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Title: USE OF GROWTH HORMONE RESPONSE TO GROWTH HORMONE-RELEASING HORMONE TO DETERMINE GROWTH POTENTIAL IN BEEF HEIFERS

Author
item AUCHTUNG, T. - UNIVERSITY OF MARYLAND
item Connor, Erin
item BARAO, S. - UNIVERSITY OF MARYLAND
item DOUGLASS, L. - UNIVERSITY OF MARYLAND
item DAHL, G. - UNIVERSITY OF MARYLAND

Submitted to: Journal of Animal Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/6/2001
Publication Date: N/A
Citation: N/A

Interpretive Summary: Growth hormone (GH) response to a challenge of growth hormone-releasing hormone (GHRH) was measured in 67 Angus heifers. Blood samples were collected immediately prior to and 10 min following a clearance dose of 4.5 mg GHRH/100 kg BW (injected i.v.) and, 3 h later, immediately prior to and 10 min following a challenge dose of either 1.5 or 4.5 mg GHRH/100 kg BW. Each animal received both challenge doses, which were randomly assigned across the 2 d of blood collection. Concentrations of GH in serum were measured by RIA. Body weight was measured every 28 d and hip height was measured at weaning and at the end of a 140-d growth test. Plasma was collected every 28 d during the growth test and assayed for IGF-I by RIA. Average daily gain (ADG) was calculated at the end of the growth test and carcass measurements were estimated by ultrasound 2 wk after the end of the growth test. Responses to the two GHRH challenges were dose-dependent. An ninverse relationship was observed between response to the 1.5 mg GHRH/100 kg BW dose versus ADG and intramuscular fat percentage. Mean plasma insulin-like growth factor I concentration was associated with increased ADG. Thus, GH response is an index of growth and carcass composition in heifers and may be an additional tool in selection of replacement beef heifers.

Technical Abstract: The hypothesis tested in this experiment was that GH response to a challenge of GHRH is predictive of rapid growth in the beef heifer. Growth hormone response to a challenge of GHRH was measured in 67 Angus heifers. Blood samples were collected immediately prior to and 10 min following a clearance dose of 4.5 mg GHRH/100 kg BW (injected i.v.) and, 3 hr later, immediately prior to and 10 min following a challenge dose of either 1.5 o 4.5 mg GHRH/100 kg BW. Each animal received both challenge doses, which were randomly assigned across the 2 d of blood collection. Concentrations of GH in serum were measured by RIA. Body weight was measured every 28 d and hip height was measured at weaning and at the end of a 140-d growth test. Plasma was collected every 28 d during the growth test and assayed for IGF-I by RIA. Average daily gain was calculated at the end of the growth test and carcass measurements were estimated by ultrasound 2 wk after the end of the growth test. Response to the two GHRH challenges was dose-dependent (P < 0.05). An inverse relationship (R2 = 0.05; P = 0.06) was observed between response to the 1.5 mg GHRH/100 kg BW dose and ADG. A similar inverse relationship (R2 = 0.06; P = 0.02) was observed between response to the 1.5 mg GHRH/100 kg BW dose and intramuscular fat percentage. Mean plasma IGF-I concentration was associated with increased ADG (R2 = 0.06; P < 0.0001). Thus, GH response is an index of growth and carcass composition in heifers and may be an additional tool in selection of replacement beef heifers.