves 4 to 6 h after the morning feeding (Diaz et al., 2001).
The concentration of glucose in plasma was greater for calves fed at 1.75% of BW daily than for calves fed at 1.25% of BW daily and tended (P = 0.08) to increase linearly with increasing dietary CP (Table 7). The concentration of NEFA was not affected by feeding rate or dietary CP concentration. Insulin concentrations generally reflected the changes in plasma glucose concentrations. Insulin, reflecting energy intake, and leucine, reflecting AA supply, converge at a common step in signal transduction mechanisms (the mammalian target of rapomycin, mTOR) to directly stimulate protein translation (Schmelzle and Hall, 2000; Meijer, 2003).
The concentration of urea N in plasma was less for calves fed at 1.75% of BW daily and increased with increasing dietary CP (Table 7). However, the interaction of feeding rate with the quadratic effect of dietary CP demonstrated that the urea N concentration increased sharply for calves fed milk replacer with 26% CP at the lower feeding rate, whereas urea N concentration for calves fed at 1.75% of BW daily increased in an essentially linear manner as dietary CP increased. The interaction supports the interpretation that calves fed the 26% CP milk replacer at 1.25% of BW daily consumed protein in excess of requirements, which resulted in greater AA catabolism.
The concentration of total protein in plasma was less for calves fed at 1.75% of BW daily and increased linearly as CP increased; however, the interaction of feeding rate and the linear effect of dietary CP indicates that total protein concentrations were unaffected by dietary CP in calves fed at 1.25% of BW daily but were less for calves fed 14 or 18% CP diets at 1.75% of BW daily (Table 7). Decreased plasma protein is a well-known consequence of protein malnutrition (Swenson, 1993); therefore, the lesser values for calves fed 14 or 18% CP diets at 1.75% of BW daily in our study likely reflect a state of protein supply being more limiting than energy availability for growth.
Use of Energy
Calves fed at 1.75% of BW daily had greater estimated ME requirements for maintenance than did calves fed at 1.25% of BW daily because of greater BW (Table 8). The amount of ME available for gain and actual retained energy (RE) also were greater for calves fed at 1.75% of BW daily. Gross efficiency of energy use (RE:GE intake) was greater for calves at the greater feeding rate (Table 8), consistent with results of others (Diaz et al., 2001) but was not affected by dietary CP concentration. Greater gross efficiency with increased intake represents the effect of dilution of maintenance energy costs at greater rates of gain. In contrast, net efficiency of use of ME above maintenance for RE was greater for calves fed at 1.25% of BW daily (65.8%) than for calves fed at 1.75% of BW daily (58.1%) but was not affected significantly by dietary CP (Table 8). Greater net efficiency for calves fed at the lower feeding rate may reflect the more energetically expensive cost of greater protein synthesis in calves at greater growth rates and the fact that fat deposition is energetically more efficient than protein deposition. Net efficiency of ME use for RE would be expected to decrease as dietary CP concentration increased because protein deposition increased and fat deposition decreased (Donnelly, 1983; Tikofsky et al., 2001) and because the lower CP diets by necessity were greater in fat, which is deposited efficiently as body fat (Tikofsky et al., 2001). The lack of these changes in efficiency attributable to dietary CP concentration in our study may have been a result of the experimental design in which calves were slaughtered at constant age rather than constant BW (Donnelly, 1983; Tikofsky et al., 2001).
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but was slightly greater than data from an earlier study from our group (58%; Blome et al., 2003) and from other studies in preruminant Holstein calves fed whey-based milk replacers (Diaz et al., 2001; Tikofsky et al., 2001). Data used by the NRC (2001) were mostly from older studies in which calves were fed whole milk or skim milk-based milk replacers more typical for veal production, with measurements made at heavier BW and greater body fatness.
Although estimates of efficiency of ME use for body energy gain are affected by calculation of maintenance energy, estimates of maintenance ME requirements obtained from the complete data set in this experiment (data not shown), as well as estimates made from the data set of Diaz et al. (2001), agree closely with maintenance ME values established by NRC (2001). Consequently, for Holstein calves of current genotype fed whey-based milk replacers with moderate fat concentration, the efficiency of ME use for body energy gain on average is overestimated by NRC (2001) equations. Furthermore, the assumption by NRC (2001) of constant partial efficiency of ME use is incorrect; efficiency of ME use depends on the rate of gain.
The energy content of EBW gain (Table 6) was greater for calves fed at 1.75% of BW daily (2.33 Mcal/kg) than for calves fed at 1.25% of BW daily (2.14 Mcal/kg) and decreased as dietary CP increased for either feeding rate. Changes in energy content of EBW gain reflected the corresponding changes in lean tissue and fat. Values were generally similar to those reported by Diaz et al. (2001) and Blome et al. (2003). Energy gain calculated from gains of fat and protein, using energetic values for fat and protein from calf tissue of 9.290 kcal/g of fat and 5.545 kcal/g of protein (Donnelly and Hutton, 1976b), were slightly larger than those obtained by calorimetric methods (data not shown). However, means obtained by either method were within the 95% confidence limits of each other. Diaz et al. (2001) compared their experimentally determined values for energy content of EBW gain with those predicted by the Toullec (1989) equation, later adopted by NRC (2001), and found that the Toullec (1989) equations overpredicted energy of BW gain when protein was not limiting growth.
Our data highlight the fact that energy content of tissue gain is dependent on dietary composition, which is not modeled in the current NRC (2001) system. Using our mean BW, ME intakes, and diet composition in the NRC (2001) equations, calves fed milk replacers with 14, 18, 22, and 26% CP had predicted ME-allowable gains of 0.36, 0.37, 0.38, and 0.37 kg/d when fed at 1.25% of BW daily, and 0.72, 0.70, 0.71, and 0.73 kg/d when fed at 1.75% of BW daily. The predicted apparently digestible protein (ADP)-allowable gains were 0.22, 0.34, 0.45, and 0.50 kg/d when fed at 1.25% of BW daily and 0.40, 0.55, 0.70, and 0.82 kg/d when fed at 1.75% of BW daily. Therefore, calves fed milk replacers containing 14 or 18% CP at 1.25% of BW daily may have been limited by ADP supply, whereas calves fed 22 or 26% CP at 1.25% of BW daily likely were limited by ME intake. For calves fed at 1.75% of BW daily, ADP supply likely limited calves fed either 14 or 18% CP, whereas ME and ADP intakes were equally limiting for calves fed 22% CP, and ME may have limited gains for calves fed 26% CP.
Use of Protein
Gross efficiency of dietary CP use (empty-body protein gain as a proportion of dietary CP intake) averaged 48.4% for calves fed at 1.25% of BW daily and was greater (56.0%) for calves fed at 1.75% of BW daily (Table 8); gross efficiency was not affected by dietary CP concentration. Gross efficiency ranged from 0.482 for calves fed 14% CP at 1.25% of BW daily to 0.576 for calves fed 22% CP at 1.75% of BW daily. Capture of dietary N in body tissue, represented here by gross use of dietary CP, is the most relevant measure of N efficiency for use in whole-farm N budgets. Consequently, DM feeding rates greater than traditional industry standards of 1.0 to 1.25% of BW daily (8 to 10% of BW daily as liquid) could increase efficiency of N use in dairy enterprises.
Digestible CP intake, calculated using the apparent CP digestibility (93%) assumed by NRC (2001), followed patterns of CP intake as reported in Table 2. The ratio of protein gain to digestible CP intake (equivalent to apparent biological value) was greater for calves fed at 1.75% of BW daily but was not significantly affected by dietary CP concentration. Efficiency of dietary digestible CP use for body protein gain averaged 52.1 and 60.3% for calves fed at 1.25 and 1.75% of BW daily (Table 8), again indicating that increasing the energy supply improved use of dietary protein, as observed by others (Donnelly and Hutton, 1976a; Diaz et al., 2001). Our values compare favorably with the efficiency determined from N retention (66%; Blome et al., 2003) or whole-body protein deposition (63%; Diaz et al., 2001) in recent experiments.
The maintenance requirement for ADP, calculated according to NRC (2001), was greater for calves fed at the greater feeding rate but was not affected significantly by dietary CP concentration (Table 8). By subtracting the calculated ADP required for maintenance from digestible CP intake we were able to estimate the net efficiency of ADP use for gain (Table 8). Average values were 73.5 and 75.8% for calves fed at 1.25 and 1.75% of BW daily, which were not significantly different. Efficiency decreased as dietary CP increased, in agreement with results by Donnelly and Hutton (1976a); this can be attributed, at least in part, to lower efficiencies in those situations where dietary CP supply exceeded requirements.
To better determine efficiency of body protein deposition from dietary protein supply, we used regression analysis for all data except those for calves fed 26% CP at 1.25% of BW daily. As discussed previously, calves fed 26% CP at 1.25% of BW daily consumed an excess of dietary protein relative to their requirements for growth as limited by ME supply. Dietary CP intake accounted for 85.3% of the variation in empty-body protein gain (Figure 3, top panel). Use of estimated digestible CP intake rather than CP intake increased the regression coefficient to 0.665 from 0.618 (data not shown). Values for efficiency of absorbed N over maintenance for tissue protein deposition depend on the estimation of maintenance N requirements. The intercept from the regression shown in Figure 3 (–12.3 g of protein/d) represents endogenous protein loss from the body at zero dietary CP intake (or zero digestible CP intake; see caption for Figure 3). This value, equivalent to 2.0 g/d of N, is smaller than the value for calves of this average BW calculated according to NRC (2001) as average endogenous urinary N (4.0 g/d), which represents the major portion of maintenance N requirements. Values calculated from Agricultural Research Council (1980) equations are somewhat lower (3.5 g/d) than NRC (2001) estimates but still greater than our calculated value. Estimates of basal urinary N excretion have largely been made in heavier calves fed whole milk or skim milk-based milk replacers and should be reevaluated for whey protein-based milk replacers.
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) is shown by the regression analysis in Figure 3 (bottom panel). This equation, which accounted for 85.1% of the variance, estimated that the marginal efficiency of additional digestible CP intake for empty-body protein gain was 0.685. By definition, however, this equation should have a zero intercept; when the regression was forced through the origin the estimated efficiency was 0.743 (Figure 3, bottom panel). The average net efficiency of digestible CP use above estimated maintenance for body protein gain across all treatments was 74.7% (Table 8). These values are slightly lower than the constant efficiency (i.e., biological value) assumed by current (80%; NRC, 2001) and previous (77%; NRC, 1978) standards, which were derived on the basis of studies using whole milk or skim milk-based milk replacers.
Values for efficiency of absorbed N use over maintenance also depend on the determined or assumed digestibility of dietary CP. For example, using the CP digestibility (87.7%) of similar milk replacers fed to calves of similar age and BW from our earlier study (Blome et al., 2003) instead of the NRC (2001) assumption of 93% digestibility increases estimates of marginal efficiency of absorbed protein use above maintenance from 74.3% (Figure 3, bottom panel) to 78.4% (not shown).
Based on daily accretion rates of body protein, the N content of EBW gain was 27.7, 28.6, 32.3, and 33.8 g of N/kg of EBW gain for calves fed milk replacers with increasing CP concentrations at 1.25% of BW daily, and 25.6, 28.3, 29.3, and 30.9 g of N/kg of EBW gain for calves fed respective CP concentrations at 1.75% of BW daily. The NRC (2001) assumes a constant value of 30 g of N/kg of live BW gain in calculation of net protein requirements for growth. Diaz et al. (2001) reported that N content of EBW gain ranged from 31 to 41 g of N/kg of EBW gain in calves that were fed an excess of CP at varying energy intakes. The protein content of the empty body of calves in our study was less than that determined by Diaz et al. (2001) but similar to values reported by Tikofsky et al. (2001) and by Donnelly and Hutton (1976b). Regardless, our data demonstrate that the N content of body tissue gain is not a constant, at least when feeding rate or dietary protein supply limit calf growth.
Conclusions
The results of this experiment describe responses of preruminant calves to increased feeding rate and increased protein to energy ratios in milk replacers. Body composition and the composition of EBW gain can be markedly altered by manipulating dietary protein to energy ratios. Increasing the feeding rate increases ADG and efficiencies of gain, with only small impact on the composition of EBW gain if dietary protein is adequate. Effects of dietary CP interacted with feeding rate for gains of lean tissue (water plus protein); these data indicated that >22% CP was unnecessary when calves were fed at 1.25% of BW daily but that lean tissue gain continued to increase with dietary CP up to 26% when calves were fed at 1.75% of BW daily. Body fat gain varied inversely to lean tissue gain. Plasma IGF-I seems to be more closely related to dietary protein intake than to energy intake in preruminant calves. The effects of diet and feeding rate on body composition and overall growth in our study were not well predicted by NRC (2001). Our data provide a basis for further development of improved prediction equations that would account for diet-induced differences in body composition.
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONIMPLICATIONSLITERATURE CITED |
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2 Present address: Cornell Cooperative Extension, Bath, NY.
3 Corresponding author: drackley{at}uiuc.edu
Received for publication September 7, 2005. Accepted for publication January 11, 2006.
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LITERATURE CITED |
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONIMPLICATIONSLITERATURE CITED |
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= 18% CP; 
= 22% CP; 
= 26% CP. The largest SEM for any treatment x week least squares mean = 20.1 ng/mL. Interactions of feeding rate x quadratic effect of week (P < 0.001; largest SEM = 16.2 ng/mL) and the linear effect of CP x linear effect of week (P < 0.05; largest SEM = 17.6 ng/mL) also were significant.
