The relationship between composition of gain and circulating hormones in growing beef bulls fed three dietary crude protein levels.
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.
Anderson
P T
PT
Animal Health Diagnostic Lab., Michigan State University, East Lansing 48824.
Bergen
W G
WG
Merkel
R A
RA
Enright
W J
WJ
Zinn
S A
SA
Refsal
K R
KR
Hawkins
D R
DR
D000818:Animals; D001823:Body Composition; D002417:Cattle / Q000254:growth & development*; D004044:Dietary Proteins / Q000008:administration & dosage*; D013006:Growth Hormone / Q000097:blood*; D006854:Hydrocortisone / Q000097:blood*; D007328:Insulin / Q000097:blood*; D007334:Insulin-Like Growth Factor I / Q000097:blood*; D008297:Male; D013002:Somatomedins / Q000097:blood*; D013739:Testosterone / Q000097:blood*; D013974:Thyroxine / Q000097:blood*; D014284:Triiodothyronine / Q000097:blood*
D004044:Dietary Proteins; D007328:Insulin; D013002:Somatomedins; D014284:Triiodothyronine; D013739:Testosterone; D007334:Insulin-Like Growth Factor I; D013006:Growth Hormone; D013974:Thyroxine; D006854:Hydrocortisone