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ANIMAL NUTRITION |
USDA, ARS, US Meat Animal Research Center, Clay Center, NE 68933
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Abstract |
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 Top Abstract INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONLITERATURE CITED |
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Hereford, Angus, Red Poll, and Pinzgauer) that were between 3 and 53 d in milk. Cows’ BW were 481 ± 4 kg. Metabolizable energy intake ranged from 14.8 to 28.9 Mcal/d. Milk yields ranged from 4.7 to 13.3 kg/d. Recovered energy (RE) increased linearly with increased ME intake. Forty-seven observations were obtained with cows in negative tissue energy (TE) balance, and 20 observations were obtained with cows in positive TE balance. Estimated zero RE from regression analysis of RE on ME intake was 146 kcal of ME/kg of BW0.75. Efficiency of conversion of ME to lactation energy (LE) was 72%. The efficiency for conversion of ME to TE and the conversion of TE to LE was 78%. Our findings suggest that, even though their milk production is lower, the overall efficiency of energy retention in young beef cows is similar to that of dairy cows.
Key Words: cow • energy • heat production
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INTRODUCTION |
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONLITERATURE CITED |
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; DeRouen et al., 1994). The energy needs of the cow are increased by lactation, and additional energy is required for growth in the primiparous cow. Determining energy metabolism during the first 40 to 60 d postpartum is critical to developing feed programs that will allow cows to become pregnant with a second calf. Energy must be provided for maintenance, milk production, and tissue growth. The need of primiparous beef cows to grow and lactate makes them different than the multiparous cow. The objective of the study was to determine the efficiency of energy use for growth and lactation in primiparous beef cows. |
MATERIALS AND METHODS |
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONLITERATURE CITED |
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Twenty-five heifers (4-breed composite: Hereford, Angus, Red Poll, and Pinzgauer; MARC III) that were trained for use in nutrient balance studies were bred to a MARC III bull. Heifers were fed the same diet throughout the study (corn silage, 67.3%; chopped alfalfa hay, 27%; corn, 5.5%; and sodium chloride, 0.2%; DM basis). The diet contained 1.81% N and had a GE of 4,419 cal/g, as DM.
During pregnancy, heifers were fed to limit maternal BW gain. Diets were fed to provide ME (Mcal/d) for maintenance (0.135 kcal of ME x metabolic body size [MBS; kg of BW0.75]), and for conceptus growth (MEy). Body weight was the cows’ BW minus the weight of the conceptus, which was estimated as: 40/38.5 x 0.7439e(0.0199694–0.0000143t)t, where t represents days post-mating (NRC, 2000). Metabolizable energy for conceptus growth was calculated as MEy = [40 x (0.4504 –0.000766t)e(0.03233 –0.0000275t)t]/1,000, where t is days postmating (NRC, 2000). At parturition, cows were assigned to 1 of 3 nutrient levels. During lactation, cows were fed 69.9, 87.4, or 105.0 g of DM/kg of MBS at parturition. Nine cows were assigned to the lowest level and 8 were assigned to each of the other levels.
Cows were fed individually by use of Calan electronic headgates (American Calan, Inc., Northwood, NH) in group pens of 4 cows each. Sixty-seven balance trials were conducted on the 25 cows that were between 3 and 53 d in milk. Three balance trials were conducted on 18 cows, 2 balance trials were conducted on 6 cows, and 1 balance trial was conducted on 1 cow. During the balance trials, cows were placed in metabolism stanchions, and their calves were placed in a pen (1.8 x 2.1 m) next to the stanchion. Calves had full access to cows during the collection period, which allowed nursing. Calves did not have access to alternative water. Total fecal and urine collections were conducted over 96 h on the cows, as previously described by Freetly and Nienaber (1998).
On the morning of the calorimetry measurement, each cow’s head was placed in a portable respiration box. The daily meal was provided and the box was closed. Flow through the box was allowed to stabilize before O2, CO2, and CH4 exchanges were determined for the next 23 h. The portable respiration box was 0.76 x 0.76 x 1.78 m and was constructed with an aluminum frame and covered with 5-mm, clear acrylic sheets. The box had a 28- x 117-cm opening fitted with a vinyl hood that attached around the animal’s neck to provide a seal between the box and animal. The bottom of the box was constructed with a hopper for feeding, and boxes were plumbed with a water bowl. Oxygen, CO2, and CH4 exchange were determined by pulling air through the box across a temperature-compensated, dry test meter to determine airflow exiting the box. Real-time air temperature and humidity were determined with a Pace Temperature/Relative Humidity Sensor (TRH-100) attached to a Pace Data Logger (XR440, Pace Scientific, Mooresville, NC). Proportional samples of air entering the box and exhausted from the box were collected into gas bags to give a composite air sample for the collection period for each box. Gas bags were constructed of a polyethylene-aluminum-Mylar laminate. System recoveries of O2 and CO2 were routinely determined by using combustion of ethanol within the box.
Air samples were analyzed for O2, CO2, and CH4 as described by Nienaber and Maddy (1985). Measurements from the 23-h period were adjusted to daily measurements by multiplying by 24/23. Heat production was calculated from indirect calorimetry using the equation described by Brouwer (1965). Energy and N analyses were conducted as described by Freetly and Nienaber (1998).
Milk yields were estimated during the 96-h balance study using D2O dilution. Calves were weighed at the beginning of the balance trial and at the end of the balance trial. Based on initial weight, calves were gavaged with 1.3 g of D2O/kg of BW and separated from the cow for 4 h to allow the D2O to equilibrate. The 4-h equilibration was based on time-course data demonstrating that equilibrium was reached within 4 h after gavage (data not shown). A 5-mL blood sample was collected, and the calf was returned to the cow at the beginning of the balance trial. At the end of the 96-h balance trial, calves were removed from cows for 4 h, and a 5-mL blood sample was collected by jugular puncture. At the end of the collection period, cow-calf pairs were returned to their group pens.
Three days after the energy and N balance collection, calves were removed from the cows, and the cows were injected with oxytocin. Cows were milked completely by machine and were kept separated from their calves for 6 h. At the end of the 6-h period, cows were injected with oxytocin, milked completely by machine, and a bucket sample (100 mL) was collected for the determination of milk DM and milk energy. Milk samples were freeze-dried, and energy content was determined by bomb calorimetry (AOAC, 1984). An additional bucket sample (50 mL) was collected and analyzed for protein, fat, and lactose by infrared spectrophotometer (B-2000, Bentley Instruments, Chaska, MN).
Blood D2O concentrations were determined with a gas chromatograph-mass spectrometer using the method of Previs et al. (1996), with modifications. Enriched acetylene was produced by combining 50 µL of sample with 100 mg of ground calcium carbide in a gas chromatograph sample vial. The sample vial headspace was sampled on a gas chromatograph (model 6890, Agilent Technologies, Palo Alto, CA). The inlet temperature was 250°C. The carrier gas was helium at 1.073 kg/cm2. The inlet had a split ratio of 40:1, with a split flow of 80 mL/min. Samples were injected on an Agilent column (Model HP-5MS, 30 m x 0.25 mm x 0.25 µm; 5% methylpoly siloxane). Oven temperature was set at 30°C. Mass 26 and 27 ions were selectively monitored on an Agilent 5973N mass spectrometer.
Milk Calculations
Milk yield was calculated using the algorithm reported by Oftedal et al. (1987):
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where total water intake (TWI) = 18 x {[k x (Sn + S0)/ 2] + (Sn – S0)}, and where k is the daily turnover rate of the label, S0 is the initial pool size (moles) of water, and Sn is the pool size (moles) of water at time n. The denominator consisted of milk constituents expressed as percentages (water, fat, protein, and lactose). The variables k, S0, and Sn were estimated as follows:
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where D0 is the amount of label (moles) at time zero, Dn is the amount of label (moles) at time n, t is the time in days between time zero and time n, D is the amount of label with which the calf was dosed, [D0] is the concentration of label at time zero, [Dn] is the concentration of label at time n, W0 is the calf BW at time zero, and Wn is the calf BW at time n. Lactation energy (LE) was calculated as the product of daily milk yield times energy content of milk. In addition, we assumed that the composition of gain between birth and 3 months of age was 12% fat and 19% protein (Buckley et al., 1990), which we calculated using the following equations:
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Energy Balance Calculations
Energy balance was determined using the following equations:
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Metabolizable energy for maintenance (MEm) is the MEi at zero RE, and was calculated using the regression of RE on MEi (Figure 1). For cows in positive energy balance, LE that was derived from feed ME (LEME) was equal to LE, and for cows in negative TE balance, LEME was equal to LE plus TE x 0.84 (Moe et al., 1970). For cows in negative energy balance, ME available for LE (MELE) was equal to MERE, and for cows in positive energy balance, MELE was equal to MERE minus TE/0.726 (Moe et al., 1970).
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Nitrogen balance was determined using the following equations:
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Statistical Analysis
Maintenance requirement was calculated by regressing RE scaled for MBS on MEi scaled for MBS, and solving for MEi when RE equals zero (Figure 1
). A multiple regression analysis was used to determine the relationships between MERE and energy in milk and tissue, with the intercept set at zero (Table 1). Differences in regression estimates for animals in positive and negative energy balance were tested with the following model: MERE = status, LE, TE, status x LE, and status x TE, where status was a fixed effect. Subclass 1 was where TE < 0 and subclass 2 was where TE > 0. The relationship between LEME and MELE was determined by regression analyses with the intercept fixed at zero (Figure 2). Tissue energy recovered in protein was regressed on total TE recovered to establish the relationship between the 2 (Figure 3). Reported SE are those associated with the estimates of the regression coefficients and intercepts.
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RESULTS AND DISCUSSION |
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONLITERATURE CITED |
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). The proportion of intake energy recovered increased with increased intake energy (Figure 4).
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). Forty-seven of the observations were collected while cows were losing TE, and 20 observations were collected while cows were gaining TE. Regression analyses (Figure 1) estimated zero RE at intakes of 146 kcal of ME/MBS. Coefficients for the contribution of LE and TE to MERE did not differ with TE status (Table 1).
Although considerable data on energetics of lactating dairy cows have been reported, less data have been reported for lactating beef cows. The lack of information in the beef cow has been partially due to the difficulty of estimating milk production in a cow that is suckled by a calf. A number of approaches have been used to measure milk production in beef cattle. Machine milking gives a measure of the milk capacity but does not represent the actual milk consumed by a calf. Accurate measures of milk production with machine milking require animals to be trained. Machine milking during balance trials deprives the calf of milk that would normally be consumed. Estimating milk consumption with weigh-suckle-weigh methods in young calves is complicated by the ability of the calf to consume the milk stored in the mammary gland in a single meal and the effect of length of separation between weights (Williams et al., 1979). Both machine milking and weigh-suckle-weigh methods change the normal suckling behavior; therefore, we used deuterium-enriched water to estimate milk consumption to allow the calf to express normal suckling behavior during the balance trials.
Flatt et al. (1967) reported that the efficiency of ME conversion into RE (LE + TE) ranged from 64% for cows on diets that were 60% alfalfa hay + 40% concentrate to 67% for cows on diets that were 20% alfalfa hay + 80% concentrate. When Flatt et al. (1967) pooled their data, their efficiency estimate was 66%, and zero RE was predicted at intakes of 140 kcal of ME/MBS. Even though milk yield in our study was less than half of that of Flatt et al. (1967), our estimate of MEi at zero RE (146 kcal of ME/MBS; Figure 1) was consistent with their observations.
In our study, RE is the sum of TE gain or loss and LE. Establishing the efficiency of ME use for TE and LE historically has been accomplished through regression analyses (Flatt et al., 1967; Van Es, 1975; Vermorel et al., 1982). A number of different regression models have been used (Moe et al., 1971) to estimate efficiency of ME use. The models consist of multi-regression analyses. The equations either predict ME use as a function of LE and TE; ME use as a function of body size, LE, and TE (gain/loss); or LE as a function of MEi and TE. In all cases, the equations result in a set of coefficients used to estimate ME use and an intercept. The coefficients are not independent of the intercept in these models. In our study, we eliminated the intercept by subtracting the MEm and forcing the regression of MERE on LE and TE through zero. The regression was forced through zero to account for a RE of zero when no MERE was provided. Based on the inverse of the coefficients for the zero-intercept regression, we estimated that the ME use for LE was 72%.
An alternative approach to calculating efficiency of MEi use for LE is to regress LEME on MELE (Figure 2). The intercept was set to zero to account for zero LE being synthesized from ME when zero ME is available for synthesis. The resulting efficiency was 73%, which is consistent with the 72% observed using the multiple regression approach. Our estimates of efficiency of ME conversion to LE are greater than the 60 to 67% reported in dairy cows (Yan et al., 1997). However, our greater efficiency estimate is consistent with the metabolic stoichometeric arguments of Baldwin (1968). Using Baldwin’s (1968) estimates of efficiency for synthesis of milk constituents and the chemical composition of our milk, we estimated that the efficiency of ME to LE would be 78%.
The greater estimates of efficiency of ME use for LE and TE in our study, compared with those of Moe et al. (1970) and Patle and Mudgal (1977), are most likely due to our greater estimate of maintenance. Mature nonpregnant, nonlactating cows of the same breed type (Freetly and Nienaber, 1998) have a lower maintenance requirement (134 kcal of ME/MBS) than was calculated with regression analyses in our study. When this lower value for maintenance was used to calculate MERE from MEi, which was then regressed on LE and TE, the estimate for efficiency of ME to LE (63%) was closer to those reported by Moe et al. (1970) and Patle and Mudgal (1977). The discrepancy in estimates of the maintenance requirement is primarily a result of whether the increased energy expenditure associated with tissues that support lactation is assigned to the maintenance or the efficiency estimate. For example, liver metabolism increases during lactation and increases with increased milk production (Freetly and Ferrell, 1997). When the value generated in the nonpregnant, nonlactating cow is used as the estimate for maintenance, then the increase in hepatic metabolism is accounted for in the efficiency estimate, resulting in a lower efficiency estimate than if the value for maintenance is taken from the regression equation. When the regression equation is used, part of the increase in hepatic metabolism is accounted for in the maintenance value, resulting in an increased value for efficiency.
Estimates of efficiency for ME conversion to TE vary. Patle and Mudgal (1977) reported an efficiency of 65% and Moe et al. (1970) reported an efficiency of 75%. Geay (1984) developed an equation to predict efficiency of TE gain based on the fraction of the energy gain that is protein. In our study, we estimated that 13% of the TE gain was protein (Figure 3). Using Geay’s (1984) equation, we would predict that our efficiency would be 56%; however, in the study of Geay (1984), energetic efficiency of fat deposition was 75% and that of protein deposition was 20%. Using those reported efficiencies and weighting the results based on observed caloric gains of fat (87%) and protein (13%; Figure 3), we would estimate an overall caloric efficiency of gain of 68%, which is near the 71% calculated in our study using the zero-intercept model for cows with positive TE. However, if we use the pooled value then our estimate (78%) is greater. When using the multiple regression approach as in Table 1, pooling data across negative and positive TE results in the inherit assumption that the efficiency of ME conversion to TE is the same as the efficiency of TE conversion to LE. Moe et al. (1970) reported that the efficiency of TE conversion to LE is 82%. Our study concurs that cows in negative TE balance have a high efficiency of conversion of TE to LE (88%; Table 1).
Our findings suggest that overall efficiency of energy retention in young beef cows is similar to that of dairy cows, even though their milk production is lower (Flatt et al., 1967). The efficiency of conversion of dietary ME to LE is greater than reported for dairy cows, but this is most likely due to differences in the way we accounted for maintenance. The efficiencies of conversion of dietary ME to LE and to TE were similar for cows that were in positive TE balance, and TE conversion to LE is highly efficient.
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Footnotes |
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2 Corresponding author: freetly{at}email.marc.usda.gov
Received for publication September 19, 2005. Accepted for publication March 17, 2006.
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LITERATURE CITED |
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONLITERATURE CITED |
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= cows in positive tissue energy balance, 
= cows in negative tissue energy balance. Recovered energy = 0 at 146 kcal/kg of BW0.75.
; 0.184 ± 0.020x + 93 ± 8, R2 = 0.55, n = 67), all scaled for metabolic body size (kcal/kg of BW0.75), on energy intake scaled for metabolic body size (x; kcal/kg of BW0.75).