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Growth Hormone Response to Growth Hormone Releasing Hormone in Calves that Differ in Genetic Merit for Milk Yield

W. J. Weber¹, L. H. Baumgard^1,, G. W. Kazmer², S. A. Zinn², L. B. Hansen¹, H. Chester-Jones¹ and B. A. Crooker¹

¹ Department of Animal Science, University of Minnesota, St. Paul 55108
² Department of Animal Science, University of Connecticut, Storrs 06269

Corresponding author: B. A. Crooker; e-mail: crook001{at}umn.edu.

Holstein heifer, steer, and bull calves from control (CL) and select (SL) lines of cows that differed by more than 4000 kg of milk during a 305-d lactation (SL > CL) were used to determine growth hormone (GH) response to 5 doses of GH releasing hormone (GHRH) and how this response was affected by gender, period (age), and genetic merit for milk yield. Doses (0, 2.5, 5, 10, and 20 µg/100 kg of BW) of a GHRH analog were assigned randomly to each heifer (4 CL, 4 SL), steer (4 CL, 4 SL), and bull (3 CL, 3 SL) calf and administered on consecutive days at approximately 3, 6, and 10 mo of age (periods; P1, P2, and P3). Jugular blood samples (n = 15) collected between –30 and 240 min relative to GHRH administration were used to quantify area under the GH response curve (AUC) after subtracting mean prechallenge GH concentrations. Estimates of maximum response (R_max) and sensitivity (ED₅₀) to GHRH were obtained from the hyperbolic dose response curves (AUC vs. dose). Data were analyzed for effects of dose, line, period, gender, and their interactions with period as the repeated effect. Prechallenge GH concentrations were not affected by genetic line, gender, or period. The AUC was not affected by line, but decreased with period and increased with GHRH dose. The R_max did not differ between lines or among genders, but decreased with period. The ED₅₀ did not differ between lines or among periods, but heifers were more sensitive to GHRH than steers or bulls. Although GH response to GHRH has been identified as a potential indicator of genetic merit, it did not differ between these substantially different genetic lines.

Key Words: calf • genetic selection • milk yield • growth hormone

Abbreviation key: AUC = area under the GH response curve, CL = control line, ED₅₀ = effective dose required to elicit a 50% maximal response, GH = growth hormone, GHRH = growth hormone releasing hormone, P = period, R_max = maximum response, SL = select line

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				W. J. Weber, C. R. Wallace, L. B. Hansen, H. Chester-Jones, and B. A. Crooker Effects of Genetic Selection for Milk Yield on Somatotropin, Insulin-Like Growth Factor-I, and Placental Lactogen in Holstein Cows J Dairy Sci, July 1, 2007; 90(7): 3314 - 3325. [Abstract] [Full Text] [PDF]