J. Anim. Sci. 2002. 80:2759-2763
© 2002 American Society of Animal Science
Relationship between aging and nutritionally controlled growth rate on heat production of ewe lambs1
H. C. Freetly2,
J. A. Nienaber and
T. M. Brown-Brandl
USDA-ARS, U.S. Meat Animal Research Center, Clay Center, NE 68933
2 Correspondence:
P.O. Box 166 (phone: 402/762-4202; fax: 402/762-4209; E-mail:
freetly{at}email.marc.usda.gov).
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Abstract
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The hypothesis of this study is that ewes that have equal body weights, but differ in chronological age due to nutrient restriction, do not differ in metabolic rate. The objective of this study was to determine how reducing growth rate nutritionally alters the relationship between heat production per unit body weight and aging. Fasting heat production of 12 Dorset ewe lambs at 114 ± 2 d of age was determined, and ewes were assigned to treatments. Treatments consisted of two different feeding levels of the same diet (ME = 2.5 Mcal/kg DM and 16.6% CP). The High treatment was offered 4.5% of their weekly BW per day, and the Low treatment was offered 2.5% of their weekly BW per day. Each treatment consisted of six animals that remained within treatment for the remainder of the study. Indirect-calorimetry measurements were repeated every 6 wk. Treatments differed in both the linear and quadratic term for fasted BW on age (P < 0.001). The rate of BW gain decreased as ewes aged in the High treatment, and the rate of BW gain increased as ewes aged in the Low treatment. The heat production:BW (HP:BW) ratio decreased in the High treatment as ewes aged and was described well by a previously reported prediction equation, but the ratio in the Low treatment was not described by this same equation. Describing the HP:BW ratio on age response with treatment-specific decay functions fit the data better than the pooled treatment function (P < 0.001). The HP:BW ratio decreased rapidly in the Low treatment following feed restriction, but remained elevated compared to the High treatment as animals aged. After excluding the initial measurements in the Low treatment that were taken before nutritional treatments were imposed, the HP:BW ratio was best described by a linear decrease. In conclusion, this study suggests that a previous model taking into account proportion of mature body size is a reasonable predictor for heat production across breeds of sheep growing in nutritionally adequate environments; however, it cannot be extended to sheep that are proportionally smaller in their mature BW due to nutritional restriction.
Key Words: Energy • Growth • Maintenance • Sheep
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Introduction
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Current models used to estimate the energy required by growing lambs typically apportion energy between that required for maintenance and that required for growth (NRC, 1985; CSIRO, 1990; AFRC, 1993). Most models predict maintenance energy requirements solely (NRC, 1985) or partially (CSIRO, 1990; AFRC, 1993) as a function of BW. However, a number of factors have been demonstrated to modify the relationship between resting heat production (HP) and BW. Freetly et al. (2002) reported that this ratio is modified by the animal’s age and that effects of aging differ with breed type. Graham (1964) and Graham et al. (1974) reported that, in general, as adequacy of previous nutrition decreases, so does the fasted metabolic rate. The extent to which aging affects fasting metabolic rate is modified by plane of nutrition has been less extensively studied. Lamb production in the United States occurs over very diverse management environments, and it is not uncommon to have lambs of equal weight, but of different ages, resulting from differences in nutrient availability. Treating these lambs equally and estimating metabolic rate based on BW will result in biased estimates for the older lamb. The objective of this study was to determine how nutritionally reducing growth rate alters the relationship between heat production per unit body weight and aging.
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Materials and Methods
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Twelve Dorset ewe lambs were weaned when they were 67 ± 2 d of age and weighed 22 ± 1 kg. Eleven days after weaning, lambs were individually penned (1.44 m2). Lambs were fed a pelleted diet that consisted of 50.8% alfalfa hay, 39.1% corn, 9.9% soybean meal, and 0.2% sodium chloride as a percent of dry matter (DM = 91%). On a DM basis, the diet had a calculated ME of 2.5 Mcal/kg and a CP of 16.6% (United States–Canadian Tables of Feed Composition, 1982). Lambs had continuous access to water. Lambs were weighed weekly and feed offered was adjusted weekly based on their full BW. Lambs were offered 4.5% of their BW of the above diet on an as-fed basis. Fresh feed was offered daily, and daily feed refusals were measured.
When lambs were 114 ± 2 d of age, fasting HP was determined by indirect calorimetry. Fifty-five hours before the beginning of calorimetry, feed was removed and withheld until the end of calorimetry. Ewes were weighed and transferred to calorimeter chambers 7 h before HP was recorded. Individual ewe gaseous exchange was determined from 55 through 71 h of feed removal. Measurement of gaseous exchange and calculation of HP were performed as previously described by Nienaber and Maddy (1985). Chamber temperature was 21°C, and relative humidity was 54%. Reported BW is the average of BW taken at 48 and 71 h of feed removal. Ewes had 6-wk fleeces at the first calorimetry measurement, and ewes were sheared each time they were removed from the calorimeter so that they would have a 6-wk fleece at each measurement.
After the first calorimetry measurement, ewes were randomly assigned to one of two treatments. Treatments consisted of two different feeding levels of the same diet. The High treatment animals continued to be offered 4.5% of their weekly BW, and the Low treatment animals were offered 2.5% of their weekly BW. Each treatment consisted of six animals, and animals remained within treatment for the remainder of the study. Calorimetry measurements were repeated every 6 wk for an additional eight sample periods. Experimental procedures were conducted in accordance with the U.S. Meat Animal Research Center Animal Care Guidelines and the Guide for the Care and Use of Agricultural Animals in Agricultural Research and Teaching (FASS, 1999).
The relationship between BW and age was fitted with a quadratic equation. Treatment differences for BW on age were tested with a split-plot model. The model consisted of treatment, animal nested in treatment, age2, age, age2 x treatment, and age x treatment. Treatment was tested using the animal nested in treatment as the error term. Data were analyzed using the GLM procedure in SAS v. 6.1 (SAS Inst. Inc., Cary, NC).
The relationship between HP/BW and age was fitted to a decay function: HP/BW = f (t) = A + Bekt, where t is age in weeks. Parameters for the decay equation were estimated using the Gauss-Newton method to fit the nonlinear regression model. The nonlinear procedures in SAS v. 6.1 were used to calculate estimates. Treatment differences between the nonlinear models were tested using a calculated F-ratio (Freetly et al., 1995b) to test whether estimation of parameters specific for each treatment significantly improved fit of the data relative to estimation of parameters from a pooled data set, ignoring treatment.
The relationship between HP per unit BW and proportion mature BW was tested with the equation developed by Freetly et al. (2002):
where BW is equal to fasted BW and matBW is equal to mature fasted BW. Mature fasted BW was estimated as 90% of average mature BW of the ewe flock from which lambs were obtained.
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Results
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Due to equipment failure during Period 8, no data was collected. One animal in the High treatment was sick during Period 9 and was not sampled.
For the High treatment, average daily feed consumption during the 7 d before feed restriction was started as follows: Period 1, 1430 ± 56 g; Period 2, 1689 ± 89 g; Period 3, 1985 ± 93 g; Period 4, 2057 ± 89 g; Period 5, 2156 ± 103 g; Period 6, 1937 ± 102 g; Period 7, 1940 ± 112 g; Period 9, 1540 ± 110 g; and for the Low treatment was as follows: Period 1, 1363 ± 63 g; Period 2, 869 ± 20 g; Period 3, 929 ± 16 g; Period 4, 1018 ± 12 g; Period 5, 1127 ± 26 g; Period 6, 1246 ± 26 g; Period 7, 1354 ± 38 g; Period 9, 1739 ± 52 g.
Treatments differed in both the linear and quadratic term for fasted BW on age (P < 0.001). The rate of BW gain decreased as ewes aged in the High treatment (f (t) = -0.02686t + 2.262; t = wk), and the rate of BW gain increased as ewes aged in the Low treatment (f (t) = 0.01062t + 0.256; Figure 1
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Heat production per unit BW decreased in both treatments as ewes aged (Figure 2). Describing the data with treatment specific decay functions for HP:BW ratios on age fit the data better than the pooled function (P < 0.001; Figure 2). After excluding the initial measurements in the Low treatment, which were taken before nutritional treatments were imposed, the HP:BW ratio was best described by a linear decrease (Figure 2b). Heat production decreased rapidly following feed restriction in the Low treatment. This rapid decrease in heat production was followed by a slower rate of decline.
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Figure 2. Relationship between daily fasting heat production (kcal/kg) and age (wk). Dorset ewes fed 4.5% of BW: f(t) = 17.4(± 2.3) + 42.9(± 3.8)e-0.03918t(a); and (b) Dorset ewes fed 2.5% (with prerestricted data); [; f(t) = 27.1 (± 0.6) + 164.0(± 92.4)e-0.15428(±0.03491)t]; and without prerestricted data [- - -; f(t) = 34.0 (± 1.06) - 0.135(± 0.24)t], where t is equal to weeks of age.
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Equation [1] reasonably estimated HP in the High treatment (Figure 3a), but the equation tended to underpredict HP at the oldest age, when predicted HP was at its lowest (Figure 3b). This same equation consistently overpredicted HP of ewes on the Low treatment after the feed restriction was imposed and before they reached their mature BW (Figure 4).
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Figure 3. Relationship between the ratio of daily heat production:BW and proportion of mature BW of Dorset ewes fed 4.5% of their BW and the residuals resulting from the following function used to predict heat production: Heat production (kcal/kg•d-1) = f(BW, MatBW) = 59.5e-0.797(BW/MatBW), where BW is equal to fasted BW and MatBW is equal to mature fasted BW (Freetly et al., 2002).
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Figure 4. Relationship between the ratio of daily heat production:BW and proportion of mature BW of Dorset ewes fed 2.5% of their BW and the residuals resulting from the following function used to predict heat production: Heat production (kcal/kg•d-1) = f(BW, MatBW) = 59.5e-0.797(BW/MatBW), where BW is equal to fasted BW and MatBW is equal to mature fasted BW (Freetly et al., 2002).
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Discussion
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We previously reported (Freetly et al., 2002) that HP decreased as lambs aged and that the rates of decrease differ between breeds of ewes. In that study, we found that breed differences could be accounted for by differences in the rate of maturation, and a single equation was developed for the prediction of fasted HP (Eq. [1]). In the current study, lambs on the High treatment exhibited a normal pattern of growth, while BW increased as the animals aged and the rate of increase in BW slowed as the ewes aged. As in the previous study (Freetly et al., 2002), HP per unit of BW in the High treatment decreased exponentially as the animals aged. A residual analysis of the Dorset ewe data using predicted values generated by the equation developed with Suffolk, Texel, Finnsheep, and Rambouillet ewes indicates that the multibreed equation is sufficiently robust to describe HP in Dorsets. Residuals were evenly distributed across predicted HP, except in the lower predicted HP which occurred in animals that had exceeded their estimated mature BW. Equation [1] tended to underpredict HP in ewes that had exceeded estimated mature BW. This underprediction of HP may be partially due to the relative body composition of the animals at this age. In our earlier study, sheep were maintained in their normal production environments before measurements were taken. In the current study, ewes were maintained in metabolism cages, and by Period 9, visual appraisal would suggest that they had a higher proportion of body fat than animals of similar ages in the previous study (Freetly et al., 2002). While ewes in the High treatment exceeded mature BW, Eq. [1] provided a reasonable estimate of HP until they were in excessive body condition.
As sheep of different breeds mature at different rates (Freetly et al., 2002), two sheep of the same age, but of different breeds, may have achieved different fractions of their mature BW. Equation [1] was developed to take into account these discrepancies in rates of maturation. Level of nutrition also can alter the proportion of mature weight obtained at a given chronological age. In the case of breed differences, ewes exhibited normal growth patterns; however, in the case of chronic nutrient restriction, growth rate is retarded. Following feed restriction in the Low treatment and before ewes reached their mature BW, Eq. [1] overpredicted HP. During this period of depressed HP, feed intake as a percent of BW was lower for ewes in the Low treatment at any given level of maturity. The rapid decrease in HP following feed restriction in the Low treatment probably represents the acute adaptation to reduced feed intake. Previous studies in lambs have demonstrated that oxygen consumption of splanchnic tissues decreases rapidly following feed restriction and that splanchnic tissue can account for approximately 50% of the whole-body oxygen consumption (Burrin et al., 1989; Freetly et al., 1995a). Six weeks following feed restriction, HP in the Low treatment was 8% less than that of the High treatment. Thus, a 16% decrease in splanchnic tissue metabolism would be required, which is consistent with the 30% decrease observed by Burrin et al. (1989) after 21 d of feed restriction, and the 30% decrease observed by Freetly et al. (1995a) after 38 d of feed restriction, suggesting that a decrease in splanchnic tissue energy expenditure could account for the rapid decrease observed in the Low treatment. In addition to reductions in visceral energy expenditure, chronic feed restriction of growing animals decreases protein turnover in peripheral tissues (Savary et al., 2001). The decrease in energy expenditure per unit of BW in nutritionally restricted sheep suggests that as growth is slowed, the metabolic rate of tissues is decreased. This decrease in metabolic rate leads to a lower HP per unit BW and results in an overprediction of HP in feed-restricted lambs, as was observed in this study.
In growing ewes with adequate nutrition, the rate of growth decreases as ewes age. Subsequently, there is a high correlation between rate of growth and level of maturity. Equation [1] describes this relationship with a decay function; however, when the correlation between growth rate and level of maturity is disassociated, the ability of Eq. [1] to predict HP is diminished. In the current study, as ewes in the Low treatment approached mature weight, the relationship between HP and level of maturity followed a pattern similar to that of the High treatment, suggesting that HP per unit BW becomes constant at maturity, regardless of the pattern of growth.
Several functions have been proposed to account for the decrease in HP per unit BW with aging. Based on indirect-calorimetry measurements, Brody (1945) presented a decay function (HP/BW = f(t) = A + Be-kt) to describe the rate of decrease, and proposed separate functions for males and females. Residual analyses of Brody’s equations for females suggest that his function was a reasonable predictor of HP in the current study; however, in our previous study (Freetly et al., 2002), there were residual biases. Rattray et al. (1973) estimated fasting HP using comparative slaughter techniques and described HP with an allometric equation where BW was scaled to the interspecies coefficient of 0.75 (HP = f(BW) = A(BW)0.75). The function of Rattray et al. (1973) consistently overpredicts HP in the current data set. In our previous study (Freetly et al., 2002), and in this study, solving for A in the allometric equation with the coefficient fixed at 0.75 resulted in biased residuals when the functions were used to predict HP. Using indirect-calorimetry data, Graham et al. (1974) proposed a function to predict HP that accounted for age with a log decay function and the function included adjustments for BW, feeding level, and growth rate (HP = f(BW, yr, Gain, DE) = A(BW0.75)e-kyr + B(Gain) + C(DE)). The function of Graham et al. (1974) predicts that HP per unit BW raised to the 3/4 power decays as the animal ages, and that there is a linear additive effect due to rate of gain and feed level. Implicit in this equation is that the reduction in HP due to aging continues regardless of the rate of growth or feed level. When this function is used to predict HP in the current data set, there is a bias in the residuals such that HP is overpredicted as predicted HP increases. Differences in the two studies partially may be due to differences in levels of feed restriction and refeeding. In the current study, both treatments were allowed to grow continuously, but in the studies of Graham et al. (1974), some of the lambs were held at constant weights for 4 or 6 mo, followed by periods of refeeding.
In conclusion, this study suggests that a model taking into account proportion of mature body size is a reasonable predictor for HP across breeds of sheep growing in nutritionally adequate environments; however, it cannot be extended to sheep that are a lower proportion of their mature BW due to nutritional restriction. Our finding, coupled with the earlier observations of Graham et al. (1974), suggests that development of a robust predictor of HP in sheep will need to include provisions for breed type by aging interactions as well as indicators of current rates of growth and dietary history.
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Implications
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This study suggests that relying solely on BW to predict metabolic rate of growing sheep will result in erroneous estimates. The amount of heat produced per unit of body weight is influenced by the sheep’s age, genetics, and previous nutritional history. The accuracy of predicting HP across different breeds can be improved by considering the fraction of mature BW they have obtained; however, if growth patterns are altered through nutrient restriction, the ability to predict HP based on the ewe’s weight relative to mature weight is no longer valid with our current mathematical functions. Use of mathematical functions that assume normal growth in nutritionally growth-retarded ewes may result in inaccurate estimates of metabolic rate and subsequent errors in predicting the ability of ewes to grow on a given feed resource.
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Footnotes
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1 The authors would like to acknowledge the technical support of Chris Haussler and Dan Marintzer. 
Received for publication April 15, 2002.
Accepted for publication June 24, 2002.
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Literature Cited
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AFRC. 1993. Energy and protein requirements of ruminants. An advisory manual prepared by the AFRC Technical Committee on Responses to Nutrients. CAB International, Wallingford, UK.
Brody, S. 1945. Bioenergetics and Growth. Reinhold Publishing, New York.
Burrin, D. G., C. L. Ferrell, J. H. Eisemann, R. A. Britton, and J. A. Nienaber. 1989. Effect of level of nutrition on splanchnic blood flow and oxygen consumption in sheep. Br. J. Nutr. 62:23–34.[Medline]
CSIRO. 1990. Ruminants. Feeding Standards for Australian Livestock. CSIRO Publications, Melbourne, Australia.
FASS. 1999. Guide for the Care and Use of Agricultural Animals in Agricultural Research and Teaching. 1st ed. Fed. Anim. Sci. Soc., Savoy, IL.
Freetly, H. C., C. L. Ferrell, T. G. Jenkins, and A. L. Goetsch. 1995a. Visceral oxygen consumption during chronic feed restriction and realimentation in sheep. J. Anim. Sci. 73:843–852.[Abstract]
Freetly, H. C., J. A. Nienaber, and T. Brown-Brandl. 2002. Relationships among heat production, body weight, and age in Finnsheep and Rambouillet ewes. J. Anim. Sci. 80:825–832.[Abstract/Free Full Text]
Freetly, H. C., J. A. Nienaber, K. A. Leymaster, and T. G. Jenkins. 1995b. Relationships among heat production, body weight, and age in Suffolk and Texel ewes. J. Anim. Sci. 73:1030–1037.[Abstract]
Graham, N. M. 1964. Energetic efficiency of fattening sheep. II. Effects of undernutrition. Aust. J. Agric. Res. 15:113–126.
Graham, N. M., T. W. Searle, and D. A. Griffiths. 1974. Basal metabolic rate in lambs and young sheep. Aust. J. Agric. Res. 25:957–971.
Nienaber, J. A., and A. L. Maddy. 1985. Temperature controlled multiple chamber indirect calorimeter-design and operation. Trans. Am. Soc. Agric. Eng. 28:555–560.
NRC. 1985. Nutrient Requirements of Sheep. 6th. ed. Nat. Acad. Press, Washington, DC.
Rattray, P. V., W. N. Garrett, N. E. East, and N. Hinman. 1973. Net energy requirements of ewe lambs for maintenance, gain and pregnancy and net energy values of feedstuffs for lambs. J. Anim. Sci. 37:853–857.[Abstract/Free Full Text]
Savary, I. C., S. O. Hoskin, N. Dennison, and G. E. Lobley. 2001. Lysine metabolism across the hindquarters of sheep: Effect of intake on transfers from plasma and red blood cells. Br. J. Nutr. 85:565–573.[Medline]
United States–Canadian Tables of Feed Composition 3rd ed. 1982. National Academy Press, Washington, DC.
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Abstract
Introduction
, —) of their BW: f(t) = -0.01343(± 0.0030)t2 + 2.262(± 0.2624)t - 4.012(± 5.132; R2 = 0.91); or fed 2.5% (
, - - -) of their BW: f(t) = 0.00531(± 0.00108)t2 + 0.256(± 0.096)t + 24.43(± 1.89; R2 = 0.97), where t is equal to weeks of age.