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The Relationship between Composition of Gain and Circulating Hormones in Growing Beef Bulls Fed Three Dietary Crude Protein Levels^1,2,

Michigan State University, East Lansing 48824

Abstract

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 concentrations 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 < .05) from d 0 to d 202. Free insulin-like growth factor I (IGF-I) was lowest (P < .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 < .05). Testosterone (T) increased from d 0 to d 66, then declined. Cortisol (C) was lowest on d 66. Thyroid hormones increased (P < .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.

¹ Michigan Agric. Exp. Sta. Rep. No. 12488.

² Dept. of Anim. Sci.

³ Animal Health Diagnostic Lab.

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