Evidence was found that supports the existence of a major gene (designated as the slick hair gene), dominant in mode of inheritance, that is responsible for producing a very short, sleek hair coat. Cattle with slick hair were observed to maintain lower rectal temperatures (RT). The gene is found in Senepol cattle and criollo (Spanish origin) breeds in Central and South America. This gene is also found in a Venezuelan composite breed, the Carora, formed from the Brown Swiss and a Venezuelan criollo breed. Two sets of backcross matings of normal-haired sire breeds to Senepol crossbred dams assumed to be heterozygous for the slick hair gene resulted in ratios of slick to normal-haired progeny that did not significantly differ from 1:1. Data from Carora x Holstein crossbred cows in Venezuela also support the concept of a major gene that is responsible for the slick hair coat of the Carora breed. Cows that were 75% Holstein: 25% Carora in breed composition segregated with a ratio that did not differ from 1:1, as would be expected from a backcross matinginvolving a dominant gene. The effect of the slick hair gene on RT depended on the degree of heat stress and appeared to be affected by age and/or lactation status. The decreased RT observed for slick-haired crossbred calves compared to normal-haired contemporaries ranged from 0.18 to 0.4 degrees C. An even larger decrease in RT (0.61 degrees C; P < 0.01) was observed in lactating Carora x Holstein F1 crossbred cows, even though it did not appear that these cows were under severe heat stress. The improved thermotolerance of crossbred calves due to their slick hair coats did not result in increased weaning weights, possibly because both the slick and normal-haired calves were being nursed by slick-haired dams. There were indications that the slick-haired calves grew faster immediately following weaning and that their growth during the cooler months of the year was not compromised significantly by their reduced quantity of hair. In the Carora x Holstein crossbred cows there was a positive effect of slick hair on milk yield under dry, tropical conditions.

Two trials were conducted with heifers to determine heat tolerance among temperate Bos taurus (Angus, Hereford), Bos indicus (Brahman), tropical Bos taurus (Senepol, Romosinuano), and the reciprocal crosses of Hereford and Senepol. Differences among breeds in temperament score, circulating concentrations of cortisol, and blood packed cell volume were also investigated. Trial 1 used 43 Angus, 28 Brahman, 12 Hereford, 23 Romosinuano, 16 Senepol, 5 Hereford x Senepol (H x S), and 5 Senepol x Hereford (S x H) heifers. Trial 2 used 36 Angus, 31 Brahman, 9 Hereford, 14 Senepol, 19 H x S, and 10 S x H heifers. On the hottest summer date in Trial 1, rectal temperature of Angus was greater (P < .001) than that of Brahman, Senepol, or Romosinuano. Rectal temperature and plasma cortisol were significantly less in Senepol than in Brahman, suggesting that the differences in rectal temperature between these breeds may be due to differences in stress response possibly related to differences in temperament. Reciprocal crosses of Hereford and Senepol had rectal temperatures nearly as low as that of Senepol and displayed substantial heterosis (-9.4%, P < .05) in log10 rectal temperature on the hottest summer date. On both the hottest and coolest dates in Trial 1, Angus heifers had significantly faster respiration rates than Brahman, Romosinuano, or Senepol heifers, and Brahman had significantly slower respiration rates than Romosinuano or Senepol. On the hottest summer date in Trial 2, rectal temperature in Angus heifers was greater (P < .001) than in Brahman or Senepol had rectal temperatures similar to that of Senepol, or heterosis for log10 rectal temperature was similar to that in Trial 1 (-9.8%, P < .05). Considering rank order among breeds, Brahman always had the slowest respiration rate and greatest packed cell volume. Brahman had significantly greater temperament scores and plasma cortisol concentrations than Angus or Senepol, except that plasma cortisol was not different between Brahman and Senepol on the hottest summer date. On this date, rectal temperature did not differ between Brahman and Senepol, which supports the hypothesis that there is a relationship between response to stress and rectal temperature that helps explain differences in rectal temperature between Brahman and Senepol. The results of these trials demonstrate heat tolerance of the Senepol and Romosinuano, two Bos taurus breeds. Furthermore, the results suggest a substantial level of dominance of the Senepol's ability to maintain constant body temperature in a hot environment as measured by rectal temperature in crosses with a non-adapted breed.

Heritabilities and genetic and phenotypic correlations were estimated from feedlot and carcass data collected from Brahman calves (n = 504) in central Florida from 1996 to 2000. Data were analyzed using animal models in MTDFREML. Models included contemporary group (n = 44; groups of calves of the same sex, fed in the same pen, slaughtered on the same day) as a fixed effect and calf age in days at slaughter as a continuous variable. Estimated feedlot trait heritabilities were 0.64, 0.67, 0.47, and 0.26 for ADG, hip height at slaughter, slaughter weight, and shrink. The USDA yield grade estimated heritability was 0.71; heritabilities for component traits of yield grade, including hot carcass weight, adjusted 12th rib backfat thickness, loin muscle area, and percentage kidney, pelvic, and heart fat were 0.55, 0.63, 0.44, and 0.46, respectively. Heritability estimates for dressing percentage, marbling score, USDA quality grade, cutability, retail yield, and carcass hump height were 0.77, 0.44, 0.47, 0.71, 0.5, and 0.54, respectively. Estimated genetic correlations of adjusted 12th rib backfat thickness with ADG, slaughter weight, marbling score, percentage kidney, pelvic, and heart fat, and yield grade (0.49, 0.46, 0.56, 0.63, and 0.93, respectively) were generally larger than most literature estimates. Estimated genetic correlations of marbling score with ADG, percentage shrink, loin muscle area, percentage kidney, pelvic, and heart fat, USDA yield grade, cutability, retail yield, and carcass hump height were 0.28, 0.49, 0.44, 0.27, 0.45, -0.43, 0.27, and 0.43, respectively. Results indicate that sufficient genetic variation exists within the Brahman breed for design and implementation of effective selection programs for important carcass quality and yield traits.

To determine breed differences in ovarian function and endocrine secretion, daily rectal ultrasonography was conducted on multiparous lactating Angus (temperate Bos taurus; n = 12), Brahman (tropical Bos indicus; n = 12), and Senepol (tropical Bos taurus; n = 12) cows during an estrous cycle in summer. Blood was collected daily to quantify plasma concentrations of FSH, LH, progesterone, estradiol, GH, insulin-like growth factor (IGF)-I, IGF-II, IGF binding proteins (IGFBP), insulin, glucose, and plasma urea nitrogen (PUN). Numbers of small (2 to 5 mm), medium (6 to 8 mm), and large follicles (> or = 9 mm) were greater (P < .05) in Brahman than in Angus and(or) Senepol cows. Length of the estrous cycle (SEM = .6 d) was similar (P > .10) among Senepol (20.4 d), Angus (19.5 d), and Brahman (19.7 d) cows. Senepol cows had greater (P < .05) diameters of the corpus luteum (CL) and a delayed regression of the CL as compared with Angus cows. The secondary surge of FSH (between d 1 and 2; d 0 = estrus) was greater in Angus than Brahman or Senepol cows (breed x day, P < .05). Between d 2 and 14 of the estrous cycle, concentrations of progesterone, LH, IGF-II, and binding activities of IGFBP-3, IGFBP-2, and the 27- to 29-kDa IGFBP in plasma did not differ (P > .10) among breeds. Concentrations of GH, IGF-I, insulin, and PUN were greater (P < .001) and binding activities of the 22-kDa and 20-kDa IGFBP tended (P < .10) to be greater in plasma of Brahman than in Angus or Senepol cows. Plasma glucose concentrations were greater (P < .05) in Senepol than in Brahman or Angus cows. In conclusion, Brahman (Bos indicus) and Senepol cows (tropical Bos taurus) had greater numbers of follicles in all size categories and greater diameter of CL than Angus (temperate Bos taurus) cows. These ovarian differences may be due to changes in the pattern of secretion of FSH, insulin, IGF-I, and GH but not LH, IGF-II, or IGFBP-2 or -3.

The effects of nutrition on plasma concentrations of insulin-like growth factor-I (IGF-I) were characterized in steers under basal conditions and following single i.m. injection of bovine growth hormone (bGH, .1 mg/kg BW). Nutritional effects on IGF-I were studied in three trials. In all trials steers were individually fed and penned Angus or Hereford x Angus (280 kg). In the first trial, two diets (LPLE1: 8% CP and 1.96 Mcal ME/kg, 4.5 kg.hd-1.d-1; MPHE1: 11% CP, 2.67 Mcal ME/kg, 6.5 kg.hd-1.d-1) were fed (n = 5/diet). Plasma IGF-I concentrations averaged 74 (LPLE1) and 152 (MPHE1) ng/ml (P less than .02). Following bGH injection, IGF-I increased to peak concentrations between 12 and 24 h (averaging 105 and 208 ng/ml at peak for LPLE and MPLE, respectively, P less than .01). In the second trial, steers were fed diets composed of 8, 11 or 14% CP and 1.96 or 2.67 Mcal ME/kg dry matter (6.35 kg.hd-1.d-1 in a factorial arrangement for 84 d, n = 4/diet). Within the low ME diet groups, plasma IGF-I was similar in steers fed 11 and 14% CP but greater at these two CP levels than in steers fed 8% CP (P less than .05). Within the high ME diet groups, plasma IGF-I increased linearly with CP (P less than .01). In the third trial, steers were fed diets to result in a negative N status. Insulin-like growth factor-I was lower (P less than .02) during feed restriction than when steers were full-fed. The IGF-I response to bGH was diminished or absent in underfed steers (P less than .01). These data are interpreted to suggest that diet composition and intake affect plasma concentrations of IGF-I in steers. In cattle, CP may be the primary nutritional determinant of basal IGF-I, but the IGF-I response to CP may be affected by the available ME. Undernutrition can attenuate the IGF-I response to GH and uncouple the regulation of IGF-I normally ascribed to GH.