The relationship between composition of gain and circulating hormones in growing beef bulls fed three dietary crude protein levels

• Published in: Journal of animal science Vol. 66; no. 12; pp. 3059 - 3067
• Main Authors: Anderson, P.T, Bergen, W.G, Merkel, R.A, Enright, W.J, Zinn, S.A, Refsal, K.R, Hawkins, D.R
• Format: Journal Article
• Language: English
• Published: United States Am Soc Animal Sci 01-12-1988
• Subjects: Animals; BEEF CATTLE; BLOOD; Body Composition; BOVIN DE BOUCHERIE; BULLS; CARCASS COMPOSITION; Cattle - growth & development; COMPOSICION DE LA CANAL; COMPOSITION DE LA CARCASSE; CRECIMIENTO; CROISSANCE; CRUDE PROTEIN; DIET; DIETA; Dietary Proteins - administration & dosage; GAIN DE POIDS; GANADO DE CARNE; GANANCIA DE PESO; GLUCOCORTICOIDE; GLUCOCORTICOIDES; GLUCOCORTICOIDS; GROWTH; Growth Hormone - blood; HORMONAS DE LA TIROIDE; HORMONAS PITUITARIAS; HORMONE HYPOPHYSAIRE; HORMONE THYROIDIENNE; Hydrocortisone - blood; Insulin - blood; Insulin-Like Growth Factor I - blood; insulin-like growth factors; Male; PITUITARY HORMONES; PROTEINA BRUTA; PROTEINE BRUTE; REGIME ALIMENTAIRE; SANG; SANGRE; Somatomedins - blood; TAUREAU; TESTOSTERONAS; TESTOSTERONE; Testosterone - blood; THYROID HORMONES; Thyroxine - blood; TORO; Triiodothyronine - blood; WEIGHT GAIN
• ISSN: 0021-8812; 1525-3163; 0021-8812
• DOI: 10.2527/jas1988.66123059x

Twenty-one Simmental crossbred bulls (311 +/- 11 kg, 9 mo of age) were used to determine the effect of feeding 10, 12 or 14% CP on concentration of hormones in blood and the relationship of these hormones to composition of gain. Six bulls were slaughtered on d 0 to provide an estimate of initial carcass composition (9-11 rib section). Remaining bulls were assigned to dietary treatments. Blood samples were collected every 30 min from 0800 to 2000 on d 0, 66, 136 and 202 of treatment; bulls were slaughtered on d 203. Across all treatments, growth hormone (GH) declined (P less than .05) from d 0 to d 202. Free insulin-like growth factor I (IGF-I) was lowest (P less than .05) on d 0. In four randomly selected bulls, IGF-I fluctuated during the 12-h sampling periods. Within each treatment group, insulin was greatest on d 202 (P less than .05). Testosterone (T) increased from d 0 to d 66, then declined. Cortisol (C) was lowest on d 66. Thyroid hormones increased (P less than .05) after d 0. Growth hormone and IGF-I were correlated negatively with carcass fat percentage, fat accretion rate and fat thickness. IGF-I concentrations were correlated positively with percentage of carcass protein. Testosterone:cortisol ratio was not related to composition, but high T coupled with low C may be related to carcass leanness (mean carcass fat = 24.4%). These data suggest that GH and IGF-I are the hormones most related to composition of gain in growing beef bulls.