Total Metabolizable Energy Requirements Research Articles

PSII-19 A Satellite-Based Decision-Support Tool to Optimize Profitability and Environmental Stewardship of Cow-Calf Operations

Abstract Management strategies implemented in cow-calf operations are essential to capitalize on heavier feeder calves, but field data collection can restrict their practical application. A computer program to optimize weaning weight, profitability, and environmental sustainability of grazing cow-calf operations was developed by combining an existing mechanistic nutrition model with pasture biomass estimated using satellite images. Data from the Texas A&M University McGregor Research Center was used as a testbed for the computer model. Pasture forage data from 2016 to 2022 was collected via satellite imagery, and the predicted pasture biomass was adjusted based on third-party algorithms (Sigfarm Intelligence LLC). Nutrient requirements were estimated using the Ruminant Nutrition System (RNS), and a constant pasture nutritive value was obtained. The potential dry matter intake (DMI) was estimated from the initial herd size (input). Actual average weaning weight was used to predict cow milk yield and total metabolizable energy requirements, which was used to estimate the required DMI (DMR). The actual forage allowance was correlated to DMR, and we simulated the forage allowance to support increased DMI and, consequently, increased milk yield to achieve a targeted average weaning weight. Based on Intergovernmental Panel on Climate Change (IPCC) methodology, environmental impact was evaluated from greenhouse gas emission estimates. Our simulations demonstrated that increasing the target weaning weight of a smaller herd with fewer mature cows (757 cows) would produce heavier calves, resulting in a 6% decrease in calf gain per pasture area. Nevertheless, the net income increased in scenarios with heavier weaned calves. The enhanced productivity with heavier weaned calves and fewer cows resulted in a lower carbon emission per kg of produced weaned calves. Based on our simulation, a herd size between 757 and 812 mature cows yielding calves weaned between 280 and 300 kg would be an optimal management strategy for the studied operation. This computer model provides the fundamental framework for a decision-support tool for producers to optimize their cow-calf operations while producing ideal weaned calves for feedlot operations.

Maternal nutrient restriction in mid-to-late gestation influences fetal mRNA expression in muscle tissues in beef cattle

BackgroundManipulating maternal nutrition during specific periods of gestation can result in re-programming of fetal and post-natal development. In this experiment we investigated how a feed restriction of 85% compared with 140% of total metabolizable energy requirements, fed to cows during mid-to-late gestation, influences phenotypic development of fetuses and mRNA expression of growth (Insulin-Like Growth Factor family and Insulin Receptor (INSR)), myogenic (Myogenic Differentiation 1 (MYOD1), Myogenin (MYOG), Myocyte Enhancer Factor 2A (MEF2A), Serum Response Factor (SRF)) and adipogenic (Peroxisome Proliferator Activated Receptor Gamma (PPARG)) genes in fetal longissimus dorsi (LD) and semitendinosus (ST) muscle. DNA methylation of imprinted genes, Insulin Like Growth Factor 2 (IGF2) and Insulin Like Growth Factor 2 Receptor (IGF2R), and micro RNA (miRNA) expression, were also examined as potential consequences of poor maternal nutrition, but also potential regulators of altered gene expression patterns.ResultsWhile the nutrient restriction impacted dam body weight, no differences were observed in phenotypic fetal measurements (weight, crown-rump length, or thorax circumference). Interestingly, LD and ST muscles responded differently to the differential pre-natal nutrient levels. While LD muscle of restricted fetal calves had greater mRNA abundances for Insulin Like Growth Factor 1 and its receptor (IGF1 and IGF1R), IGF2R, INSR, MYOD1, MYOG, and PPARG, no significant differences were observed for gene expression in ST muscle. Similarly, feed restriction had a greater impact on the methylation level of IGF2 Differentially Methylated Region 2 (DMR2) in LD muscle as compared to ST muscle between treatment groups. A negative correlation existed between IGF2 mRNA expression and IGF2 DMR2 methylation level in both LD and ST muscles. Differential expression of miRNAs 1 and 133a were also detected in LD muscle.ConclusionsOur data suggests that a nutrient restriction of 85% as compared to 140% of total metabolizable energy requirements during the 2nd half of gestation can alter the expression of growth, myogenic and adipogenic genes in fetal muscle without apparent differences in fetal phenotype. It also appears that the impact of feed restriction varies between muscles suggesting a priority for nutrient partitioning depending on muscle function and/or fiber composition. Differences in the methylation level in IGF2, a well-known imprinted gene, as well as differences in miRNA expression, may be functional mechanisms that precede the differences in gene expression observed, and could lead to trans-generational epigenetic programming.

Energy and lysine requirements and balances of sows during transition and lactation: A factorial approach

This study aimed to quantify daily requirements for metabolizable energy (ME) and standard ileal digestible (SID) lysine in late gestating and lactating sows using a factorial approach. Metabolizable energy and SID lysine required for fetal and mammary growth, colostrum and milk production, uterine components (including uterus wall, placenta and membrane fluids) and maintenance were estimated. It was estimated that maintenance, additional heat loss, colostrum production, fetal growth, mammary growth and uterine components accounted for 66.8%, 19.3%, 7.2%, 5.0%, 1.3% and 0.5% of total ME requirements, respectively, in the last 12 days of gestation. Oxidation/transamination, fetal growth, mammary growth, colostrum production, maintenance and uterine components were estimated to account for 29.5%, 22.7%, 16.8%, 16.1%, 10.4% and 4.5% of total SID lysine requirements, respectively, in the last 12 days of gestation. After parturition, ME and SID lysine requirements increased daily until peak lactation (day 17). At peak lactation, 95% and 72% of total required SID lysine and ME, respectively, were associated with milk production (including oxidation). Relative to day 104 of gestation, ME and SID lysine requirements increased by 60% and 149% at day 115 of gestation, and by 228%, and 338% at peak lactation, respectively. A dietary SID lysine: ME ratio of 0.55 was estimated as the ideal ratio to optimize milk production at peak lactation. It was estimated that the regressing uterus released about 30g SID lysine and 14MJ ME into the endogenous plasma pool during lactation. It was concluded that dramatic changes in energy and lysine requirements and balances occur during transition and lactation; that sows with high milk yield and /or low live weight require high SID lysine: ME ratio and that it would be beneficial to feed sows with two components at each meal to match the daily requirements for maintenance and production.

Reduced fetal vitamin D status by maternal undernutrition during discrete gestational windows in sheep.

Placental transport of vitamin D and other nutrients (e.g. amino acids, fats and glucose) to the fetus is sensitive to maternal and fetal nutritional cues. We studied the effect of maternal calorific restriction on fetal vitamin D status and the placental expression of genes for nutrient transport [aromatic T-type amino acid transporter-1 (TAT-1); triglyceride hydrolase/lipoprotein uptake facilitator lipoprotein lipase (LPL)] and vitamin D homeostasis [CYP27B1; vitamin D receptor (VDR)], and their association with markers of fetal cardiovascular function and skeletal muscle growth. Pregnant sheep received 100% total metabolizable energy (ME) requirements (control), 40% total ME requirements peri-implantation [PI40, 1-31 days of gestation (dGA)] or 50% total ME requirements in late gestation (L, 104-127 dGA). Fetal, but not maternal, plasma 25-hydroxy-vitamin D (25OHD) concentration was lower in PI40 and L maternal undernutrition groups (P<0.01) compared with the control group at 0.86 gestation. PI40 group placental CYP27B1 messenger RNA (mRNA) levels were increased (P<0.05) compared with the control group. Across all groups, higher fetal plasma 25OHD concentration was associated with higher skeletal muscle myofibre and capillary density (P<0.05). In the placenta, higher VDR mRNA levels were associated with higher TAT-1 (P<0.05) and LPL (P<0.01) mRNA levels. In the PI40 maternal undernutrition group only, reduced fetal plasma 25OHD concentration may be mediated in part by altered placental CYP27B1. The association between placental mRNA levels of VDR and nutrient transport genes suggests a way in which the placenta may integrate nutritional cues in the face of maternal dietary challenges and alter fetal physiology.

Requirements for zero energy balance of nonlactating, pregnant dairy cows fed fresh autumn pasture are greater than currently estimated

Fifty-three nonlactating, pregnant Holstein-Friesian and Holstein-Friesian × Jersey cross dairy cows were grouped into 4 cohorts (n=15, 12, 13, and 13) and offered 1 of 3 allowances of fresh, cut pasture indoors for 38±2d (mean ± SD). Cows were released onto a bare paddock after their meal until the following morning. Animals were blocked by age (6±2yr), day of gestation (208±17d), and body weight (BW; 526±55kg). The 3 pasture allowances [low: 7.5kg of dry matter (DM), medium: 10.1kg of DM, or high: 12.4kg of DM/cow per day] were offered in individual stalls to determine the estimated DM and metabolizable energy (ME) intake required for zero energy balance. Individual cow DM intake was determined daily and body condition score was assessed once per week. Cow BW was recorded once per week in cohorts 1 and 2, and 3 times per week in cohorts 3 and 4. Low, medium, and high allowance treatments consumed 7.5, 9.4, and 10.6kg of DM/cow per day [standard error of the difference (SED)=0.26kg of DM], and BW gain, including the conceptus, was 0.2, 0.6, and 0.9kg/cow per day (SED=0.12kg), respectively. The ME content of the pasture was estimated from in vitro true digestibility and by near infrared spectroscopy. Total ME requirements for maintenance, pregnancy, and limited activity were 1.07MJ of ME/kg of measured metabolic BW per day. This is more than 45% greater than current recommendations. Differences may be due to an underestimation of ME requirements for maintenance or pregnancy, an overestimation of diet metabolizability, or a combination of these. Further research is necessary to determine the reasons for the greater ME requirements measured in the present study, but the results are important for on-farm decisions regarding feed allocation for nonlactating, pregnant dairy cows.

Potential contribution of Quercus durata and Adenostoma fasciculatum supplemented with Medicago sativa on intake and digestibility in sheep and goats

A digestion trial was conducted to estimate the potential contribution of the chaparral browse Quercus durata (leather oak) and Adenostoma fasciculatum (chamise) on intake and nutrient digestion in goats and sheep. Four wether Kiko goats (avg. wt. 22.9 kg) and four wether Targhee sheep (avg. wt. 39.6 kg) were housed in metabolism cages. Alfalfa pellets were used as a dietary supplement. Digestibility of the shrubs was measured by difference and metabolizable energy (ME) and ME intake (MEI) were calculated from digestibility and plants intake values. Oak and chamise had low crude protein (CP) content (6.5 and 4.3% DM) and high content of extractable condensed tannins (17.8 and 9.8% DM). In vitro organic matter digestibility (OMD) and ME for oak and chamise were 35.6 and 33.1% and 5.0 and 4.7 MJ/kg DM, respectively. In vivo OMD (calculated by difference with alfalfa) and MEI, for browse were greater (P < 0.01) for goats fed chamise compared to sheep. In oak, OMD was similar for both animal species but MEI was greater (P < 0.01) for goats. Greater (P < 0.05) DM and CP intakes (per kg BW0.75) were observed for goats fed either oak or chamise compared with sheep. When an energy source was given as supplement, the potential contribution of chamise and oak to total ME requirements for maintenance was about 58.3 and 60.4% in goats and about 17.1 and 32.9% in sheep, respectively. In consequence, metabolizable energy supplied by these shrubs may not be adequate for maintenance when, during grazing, they are consumed in high proportions with other chaparral plants that may have lower nutritional value than the alfalfa supplemented in this study. Under an appropriate supplementation program, goats could be more effective than sheep to control fuel load in California chaparral, as they consume more chamise and leather oak and obtain more nutrients from them.

The differential effects of the timing of maternal nutrient restriction in the ovine placenta on glucocorticoid sensitivity, uncoupling protein 2, peroxisome proliferator-activated receptor-γ and cell proliferation

Nutrient restriction (NR) during critical windows of pregnancy has differential effects on placento-fetal growth and development. Our study, therefore, investigated developmental and metabolic adaptations within the ovine placenta following NR at different critical windows during the first 110 days of gestation (term=147 days). Thus, the effects of NR on cell proliferation, glucocorticoid sensitivity, IGF1 and 2 receptor, peroxisome proliferator-activated receptor gamma (PPARG), and uncoupling protein (UCP)2 gene expression in the placenta were examined. Singleton bearing sheep (n=4-8 per group) were fed either 100% of their total metabolizable energy requirements throughout the study or 50% of this amount between 0-30, 31-65, 66-110, and 0-110 days gestation. A significant reduction in cell proliferation and increased gene expression for the glucocorticoid and IGF2 receptors, PPARG, and UCP2 were detected in placentae sampled from mothers who were nutrient restricted between days 66 and 110 of gestation, only, relative to controls. This window of gestation coincides with the maximum placental growth and the start of exponential growth of the fetus when there are substantially increased metabolic demands on the placenta compared with earlier in gestation. Consequently, increased glucocorticoid sensitivity and suppressed IGF2 action could contribute to a switch in the placenta from proliferation to differentiation, thereby improving its nutrient transfer capacity. Upregulation of PPARG and UCP2 would promote placental fatty acid metabolism thereby limiting glucose utilization. These compensatory placental responses may serve to maintain fetal growth but could result in adverse adaptations such as the early onset of the metabolic syndrome in later life.

Nutritional manipulation between early to mid-gestation: effects on uncoupling protein-2, glucocorticoid sensitivity, IGF-I receptor and cell proliferation but not apoptosis in the ovine placenta

In sheep, modest maternal nutrient restriction (NR) over the period of rapid placental growth restricts placentome growth and results in offspring in which glucocorticoid action is enhanced. Therefore, this study investigated the placental effects of early to mid-gestational NR on glucocorticoid receptor (GR), 11beta-hydroxysteroid dehydrogenase type 2 (11betaHSD2), uncoupling protein-2 (UCP2), and IGF type-I receptor (IGF-IR) mRNA abundance together with cell proliferation and apoptosis as determined histologically, and the mitochondrial proteins voltage-dependent anion channel and cytochrome c that are involved in apoptosis. Placenta was sampled at 80 and 140 days gestation (dGA; term ~147 dGA). NR was imposed between 28 and 80 days gestation when control and nutrient-restricted groups consumed 150 or 60% respectively of their total metabolizable energy requirements. All mothers were then fed to requirements up to term. Total fetal placentome weights were decreased by NR at 80 dGA but were heavier at 140 dGA following 60 days of nutritional rehabilitation. GR and UCP2 mRNA abundance increased whilst 11betaHSD2 mRNA decreased with gestational age. NR persistently up-regulated GR and UCP2 mRNA abundance. 11betaHSD2 mRNA was reduced by NR at 80 dGA but increased near to term. IGF-IRmRNA abundance was only decreased at 80 dGA. Placental apoptosis and mitochondrial protein abundance were unaffected by NR, whereas cell proliferation was markedly reduced. In conclusion, placental UCP2 and local glucocorticoid action are affected by the gestational nutritional status and may result in the offspring showing enhanced glucocorticoid sensitivity, thereby predisposing them to disease in later life.

Effects of maternal parity and late gestational nutrition on mRNA abundance for growth factors in the liver of postnatal sheep

The liver is a major metabolic and endocrine organ in growing neonates, but the extent to which its hormone receptor (R) sensitivity is potentially determined by maternal parity and the mother's nutritional environment is unknown. This was therefore investigated by sampling livers from postnatal sheep born to nulliparous or multiparous mothers. Offspring were sampled 1 or 30 days after birth from mothers consuming either 100 or 50% [i.e., nutrient-restricted (NR) group] of total metabolizable energy requirements from 110 days gestation to term ( approximately 147 days). Regardless of maternal diet, offspring of nulliparous mothers were lighter at birth and had smaller livers. By 1 mo of age, they exhibited catch-up growth, an adaptation not seen when mothers were NR, but they retained their lighter livers. At both sampling ages, livers from offspring born to nulliparous mothers exhibited increased mRNA abundance for growth hormone (GH) receptor, IGF-IR, plus hepatocyte growth factor (HGF); and at day 1 only IGF-I, but not IGF-IIR mRNA was decreased. In addition, mRNA for IGF-II, the HGFR, c-Met, and Bax were persistently reduced in these offspring. Effects of parity were largely unaffected by maternal nutrient restriction. Maternal parity therefore has a substantial effect on liver size during postnatal development and its receptor population that is not dependent on maternal diet. First-born offspring appear to exhibit a resetting of the endocrine control of hepatic growth within the HGF and GH-IGF axis, which could have later consequences after their growth has caught up.

Differential effects of fetal number and maternal nutrition in late gestation on prolactin receptor abundance and adipose tissue development in the neonatal lamb.

The present study examined the extent to which abundance of the prolactin receptor (PRLR) and a range of primary mitochondrial proteins are influenced by either maternal nutrition and/or fetal number in adipose tissue. Pregnant sheep were control fed [consuming 100% of total metabolizable energy (ME) requirements (taking into account requirements for both ewe maintenance and growth of the conceptus to produce a 4.5-kg lamb at term) for that stage of gestation] or were nutrient restricted (consuming 60% of total ME requirements). All ewes lambed normally at term and both perirenal adipose and hepatic tissues were sampled within 6 h of birth. Plasma membranes and mitochondria were prepared and analyzed using immunoblotting for abundance of PRLR and/or cytochrome c, voltage-dependent anion channel (VDAC), and uncoupling protein-1 (UCP1). Irrespective of maternal nutrition, abundance of specific isoforms of PRLR were significantly higher in adipose tissue sampled from twins compared with singletons and total UCP1 concentration per milligram of tissue was increased (p < 0.05). There was no effect of fetal number on PRLR abundance in the liver. Maternal nutrient restriction resulted in an increased abundance of both cytochrome c (p < 0.001) and VDAC in adipose tissue of twins but not singletons. This occurred despite maternal nutrition having no effect on either lamb body or adipose tissue weights, although both were lower (p < 0.05) in twins compared with singletons. In conclusion, fetal adipose tissue development is highly sensitive to nutritionally mediated changes in late gestation. An increase in fetal number results in greater PRLR abundance, which, in conjunction with a decrease in maternal nutrition, results in up-regulation of specific mitochondrial proteins.

Diet Energy Requirements of Growing Holsteins

The conceptual bases of recent NRC publications of nutrient requirements to meet dietary energy needs of growing Holsteins, emphasizing heifers gaining 0.75 kg/d, are described and compared with several world systems and with body composition data. Requirements of Holsteins for net energy for maintenance are generally thought to be higher (from 7 to 20%) than those of British beef breeds. Total estimates of metabolizable energy requirements are quite similar for most energy systems for cattle at 200 to 400kg of body weight. For heavier cattle, the energy density estimates based on beef breed equations project higher metabolizable energy for gain and total metabolizable energy requirements. Extrapolations from the chemical composition of growing Holstein heifers indicate that, at the slower growth that is typical of replacements, energy densities are lower than for more rapidly growing heifers and that net energy requirements are consistent with current US standards.

An Energy Model for Adult Female Caribou of the Denali Herd, Alaska

Estimates of the energy costs of resting, activity, and productive processes from the literature were applied to caribou activity budgets and movements documented during 1978-80 to estimate seasonal and annual energy requirements of adult female caribou (Rangifer tarandus granti) in Denali National Park and Preserve. Energy requirements were 45% lower in winter than summer, primarily because of a 21% reduction in resting metabolic rate and benefits of fat catabolism in winter, and high summer costs of lactation, fat deposition, and activity. Estimated energy requirements totaled 38.2 MJ . day-' during the insect season, 36.7 during summer migration, 34.2 during fall migration, 31.9 during prerut and rut, 29.3 during calving and postcalving, 23.7 during early and late winter migrations, 21.6 during early winter, 20.9 during late winter, and 20.5 during midwinter. Annual energy requirements, including costs of pregnancy (368.6 MJ) and lactation (560.3), totaled 9,870 MJ, excluding costs of cratering through snow and apparently minor costs of physiological stress. Compared to studies of food energy requirements of penned caribou, the model gives accurate predictions of total metabolizable energy requirements when allowances are made for the relative inactivity of penned animals. Modeling the energy requirements of an ungulate population is a prerequisite to estimating nutritional carrying capacities (Moen 1973, Robbins 1973, Wallmo et al. 1977, Mautz 1978). Measurements of the energy requirements of Rangifer have received emphasis in several studies (Makarova and Segal 1958, Hammel et al. 1961, Segal 1962, McEwan 1970, Luick et al. 1974, Young and McEwan 1975, White and Yousef 1978), but these measurements have not previously been used to model seasonal energy requirements of a caribou/ reindeer (Rangifer tarandus) population. Author is game research biologist, Alaska Department of Fish and Game, 1300 College Road, Fairbanks, Alaska 99701. At the time of the research, author was graduate student, Alaska Cooperative Wildlife Research Unit, University of Alaska, Fairbanks. Financial and logistical support was provided by the U.S. National Park Service (contract CX-9000-7-E080). Manuscript accepted May 28, 1985. Thomson (1977) attempted to model the year-round energy expenditure of wild reindeer in Norway using estimates of energy expenditure from the literature. However, Thomson's applications of energy costs differed in several ways from those presented here, making comparisons difficult. The most significant deviations included Thomson's exclusion of estimates for the calorigenic effect of food and his extremely high estimate for the energy cost of trotting. Gaare et al. (1975) also estimated seasonal energy requirements of wild reindeer in Norway. However, their estimates are about 45% of those reported here. Since they did not present energy-cost estimates of the various activities, it is impossible to account for the difference. White et al. (1975) estimated the summer energy requirements for caribou activity as simply 2 X basal metabolic rate (BMR). However, they used a winter-BMR value, so their summer energy estimates are substantially lower than presented here. I studied the Denali (formerly McKinley) caribou herd in Denali National Park and Preserve, Alaska during 1978-80. A model of the nutritional ecology of the herd was developed to assess the energetic/ nutritional status of this extremely reduced population (Boertje 1981). This paper demonstrates the applicability of measurements of energy costs by modeling metabolizable energy requirements of adult female caribou in the Denali herd.

The utilized metabolizable energy output of grassland on hill farms in Northern Ireland

AbstractForty hill farms in Northern Ireland were surveyed to obtain information on stocking rates and output and to identify factors affecting output. The mean farm area of 194 ha was composed of 155 ha hill land and 39 ha inbye land with a mean stocking rate of 0–7 cow‐equivalents ha−1.On the average farm most of the grassland (88%) was over 20 years old. The preferred species content of the swards averaged 22% over the whole farm and 42% in cut swards. This latter result may be attributed to the relatively high level of fertilizer N applied to these swards (131 kg ha−1) compared to the mean application rate over the whole farm of only 29 kg N ha−1 and 93 kg ha−1 applied to the inbye land.Output data calculated as utilized metabolizable energy (UME) per unit area for the whole farm and separately for hill and inbye components showed that although only 21% of the farm area was inbye land almost half the annual total metabolizable energy requirements of stock had to be met by grazing inbye. The capacity of herbage to meet stock energy demands was much lower on the hill with supplementary feed having to be provided over the winter.Calculated output from inbye land was 39 GJ ha−1 compared to 7 GJ ha−1 from the hill and 15 GJ ha−1 from the whole farm. UME output of the whole farm was negatively correlated with farm size (r=−0 55) and positively correlated with stocking rate (r = 0 89), applied N level (r = 0 63)

Relative Allocation of Energy to Growth and Development of Homeothermy in the Eastern Wood Rat (Neotoma floridana) and Hispid Cotton Rat (Sigmodon hispidus)

Allocation of energy to growth and heat production was measured by constructing energy budgets of male and female Sigmodon hispidus (x Adult body mass = ABM = 113 g ♂ 105 g ♀) and Neotoma floridana (ABM = 270 g ♂ 184 g ♀) from birth to maturity. Energy ingested and oxygen consumption per gram of body mass of post—weaning animals were not significantly different between sexes within species, but did differ significantly between the species. In all animals the relation between post—weaning ingestion (per gram body mass) and body mass follows a negative power function. Post—weaning metabolic rate as a function of body mass follows a different negative power function. The pattern of change in metabolic rate prior to weaning is different in the two species. In Sigmodon, metabolic rate increases linearly from birth to about 20 g (or about 20% of ABM). The rate at 20 g is about 5 cm3°g—1°h—1. In Neotoma metabolic rate remains constant at 1.7 cm3°g—1°h—1 from birth to about 26 g (or about 11% of ABM), then increases linearly to about 2.2 cm3°g—1°h—1 at 65 g (29% ABM). In both species, the peak in metabolic rate and nutritional weaning coincide with development of homeothermy, (measured as ability to maintain adult—level body temperatures at an ambient temperature of 20°C). Young Sigmodon become efficient homeotherms by 10—12 d of age whereas Neotoma require 19—22 d. Growth of each sex and species can be described by Gompertz equations. Growth rate constants, K, and asymptotic mass, A, differ between sexes within species and between species. Gompertz equations do not accurately describe growth of pre—weaning Neotoma which was linear from birth to 21—22 d of age. No such period of linearity is seen in Sigmodon growth. Analysis of litter—size dependence of growth of pre—weaning animals indicates that the early linear growth of Neotoma may not be a result of limited milk production, as it seems to be in Sigmodon. The relative total investment of energy in growth for a litter of Sigmodon (° = 4.8 young) between birth and 12 d of age was 416.7 kJ compared to 770.7 kJ for the first 24 d of life of the average Neotoma (x = 2.6 young). Over these same times, a litter of Sigmodon expended 1350.6 kJ on respiration compared with 1296.2 kJ in Neotoma. Thus the total metabolizable energy requirements of 1767.7 kJ and 2066.2 kJ are similar in the two species; however, a litter of Sigmodon requires about twice as much metabolizable energy per day each day they remain with their mother. Neotoma daily requirements are lower but protracted in time. During the pre—weaning period, Neotoma are investing proportionally more of their metabolizable energy in growth, whereas Sigmodon spend more on respiration, perhaps because they depend on their own heat production for thermoregulation while Neotoma rely on their mother's heat. These results are compared with data from the literature on growth and development of homothermy in a variety of small rodents. The hypothesis is advanced that, in general, larger species defer onset of active thermoregulation to larger body masses when lower metabolic rates occur, perhaps as an adaptation to permit more efficient early growth. Small species sacrifice growth efficiency in favor of rapid attainment of early independence and they pay the high energy costs of that speed. The different developmental physiologies of Neotoma and Sigmodon are explainable by inference from current ideas on the evolution of life histories, but much of the difference is also predictable simply from their difference in body size. This may indicate the importance of body size in the complex of life history traits.

An equation, suitable for computer use, based on the ARC feeding system to determine the energy requirements of growing and fattening cattle

SUMMARYBased on the Agricultural Research Council's feeding system, equations were developed that allow the calculation of the metabolizable energy requirements for maintenance and weight gain by cattle, separately or combined. A general equation was developed for the straight-forward calculation of the total metabolizable energy requirements of growing and fattening cattle for any combination of body weight, rate of weight gain, age, level of muscular work, and metabolizable energy concentration of the diet. The estimates of energy requirement made by the use of this equation are in excellent agreement with those made by the Agricultural Research Council using an iterative method.In addition to avoiding the awkward iterative process, the equations proposed are readily adaptable to computer use.