EFFECTS OF GROWTH HORMONE-RELEASING FACTOR AND(OR) THYROTROPIN-RELEASING-HORMONE ON GROWTH, FEED-EFFICIENCY, CARCASS CHARACTERISTICS, AND BLOOD HORMONES AND METABOLITES IN BEEF HEIFERS

The objective of this study was to determine the effect of long-term a dministration of a growth hormone (GH)-releasing factor analog (GRFa) and(or) thyrotropin-releasing hormone (TRH) on growth, feed efficiency , carcass characteristics, and blood hormones and metabolites in beef heifers. Crossbred heifers (n = 48; 345.9 +/- 2.8 kg) were divided int o four equal groups: control (vehicle), 1 mug of GRFa (human GRF 1-29 analog).kg BW-1.d-1, 1 mug of TRH.kg BW-1.d-1, or GRFa + TRH. Daily s. c. injections continued for 86 d. Blood samples were collected from ha lf of the heifers after injection on d 1, 36, and 78. On d 89, all hei fers were slaughtered. Treatments did not affect (P > .05) ADG but GRF a + TRH decreased (P < .05) ADFI relative to all other treatments. Fee d conversion efficiency tended (P < .10) to be improved in the groups given GRFa alone or TRH alone. Treatment with GRFa and(or) TRH did not affect carcass weight, dressing percentage, conformation score, backf at thickness, or weights of liver, kidneys, pituitary, and ovaries. Th e GRFa + TRH treatment reduced (P < .05) fat score and increased (P < .05) longissimus muscle area relative to other treatments. The GRFa tr eatments reduced (P < .05) the weight and fat percentage of the mammar y gland and increased (P < .05) heart weight. Treatment with TRH alone failed to stimulate GH on d 1, 36, and 78. Treatment with GRFa alone increased (P < .05) GH above controls on d 36, whereas GRFa + TRH incr eased (P < .05) GH on d 1, 36, and 78. Treatment with GRFa alone incre ased (P < .05) IGF-I only on d 1, whereas GRFa + TRH was without effec t on all days. Across sampling days, treatments had little effect on b lood concentrations of insulin, triiodothyronine, nonesterified fatty acids, urea nitrogen, and glucose. The GRFa alone and GRFa + TRH decre ased (P < .05) and TRH alone increased (P < .05) thyroxine concentrati ons. In conclusion, with the dose and administration regimen used, GRF a and(or) TRH yielded small but positive improvements in animal perfor mance.