Brown adipose tissue development and metabolism in ruminants1

We conducted several experiments to better understand the relationship between brown adipose tissue (BAT) metabolism and thermogenesis. In Exp. 1, we examined perirenal (brown) and sternum s.c. adipose tissue in 14 Wagyu x Angus neonates infused with norepinephrine (NE). Perirenal adipocytes contained numerous large mitochondria with well-differentiated cristae; sternum s.c. adipocytes contained a few, small mitochondria, with poorly developed cristae. Lipogenesis from acetate was high in BAT but barely detectable in sternum s.c. adipose tissue. In Exp. 2, we compared perirenal and tailhead adipose tissues between NE-infused Angus (n = 6) and Brahman (n = 7) newborn calves. Brahman BAT contained two-to-three times as many total beta-receptors as Angus BAT. The mitochondrial UCP1:28S rRNA ratio was greater in Brahman BAT than in BAT from Angus calves. Lipogenesis from acetate and glucose again was high, but lipogenesis from palmitate was barely detectable. Tail-head s.c. adipose tissue from both breed types contained adipocytes with distinct brown adipocyte morphology. In Exp. 3, three fetuses of each breed type were taken at 96, 48, 24, 14, and 6 d before expected parturition, and at parturition. Lipogenesis from acetate and glucose in vitro decreased 97% during the last 96 d of gestation in both breed types, whereas the UCP1 gene expression tripled during gestation in both breed types. At birth, palmitate esterification was twice as high in Angus than in Brahman BAT and was at least 100-fold higher than in BAT from NE-infused calves from Exp. 2. Uncoupling protein-1 mRNA was readily detectable in tailhead s.c. adipose tissue in all fetal samples. In Exp. 4, male Brahman and Angus calves (n = 5 to 7 per group) were assigned to 1) newborn treatment (15 h of age), 2) 48 h of warm exposure (22 degrees C) starting at 15 h of age, or 3) 48 h of cold exposure (4 degrees C) starting at 15 h of age. Brahman BAT adipocytes shrank with cold exposure, whereas Angus BAT adipocytes did not. Similarly, BAT from neonatal lambs (Exp. 5; n = 6 per group) was depleted of lipid in response to cold exposure, although UCP1 gene expression persisted. In Exp. 4, NE stimulated lipogenesis from palmitate in BAT incubated in vitro. Lipogenesis from palmitate was higher in Angus than in Brahman BAT, and increased with both warm and cold exposure. These studies suggest that BAT from Brahman calves may be exhausted of lipid shortly after birth during times of cold exposure.

Ontogenic development of brown adipose tissue in Angus and Brahman fetal calves

Brahman calves experience greater neonatal mortality than Angus calves if cold-stressed. To establish a developmental basis for this, three fetuses of each breed type were taken at 96, 48, 24, 14, and 6 d before expected parturition, and at parturition. Overall fetal BW tended (P = 0.08) to be greater for Angus than for Brahman fetuses. There was no difference between breed types in total brown adipose tissue (BAT) mass or grams of BAT/kg BW. Brown adipocyte density decreased 56%, whereas lipogenesis from acetate and glucose in vitro decreased 97% during the last 96 d of gestation in both breed types. Glycerolipid synthesis from palmitate declined by 85% during the last trimester but still contributed 98% to total lipid synthesis at birth. The fetal age x breed interaction was significant for lipogenesis from glucose (P = 0.05) and palmitate (P = 0.005); rates were higher at 96 d before birth in Brahman BAT but declined to similar rates by birth. Uncoupling protein-1 (UCP1) mRNA tripled during gestation in both breed types (P = 0.002), whereas mitochondrial cross-sectional area did not change (P = 0.14) during gestation. Neither the breed nor the age x breed effect was significant (P > or = 0.24) for UCP1 mRNA concentration or mitochondrial cross-sectional area. In both breed types, a marked decrease in BAT UCP1 mRNA between 24 and 14 d prepartum was associated with a similar reduction in lipogenesis from palmitate and a noticeable change in BAT mitochondrial morphology, as the mitochondria became more elongated and the cristae became more elaborate. Uncoupling protein-1 mRNA initially was elevated in Angus tailhead s.c. adipose tissue, but was barely detectable by birth, and tended to be greater overall (P = 0.09) in Angus than in Brahman BAT. If uncoupling protein activity in s.c. adipose tissue persists after birth, then s.c. adipose tissue may contribute more to thermogenesis in Angus newborn calves than in Brahman calves. In contrast, we did not observe differences in ontogenic development of perirenal BAT that could explain the documented differences in thermogenic capacity between Angus and Brahman newborn calves.

Metabolism and morphology of brown adipose tissue from Brahman and Angus newborn calves

The objective of this study was to compare adipocyte morphology and lipogenesis between breed types (Angus vs Brahman) in brown adipose tissue (BAT) and white adipose tissue (WAT) from newborn calves. The Brahman calves (n = 7) were born during the fall season, whereas the Angus calves were born in fall (n = 6) or the following spring (n = 4). At parturition, Brahman cows were lighter than fall Angus cows, but were heavier than spring Angus cows (P < .05). Birth weights and perirenal BAT weights were greater in spring-born, but not in fall-born Angus calves, than in Brahman calves (P < .05). Fall-born Angus BAT contained 63% more (P < .05) adipocytes/100 mg tissue and contained a greater proportion (P < .05) of adipocytes with mean diameters of 40 to 50 microm, and fewer adipocytes with diameters of 60 microm or greater, than Brahman BAT. Brahman BAT contained two-to-three times as many beta-receptors as Angus BAT (P < .05), although the dissociation constant (Kd) was not different between breed types. Mitochondria in Brahman BAT were primarily spherical, whereas Angus BAT mitochondria were elongated, and mitochondrial cross-sectional area tended (P = .08) to be greater in Brahman BAT than in Angus BAT. The mitochondrial uncoupling protein (UCP) mRNA concentration (per 10(6) cells) was greater in Brahman BAT than in BAT from fall-born Angus calves. Lipogenesis from acetate was greater in Angus BAT than in Brahman BAT (P < .05), and glucose and palmitate contributed a greater proportion of carbon to lipogenesis in Brahman BAT than in Angus BAT. These differences in lipogenesis between breed types were not observed in s.c. WAT. The WAT from both breed types contained adipocytes with distinct brown adipocyte morphology, suggesting an involution of BAT to WAT in utero. We conclude that differences in UCP gene expression cannot cause the greater peak thermogenesis of Angus calves; however, differences between breed types in lipid metabolism and(or) mitochondrial morphology may contribute to this phenomenon.

Copper status of ewes fed increasing amounts of copper from copper sulfate or copper proteinate

The Cu status of mature, crossbred ewes fed two sources (CuSO4 vs. Cu proteinate) and three levels (10, 20, or 30 mg/kg) of dietary Cu was determined in a 73-d feeding trial. Ewes (n = 30) were fed a basal diet containing rice meal feed, cottonseed hulls, cottonseed meal, meat and bone meal, cracked corn, and vitamin-mineral supplements at 2.5% of BW to meet NRC requirements for protein, energy, macrominerals, and microminerals, excluding Cu. The basal diet contained 5 mg/kg Cu, 113 mg/kg Fe, .1 mg/kg Mo, and .17% S. Copper sulfate or Cu proteinate was added to the basal diet to supply 10, 20, or 30 mg/kg of dietary copper in a 2x3 factorial arrangement of treatments. Ewes were housed in 3.7- x 9.1-m pens in an open-sided barn. Blood samples were collected on d 28 and 73. Ewes were slaughtered on d 74, and liver and other tissues were collected to determine Cu concentrations. An interaction (P = .08) occurred between source and level for liver Cu. The interaction existed due to an increase in liver Cu concentrations when ewes were fed increasing dietary Cu from CuSO4 but not when fed Cu proteinate diets. There was no source x level interaction (P>.10) for the blood constituents measured. On d 73, plasma ceruloplasmin activity was greater (P<.05) in ewes fed Cu proteinate than in those fed CuSO4 (33.1 vs. 26.8 microM x min(-1) x L(-1)). Increasing the concentration of dietary Cu did not affect (P>.10) plasma ceruloplasmin. Packed cell volume (PCV), red blood cell count (RBC), white blood cell count, whole blood hemoglobin (wHb), plasma hemoglobin, and plasma Cu were similar between sources of Cu. Ewes fed 20 mg/kg Cu had lower (P<.05) PCV, RBC, and wHb than those fed 10 or 30 mg/kg Cu diets. Feeding up to 30 mg/kg Cu from these sources did not cause an observable Cu toxicity during the 73-d period.

Prepartum protein restriction does not alter norepinephrine-induced thermogenesis or brown adipose tissue function in newborn calves

We examined the effect of prepartum protein restriction on thermogenesis and several aspects of perirenal (brown) adipose tissue (BAT) in newborn calves. Lipid synthesis and morphology also were compared between BAT and sternum (white) adipose tissue. During the last 140 d of gestation, heifers were fed isocaloric diets containing adequate (10.4%) or restricted (average of 6.8%, dry matter basis) levels of protein. Body condition scores and weight gain during gestation were significantly lower in heifers fed the restricted-protein diet. However, newborn calf birth weight, calf BAT weight and composition, and calf thermoneutral metabolic rates were not affected by prepartum protein restriction. Similarly, visceral organ weights, except for lung plus trachea, were not affected (P > 0.10) by prepartum protein treatment. Peak metabolic rates were not affected (P > 0.10) by prepartum protein treatment and on average were twice the thermoneutral metabolic rates. Consistent with this, BAT of calves from heifers fed adequate- or restricted-protein diets did not differ in lipid synthesis, cellularity, or uncoupling protein mRNA:28S rRNA ratios. Although both perirenal and sternum adipocytes were mostly unilocular, perirenal adipocytes contained numerous large mitochondria with well-differentiated cristae; sternum adipocytes contained a small number of mitochondria with poorly developed cristae. Fatty acid biosynthesis from acetate was high in BAT (55-57 nmol acetate incorporated.100 mg-1.h-1) but barely detectable in sternum adipose tissue. Conversely, fatty acid biosynthesis from glucose was 80-110% higher in sternum adipose tissue than in BAT (4.5 vs 2.1-2.5 nmol glucose incorporated.100 mg-1.h-1). Thus maternal protein restriction severely affected heifers but had no effect on neonatal calf thermogenesis or BAT function.

Effects of bovine somatotropin treatment and intermittent growth pattern on mammary gland development in heifers

Angus x Holstein heifers (initial age and BW: 6.0 +/- .4 mo; 159 +/- 12 kg) were assigned to one of four treatments (n = 10) in a 2 x 2 factorial arrangement to examine the effects of bovine somatotropin (bST) or vehicle treatment and continuous (CG) or intermittent growth (IG) pattern treatment on mammogenesis. Subcutaneous injections of bST (500 mg) or vehicle were administered at 14-d intervals. The CG heifers were limit-fed a diet to gain .8 kg/d throughout the experiment, whereas IG heifers underwent two successive growth restriction-compensation phases with each phase consisting of a 3-mo growth restriction period (.25 kg/d) followed by a growth compensation period. All heifers were individually fed during the experiment and slaughtered when average treatment BW reached 390 kg. Mammary gland development was measured by dissection, chemical analysis, and computed tomography scanning. Overall ADG were increased (P < .001) 20% by bST treatment of CG heifers, although ADG of IG-vehicle and IG-bST heifers were similar to ADG of CG-vehicle heifers. Dissectable extraparenchymal tissue (EPT) mass was reduced 23% (P < .05) and 36% (P < .001) by IG pattern and bST treatments, respectively, although the bST effect on dissectable EPT tended (P = .08) to be greater in CG than in IG heifers. Fat-free parenchymal tissue (PT) mass (P = .11) and PT volume (P = .08) tended to be greater in IG than in CG heifers. Fat-free PT was 60% greater (P = .01) and PT volume onefold higher (P = .01) in bST than in vehicle heifers. The bST and IG pattern treatments increased the proportional weight of fat-free PT mass, although the effect of bST was greater than IG pattern treatment. There was no evidence to suggest that the two treatments were additive in affecting mammogenesis.

Genotypic effects on norepinephrine-induced changes in thermogenesis, metabolic hormones, and metabolites in newborn calves

Heat production was measured in newborn Angus-, Brahman-, and Tuli-sired calves born to Angus (n = 20) and Brahman (n = 26) dams, before (thermoneutral metabolic rate, TMR) and after norepinephrine (NE) infusion (peak metabolic rate, PMR), to assess genotypic effects on nonshivering thermogenesis in brown adipose tissue. Calves were fed pooled colostrum, fitted with jugular catheters, and placed in a temperature-controlled (37 degrees C) water immersion system. Heat production, determined by indirect calorimetry, and tympanic temperature were measured continuously in calves from approximately 3 to 6 h of age. Blood samples were collected at birth and at 0, 5, 20, 40, 60, 80, 100, and 120 min relative to NE infusion (35 micrograms.min-1.kg BW-1 for 4 min), and plasma was analyzed for metabolites (glucose, NEFA, and urea nitrogen [PUN]) and hormones (cortisol, triiodothyronine [T3] and thyroxine [T4]). Weight-specific TMR (cal.min-1.kg-1) was not affected by breed of sire or dam, although weight-specific PMR (cal.min-1.kg-1) was lower (P < .01) in Brahman-sired calves than in Angus- or Tuli-sired calves and was lower (P < .001) in calves born to Brahman rather than Angus dams. The reduction in weight-specific PMR due to the maternal Brahman influence was sire-breed dependent, and the reduction was largest (P < .01) for Tuli-sired (34.3%), intermediate (P < .05) for Brahman-sired (15.1%), and lowest (P > .25) for Angus-sired calves (4.1%). The PMR:TMR ratio was 1.80 and 2.21 +/- .06 in calves born to Brahman and Angus dams, respectively. Peak tympanic temperature was .6 degree C lower (P < .01) in calves born to Brahman rather than Angus dams. At birth, plasma NEFA concentrations were higher (P < .001) and glucose tended (P = .13) to be higher in calves born to Brahman rather than Angus dams. Cortisol, T3, and T4 concentrations at birth were higher in calves born to Brahman dams than in those born to Angus dams. These results suggest that calves born to Brahman dams may have less thermogenically active brown adipose tissue than calves born to Angus dams, which may contribute to the relative cold intolerance of calves with Bos indicus inheritance.

Physiological and hormonal responses of lambs repeatedly implanted with zeranol and provided two levels of feed intake

Forty crossbred wethers (average weight 30 kg) were implanted with zeranol (12 mg) at 30-d intervals and fed at two levels of intake in a 2 x 2 factorial arrangement of treatments to determine performance, carcass and bone characteristics, blood metabolites, and hormones. Restricted lambs were fed to gain one-half the BW gained by lambs with ad libitum feed access. Lambs with ad libitum and restricted access to feed were slaughtered after 98 and 154 d, respectively. Zeranol increased ADG (P = .047; 20%), gain to feed (P = .023; 17%), metacarpal length (P = .004; 6%) and weight (P = .013; 13%), and tended to increase carcass crude protein gain (P = .106; 63%) while reducing kidney pelvic fat (P = .001; 33%) and dressing percentage (P = .038; 3%). Restricted feed intake increased the percentage of carcass ash and metacarpal length and weight by 27% (P = .048), 5% (P = .006), and 10% (P = .045), respectively, while reducing quality grade scores (P = .022; 5%), gain to feed (P = .001; 49%), longissimus muscle area (P = .001; 28%), the percentage of kidney pelvic fat (P = .033; 13%), and daily fat gain (P = .001; 54%). Zeranol increased pituitary weight (P = .001; 166%), plasma glucose (P = .036; 13%), mean serum growth hormone (GH; P = .011; 52%), baseline GH (P = .048; 34%), GH pulse amplitude (P = .003; 59%), and IGF-I (P = .001; 53%) concentrations. The results indicate that continuous administration of zeranol from 60 d of age to slaughter increases GH release, which directs nutrient utilization such that a carcass with more desirable lean and fat deposition patterns is obtained when nutrient availability is adequate.

Influence of calf genotype on colostral immunoglobulins in Bos taurus and Bos indicus cows and serum immunoglobulins in their calves

Purebred Bos indicus calves are documented to have lower survival rates than Bos taurus calves. Thus, this study was designed to investigate the possibility that this decreased survival rate may be attributed to dam colostral immunoglobulin (Ig) concentrations and subsequent calf serum Ig concentrations. The specific objective was to determine the effect of breed type of calf on colostrum production, immunoglobulin concentrations in colostrum and calf serum, and availability and absorption efficiency of Ig. Brahman (B) and Angus (A) cattle were reciprocally mated to produce calves of the following types: A x A (n = 8), A x B (n = 9), B x B (n = 11), and B x A (n = 11). At birth, calves were separated from their dams and a blood sample was collected before feeding pooled colostrum (30 mL/kg birth weight) at 1 and 6 h of age. From 6 to 12 h of age, each calf was placed in a box that allowed interaction with the dam but prevented suckling. At 12 h of age, each calf was fed its dam's colostrum and placed with the dam. Additional blood samples were collected at 12, 24, and 48 h after birth. Serum and colostrum samples were analyzed for IgG, IgG1, IgG2, IgM, and IgA using single radial immunodiffusion (RID) assay techniques. The cows were hand-milked after induction of milk letdown with oxytocin at 1 and 12 h after calving. Colostrum volume was recorded, and samples were collected. Brahman cows produced more (P < .001) colostrum at 1 and 12 h than A cows. Total Ig concentrations were obtained by summing IgG, IgG1, IgG2, IgM, and IgA concentrations. Total Ig (P < .02), IgG (P < .005), and IgA (P < .01) concentrations in colostrum were greater in cows producing crossbred calves. Total Ig (P < .006), IgG (P < .02), IgG1 (P < .004), and IgG2 (P < .02) available in colostrum were affected by B x B and A x B breed types of calf. Brahman cows had more Ig available at 1 and 12 h than A cows due to increased production of colostrum. Breed type influenced colostral Ig in cattle. Serum concentrations of total Ig, IgG, IgG1, IgG2, IgM, and IgA in the calf and efficiency of absorption at 6 and 12 h were not affected by breed type, sex of calf, or any interaction.

Cold thermoregulation in the newborn calf

During the fetal to neonatal transition, the newborn calf encounters severe thermolysis due to an abrupt change in thermal environment that is compounded by evaporation of fetal fluids and severe weather conditions. Maintenance of homeothermy during the neonatal period necessitates an acute and sustained thermogenic response by the newborn calf. It is now widely accepted that this thermogenic response is derived from both shivering thermogenesis in muscle tissue and nonshivering thermogenesis in brown adipose tissue (BAT). It is critical that newborn calves possess functional BAT during the neonatal period. This article focuses on the pre- and postnatal factors that influence nonshivering thermogenesis of BAT in the neonatal calf.

Effects of zeranol and two dietary levels of calcium and phosphorus on performance, carcass and bone characteristics, and calcium status in growing lambs

Seventy-two crossbred wether lambs (average initial weight, 25.1 kg) were used to determine the interaction between zeranol treatment and two dietary levels of Ca and P (.8 and .6% vs .4 and .3% Ca and P, respectively) in a 2 x 2 factorial arrangement on performance, carcass and bone characteristics, and serum concentrations of parathyroid hormone (PTH) and Ca. Lambs were implanted on d 0 and 56 with 12 mg of zeranol. Lambs had ad libitum access to feed for 105 d. On d 99, blood samples were collected. Implanted lambs had 12% greater (P less than .01) daily feed intake, 26% greater (P less than .10) ADG, and a 12% improvement in (P less than .10) feed efficiency compared with nonimplanted lambs. Zeranol-treated lambs had increased (P less than .05) bone cortical area, breaking load, and width of the metacarpal compared with nonimplanted lambs. Lambs fed the .8% Ca and .6% P diet had a higher (P less than .05) percentage of bone ash than lambs fed the .4% Ca and .3% P diet. However, there were no differences (P greater than .05) in the percentage of Ca, P, Mg, or Zn in metacarpal bones due either to higher dietary Ca and P or to implant treatments. Serum concentration of PTH was greater (P less than .10) in lambs fed .8% Ca and .6% P than in those receiving .4% Ca and .3% P. Serum concentrations of PTH and Ca pooled across treatments were greater (P less than .05) before feeding than at 1 h after feeding.(ABSTRACT TRUNCATED AT 250 WORDS)

Physical and chemical components of the empty body during compensatory growth in beef steers

The composition of carcass and noncarcass tissue growth was quantified by serial slaughter of 26 Angus x Hereford crossbred steers (initial age and weight 289 +/- 4 d and 245 +/- 4 kg) during continuous growth (CON) or compensatory growth (CG) after a period of growth restriction (.4 kg/d) from 245 to 325 kg BW. All steers were fed a 70% concentrate diet at ad libitum or restricted levels. Homogenized samples of 9-10-11th rib and noncarcass tissues were analyzed for nitrogen, fat, ash, and moisture. Growth rate from 325 to 500 kg BW was 1.54 and 1.16 kg/d for CG and CON steers. The weight of gut fill in CG steers was 10.8 kg less (P less than .05) before realimentation and 8.8 kg more (P less than .10) at 500 kg BW than in CON steers. The allometric accretive rates for carcass chemical components relative to the empty body were not affected by treatment. However, the accretive rates for CG steers were greater (P less than .01) for noncarcass protein (.821 vs .265), noncarcass water (.861 vs .507), and empty-body protein (.835 vs. .601) than for CON steers. Final empty-body fat was lower (P less than .001; 24.2 vs 32.4%) and empty-body protein higher (P less than .001; 16.6 vs 14.8%) in CG steers than in CON steers. Consequently, net energy requirements for growth (NEg) were approximately 18% lower for CG steers. We conclude that reduced NEg requirements and changes in gut fill accounted for most of the compensatory growth response exhibited in these steers.

Variation among twin beef cattle in maintenance energy requirements

The genetic variation in energy expenditures of cattle at fasting (FHP) and maintenance (MEm) was determined by using 12 pairs of monozygous twins at 20 mo of age. The pairs were of two breed types, eight Angus x Hereford (three steers, five heifers) and four Barzona x Hereford (three steers, one heifer). The heifers were 132 +/- 13 d pregnant at the time of measurement. The pairs were fed at 1.15 x maintenance energy requirements for a minimum of 30 d prior to heat production (HP) measurements in dual indirect respiration calorimetry chambers. The diet fed was cracked corn:alfalfa hay (45:55) with a determined ME of 2.47 +/- .02 Mcal/kg DM. This diet was fed individually for 7 d prior to and during two consecutive 22-h HP measurements. The animals then were fasted for 2 d and fasting heat production measurements (FHP) were made on d 3 and 4 of the fast. Metabolizable energy required for MEm was calculated iteratively by assuming a semi-log relationship between HP and metabolizable energy intake. There were no differences (P greater than .10) in measured energy expenditures due to different breed type. The FHP and efficiency of ME use for MEm (Km) were similar between sexes, although heifers had lower (P less than .025) MEm than steers. Twin pair effects were detected for FHP (P less than .005) and MEm (P less than .05) but not for Km. Broad sense heritability estimates were calculated as the intraclass correlation between members of monozygous twin pairs. Heritability estimates for MEm, FHP, and Km were .52 +/- .22, .75 +/- .13, and .34 +/- .27, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Site and extent of apparent magnesium absorption by lambs fed different sources of magnesium

Ten abomasally cannulated crossbred wether lambs (avg wt, 33 kg) were used in a replicated 5 x 5 latin square design to determine the site and extent of apparent absorption of Mg when fed different sources of Mg. Lambs were fed twice daily 220 g of chopped mixed grass hay and 180 g of a corn-based supplement (control; .13% mg, DM basis), or the control diet supplemented with Mg (.26% Mg, DM basis) from MgO, magnesium citrate (MgC), smectite-vermiculite (Mg-Mica) or MgOH. Lambs were maintained in metabolism stalls during each of the five experimental periods. Each period consisted of a 7-d dietary adjustment followed by a 3-d collection of abomasal samples, feces and urine. Abomasal contents were sampled four times daily during the 3-d collection period. The diet contained .5% chromium oxide as a digestion marker. Apparent absorption of Mg was .17, .55, .85, .78 and .82 g/d for lambs fed the control, MgO, MgC, Mg-Mica and MgOH diets, respectively. Apparent absorption of Mg (g/d) was similar (P greater than .05) in the lambs fed the supplemented diets and greater (P less than .05) than in those fed the control diet. Preintestinal absorption of Mg was .21, .57, 1.08, .14 and .92 g/d when the control, MgO, MgC, Mg-Mica and MgOH diets were fed. Lambs fed the control and Mg-Mica diets absorbed similar (P greater than .05) quantities of Mg in the preintestinal region and less (P less than .05) than lambs fed the MgO, MgC and MgOH diets.(ABSTRACT TRUNCATED AT 250 WORDS)

Endocrine and metabolic changes during altered growth rates in beef cattle

Eight steers from a group of 14 were fed ad libitum from 240 to 510 kg live weight, gaining at 1.4 +/- .2 kg/d. The six other steers were diet-restricted and grew at .37 +/- .09 kg/d from 240 to 307 kg, prior to ad libitum realimentation on the same diet to a final weight of 510 kg. Blood samples taken during the growth phases from both treatments were analyzed for insulin-like growth factor-I (IGF-I), triiodothyronine (T3), thyroxine (T4), glucose (GLU), nonesterified fatty acids (NEFA), and blood urea nitrogen (BUN) and (or) growth hormone (GH). During restricted growth, mean serum concentrations of GH were elevated (45.6 vs 23.4 ng/ml; P less than .05), serum concentrations of IGF-I decreased (108 vs 167 ng/ml; P less than .05) compared with control steers with ad libitum access to feed. Levels of T4 and GLU also were lower (P less than .05) during restricted than during normal growth. During early realimentation, levels of GLU (P less than .05), IGF-I (P less than .01), T4 and BUN (P less than .01) increased. Levels of T3 remained unchanged, whereas concentration of NEFA declined (P less than .001). Blood urea nitrogen decreased during early realimentation despite a large increase in diet protein intake and in protein storage, suggesting an increased efficiency of nitrogen use for protein synthesis. During realimentation, IGF-I levels rose above those of control steers and remained higher at the final weight of 510 kg (P less than .05).(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of prepartum protein nutrition and birth weight on basal metabolism in bovine neonates

Recipient beef heifers, pregnant with single demi-embryos, were paired to according to identical twin or full-sib embryo. Within pair, recipient heifers were assigned to one of two isocaloric diets containing a control or restricted level of protein (91 vs 55% of National Research Council recommendations) on d 190 of gestation. Following parturition, calves were weighed, fed 1 liter of colostrum, dried and placed in a metabolic chamber at 5 h of age for an 8-h determination of heat production (HP). Maternal body weight gains (P less than .001) during the last trimester and body condition scores at parturition (P less than .05) were reduced in heifers fed the protein-restricted diet. Calves born to heifers fed the protein-restricted diet had 11.4% lower (P less than .05) HP than calves born to control heifers (43.7 vs 49.3 kcal.kg-1.d-1). Birth weights, respiratory quotients and rectal temperatures of newborn calves were not significantly affected by prepartum protein restriction. Within treatment groups, the relationship between HP and weight was described by the equation: HP (kcal/d) = 2.30 wt1.86. The allometric exponent of 1.86 +/- .26 implied that weight-specific metabolic rate was higher rather than lower in larger weight calves. We concluded that the thermogenic ability of neonatal calves may be compromised by prepartum protein restriction and(or) small birth weights.