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The present study aimed to evaluate the effects of rearing system and level of dietary protein supplementation on growth performance, metabolic profile, plasma leptin, and carcass composition of young Podolian bulls. At the beginning of the finishing period (about 14 mo of age), animals were divided into 3 groups according to rearing system and dietary protein level: indoor receiving a diet with 15% CP of DM (IND); and outdoor, at pasture, receiving a supplementation with 12% CP of DM (OUT12) or with 15% CP of DM (OUT15). Plasma leptin concentration increased (P < 0.05) with age, whereas it was unaffected by rearing system and protein level. Positive correlations between plasma leptin concentration, ADG (r = 0.861, P < 0.001), and blood glucose concentration (r = 0.977, P < 0.001) were observed, whereas NEFA and triglycerides were not correlated with leptin concentration. At the end of the finishing period (about 20 mo of age), young bulls in the OUT12 group showed greater (P < 0.05) cholesterol concentrations in blood than the OUT15 group, whereas the OUT15 group showed greater urea concentrations compared with IND (P < 0.01) and OUT12 groups (P < 0.001). Average daily gains, feed conversion ratio, and final BW were similar between groups. Intramuscular lipid content of LM, semitendinosus, and semimembranosus muscles was correlated with plasma leptin before slaughtering (r = 0.77, P < 0.01; r = 0.67, P < 0.01, r = 0.79, respectively). Correlations between plasma leptin and intramuscular fat were found starting from 18 mo of age for LM (r = 0.44; P < 0.05) and from 19 mo for semimembranosus (r = 0.56; P < 0.05) and semitendinosus (r = 0.58; P < 0.05). Mean value of intramuscular fat and intramuscular fat of LM was greater in OUT15 than OUT12 (P < 0.05). No differences were found between 12 and 15% CP supplementations for growth and carcass data. Therefore, we conclude that the extensive rearing system supplemented with 12% CP may be adequate to sustain good growth performance and carcass quality from the Podolian breed. These data also provide confirmation that leptin, in cattle, is directly related to nutritional status and may represent a suitable predictor for body fat starting from 1 mo before slaughtering. Key words: carcass composition, leptin, metabolic profile, Podolian cattle, protein level, rearing system

The objective of the study was to characterize body growth, testicular development, and puberty from 8 to 14 mo of age in bulls (n = 120) produced by mating sires from Hereford, Angus, Norwegian Red, Swedish Red and White, Friesian, and Wagyu breeds to MARC III (1/4 Hereford, 1/4 Angus, 1/4 Red Poll, and 1/4 Pinzgauer) cows. Traits evaluated were birth weight, weaning weight (at 215 d), yearling weight, ADG from 8 to 14 mo of age, paired testicular volume growth from 8 to 14 mo of age, age at puberty (determined by production of 50 x [10.sup.6] sperm with 10% motility), age at freezable semen (determined by production of 500 x 106 sperm with 50% motility), and, at 15 mo of age, paired testicular weight and daily sperm production per testis pair. There was an effect of sire breed (P = 0.03) for age at puberty; animals with Wagyu and Swedish Red and White inheritance reached puberty at a later date (302 and 302 d of age, respectively) compared with Angus-sired bulls (268 d). Age at puberty for Hereford-, Norwegian Red-, and Friesian-sired bulls was 270, 271, and 278 d, respectively. Differences in BW were observed (P = 0.03) at birth; bulls with Hereford and Friesian were heavier at birth (43 and 41 kg, respectively) compared with those with Norwegian Red, Swedish Red and White, and Wagyu inheritance (39, 38, and 38 kg, respectively). Differences in BW were also observed at 1 yr of age (P = 0.001), where the heaviest animals were those sired by Angus (450 kg), whereas the lightest animals were those sired by Wagyu (403 kg). Bulls with Wagyu inheritance had the lowest (P = 0.04) ADG (1.12 kg/d) compared with bulls with inheritance from Hereford (1.22 kg/d), Angus (1.28 kg/d), Norwegian Red (1.24 kg/d), Swedish Red and white (1.25 kg/d), and Friesian (1.27 kg/d). Differences in scrotal growth rate were not significant (P = 0.99). They ranged from 1.95 in Angus-sired to 1.66 [cm.sup.3]/d in Wagyu-sired bulls. There were no differences (P = 0.80) for age at freezable semen (335 [+ or -] 10 d). At slaughter (15 mo of age), there were no differences (P = 0.62) for paired testicular weight (603 [+ or -] 28 g) and daily sperm production (10.6 x [10.sup.9] [+ or -] 0.9 x [10.sup.9] per testis pair). Growth of bulls with Wagyu inheritance was slower, and bulls with Wagyu or Scandinavian inheritance reach puberty at an older age than bulls with Angus inheritance. Key words: beef cattle, bull, puberty, semen, testes

Growth and testicular development between 7 and 15 mo of age were evaluated in bulls produced by mating sires of six breeds (Hereford, Angus, Belgian Blue, Brahman, Boran, and Tuli) to Angus, Hereford, and MARC III (four-breed composite) cows. At 12 mo of age, Angus- and Hereford-sired bulls had the heaviest body weight (P < 0.08 to 0.001), whereas Brahman- and Belgian Blue-sired bulls were intermediate, and Boran- and Tuli-sired bulls weighed the least. Bulls sired by European breeds grew more rapidly after weaning (P < 0.01) than did Brahman-, Boran-, and Tuli-sired bulls, and these differences in growth rate were maintained through 15 mo of age, indicating that offspring of heat-adapted sire breeds (Brahman, Boran, and Tuli) have lower postweaning rates of gain, particularly during winter months, than do offspring of nonheat adapted sire breeds. Testis size was smaller initially (P < 0.01) and remained smaller in offspring of heat-adapted sire breeds through yearling age. By 15 mo of age, testis size was largest (P < 0.06 to 0.001) in Angus-sired bulls and had become similar among Hereford-, Brahman-, Boran- and Belgian Blue-sired bulls but remained smaller (P < 0.02 to 0.001) in Tuli-sired bulls. Thus, offspring of heat-adapted sire breeds had delayed testicular development compared with that of nonheat adapted sire breeds, particularly through yearling age. At puberty, Angus-sired bulls were 23 to 82 d younger (P < 0.05 to 0.001) than all other sire breeds except Hereford, and Brahman-sired bulls were older at puberty (P < 0.05 to 0.001) than were bulls of all other sire breeds except Boran. Testis size at puberty was quite similar among breeds of bulls (scrotal circumference = 27.9 [+ or -] 0.1 cm) despite large breed differences in age, body weight, and hip height. Thus, measurement of yearling testis size was a reliable indicator of age at puberty among widely divergent breeds of bulls. In addition, the lower postweaning rates of gain and the smaller and slower testicular development in offspring of heat-adapted sire breeds should be noted by cattle producers considering use of such breeds in crossbreeding and breed improvement programs. Key Words: Beef Cattle, Bulls, Puberty, Semen, Testes

Carcass and growth measurements of finished crossbred steers (n = 843) and yearling ultrasound and growth measurements of purebred bulls (n = 5,654) of 11 breeds were analyzed to estimate genetic parameters. Multiple-trait restricted maximum likelihood (REML) was used to estimate heritabilities and genetic correlations between finished steer carcass measurements and yearling bull ultrasound measurements. Separate analyses were conducted to examine the effect of adjustment to three different end points: age, backfat thickness, and weight at measurement. Age-constant heritability estimates from finished steer measurements of hot carcass weight, carcass longissitaus muscle area, carcass marbling score, carcass backfat, and average daily feedlot gain were 0.47, 0.45, 0.35, 0.41, and 0.30, respectively. Age-constant heritability estimates from yearling bull measurements of ultrasound longissimus muscle area, ultrasound percentage of intramuscular fat, ultrasound backfat, and average daily postweaning gain were 0.48, 0.23, 0.52, and 0.46, respectively. Similar estimates were found for backfat and weight-constant traits. Age-constant genetic correlation estimates between steer carcass longissimus muscle area and bull ultrasound longissimus muscle area, steer carcass backfat and bull ultrasound backfat, steer carcass marbling and bull ultrasound intramuscular fat, and steer average daily gain and bull average daily gain were 0.66, 0.88, 0.80, and 0.72, respectively. The strong, positive genetic correlation estimates between bull ultrasound measurements and corresponding steer carcass measurements suggest that genetic improvement for steer carcass traits can be achieved by using yearling bull ultrasound measurements as selection criteria. Key Words: Beef Cattle, Carcass Quality, Genetic Parameters, Ultrasound

Postweaning growth, sexual development, libido, and carcass data were collected from two consecutive calf crops using 31 Brahman x Angus (B x A), 41 Senepol x Angus (S x A), and 38 Tuli x Angus (T x A) [F.sub.1] bulls. Following weaning (by mid-September) and preconditioning, at the start of the study (late September) bulls were fed concentrate (three times each week at a rate equivalent to 4.5 kg/d) on bahiagrass pasture for approximately 250 d. At the start of the study and at 28-d intervals, BW, hip height, and scrotal circumference (SC) were measured. Concurrently at 28-d intervals, when the SC of a bull was [is greater thann or equal to] 23 cm, semen collection was attempted using electroejaculation. Ejaculates were evaluated for presence of first spermatozoa (FS), 50 x [10.sup.6] sperm with at least 10% motility (PU), and 500 x [10.sup.6] sperm with at least 50% motility (PP). After all bulls reached PP they were subjected to two libido tests. Carcass data were collected on all bulls (n=110) and Warner-Bratzler shear (WBS) force values were assessed on a subset (n = 80). For both years, B x A bulls were heavier (P [is less than] 0.05) and taller (P [is less than] 0.05) than S x A and T x A bulls at the start and end of the study. However, breed type did not influence (P [is greater than] 0.10) gain in BW or hip height during the study. Scrotal circumference of T x A bulls was larger (P [is less than] 0.05) than that of B x A or S x A bulls at the start of the study, but there was no effect (P [is greater than] 0.10) of breed type by the end of the study. At PU and PP, B x A bulls were older (P [is less than] 0.05), heavier (P [is less than] 0.05), and taller (P [is less than] 0.05) and had larger (P [is less than] 0.05) SC than S x A and T x A bulls. Tuli x Angus bulls were younger (P [is less than] 0.05) than S x A bulls at PU and PP but had similar SC. Libido scores tended (P [is less than] 0.10) to be lower for B x A than for S x A and T x A bulls. Breed type affected (P [is less than] 0.05) carcass traits; B x A bulls had the heaviest (P [is less than] 0.05) hot carcass weight, greatest (P [is less than] 0.05) dressing percentage, larger (P [is less than] 0.05) longissimus muscle area than S x A bulls, and higher (P [is less than] 0.05) USDA yield grade than T x A bulls but greatest (P [is less than] 0.05) WBS force values. Breed type did not affect (P [is greater than] 0.10) USDA quality grade. In conclusion, tropically adapted [F.sub.1] bulls produced from Senepol (Bos taurus) and Tuli (Sanga) sires bred to Angus cows in Florida had lighter BW, shorter hip heights, and smaller carcasses than those from Brahman sires but reached puberty earlier and had higher libido scores and lower WBS force values. Key Words: Bulls, Carcass Quality, Cattle Breeds, Growth, Puberty, Tropics

The response of GH to GHRH at weaning is known to predict postweaning growth and body composition in beef bulls. The objective of this study was to determine whether GH response to a challenge of GHRH and plasma IGF-I can predict growth rate and body composition in the beef heifer. Growth hormone response to a challenge with two doses of GHRH was measured in 67 Angus heifers averaging 225 d of age (SD = 21) and 217 kg BW (SD = 32). Blood samples were collected at 0 and 10 min relative to an initial 'clearance dose' (4.5 [micro]g GHRH/100 kg BW) and again, 3 h later, relative to a challenge dose (1.5 or 4.5 [micro]g GHRH/100 kg BW). Each animal received each of the two challenge doses, which were randomly assigned across 2 d of blood collection. Serum GH concentration was measured by RIA. Plasma was collected every 28 d during a 140-d growth test and assayed for IGF-I by RIA. Body weight was measured every 28 d and hip height was measured at weaning and at the end of a 140-d growth test. Average daily gain was calculated on d 140 of the growth test and body composition measurements were estimated by ultrasound 2 wk after completion of the growth test. Responses to the two GHRH challenges were dose-dependent (P [is less than] 0.05). Average daily gain tended to be related to GH response to the 1.5 [micro]g GHRH/100 kg BW dose ([R.sup.2] = 0.05; P = 0.06), but no relationship was observed at the 4.5 [micro]g GHRH/100 kg BW dose ([R.sup.2] = 0.00; P = 0.93). An inverse relationship ([R.sup.2] = 0.06; P = 0.02) was observed between response to the 1.5 [micro]g GHRH/100 kg BW dose and intramuscular fat percentage. Mean plasma IGF-I concentration was positively associated with ADG ([R.sup.2] = 0.06; P [is less than] 0.01). Growth hormone response to GHRH is modestly related to body composition but not to ADG in weanling beef heifers and likely has limited use in evaluation of growth performance in replacement beef heifers. Key Words: Beef Cattle, Hypothalamic-Releasing Hormones, Selection Criteria

Effects of calpastatin genotypes on growth and IGF-I concentration were examined in 47 purebred Angus bulls divergently selected for blood serum IGF-I concentration at the Eastern Ohio Resource Development Center (EORDC). Genotyping was performed by PCR-SSCP (single strand conformation polymorphism) and RFLP (restriction fragment length polymorphism) analysis of the calpastatin segments. The PCR primers were selected from the bovine calpastatin cDNA for the unique domain L (CAST1, CAST5, and CAST6), domain 1 (CAST10), and domain IV (CAST28). The IGF-I concentration on d 28 (IGF28), 42 (IGF42), and 56 (IGF56) of the 140-d postweaning test was measured. Birth weight (BW), weaning weight (WW), on-test weight (ONW), weight on d 28 (W28), 42 (W42), and 56 (W56) of the postweaning test, and off-test weight (OFW) also were measured. Fixed effects in this analysis were genotypes, IGF-I selection line (high vs. low), age of dam, and a covariate for age of bull. Genetic polymorphisms among individuals were observed for all calpastatin segments. The CAST1 genotypes influenced WW (P=.06), W28 (P=.06), W42 (P=.04), W56 (P=.04), and W140 (P=.01), but no influence was detected on BW or IGF-I traits. The CAST6 (P=.06), CAST10 (P=.01), and CAST28 (P=-.04) genotypes explained variation in BW. The CAST5 and CAST10 genotypes accounted for significant variation in the IGF-I traits. Genetic variants discovered in this study may provide useful information for future selection programs. Key Words: Calpastatin, IGF-I, PCR- SSCP