Auchtung T L, Barao S M, Dahl G E
Department of Animal and Avian Sciences, University of Maryland, College Park 20742-2311, USA.
J Anim Sci. 2001 Aug;79(8):2217-23. doi: 10.2527/2001.7982217x.
Previously, GH response to GHRH challenge at weaning has been shown to be indicative of ADG during a standard postweaning growth performance test in Angus cattle. In this study, we tested the hypothesis that GH response to GHRH before weaning would predict postweaning ADG. Bulls with the highest and lowest GH responses to GHRH over a 3-yr period, relative to their contemporaries, were used as sires, to allow for examination of the persistence of GH response to GHRH through selection. The selected calves in this study were sired by one of four Angus bulls chosen based on their GH response to GHRH (high response, n = 2; low response, n = 2). Forty-nine Angus calves (bulls, n = 24; heifers, n = 25) were challenged with GHRH at approximately 60, 105, and 150 d of age and at weaning (219 d; SD = 25). Blood samples were taken immediately prior to and 10 min following an i.v. clearance dose of 4.5 microg of GHRH/100 kg BW and, 2 h later, immediately prior to and 10 min following a challenge dose of either 1.5 or 4.5 microg of GHRH/100 kg BW. Two hours later, the procedure was repeated, with each calf receiving the other challenge dose. Body weight was measured every 28 d and ADG was calculated over a 140-d growth performance test (heifers and bulls maintained separately). Data were log-transformed for statistical analyses. In the selected bulls and heifers, response of GH to 1.5 microg of GHRH/100 kg BW at 60 and 105 d of age was positively related (P < 0.05) to postweaning ADG. Response to 4.5 microg of GHRH/100 kg BW at 105 d of age and at weaning was positively related (P < 0.01) to postweaning ADG. Inclusion of sire in the analysis improved the relationship between GH response and ADG for calves of sires with high GH responses from R2 = 0.18 (P = 0.01) to R2 = 0.33 (P = 0.02). When the GH response to GHRH of the unselected calves at weaning was added to the data from the selected animals and analyzed, the GH response of the bulls was related to postweaning ADG (R2 = 0.09; P = 0.04). In conclusion, GH response to GHRH as early as 60 d of age is indicative of postweaning ADG in beef cattle. In addition, the relationship between GH response to GHRH and postweaning ADG is improved with selection for greater GH response to GHRH.
此前研究表明，在安格斯牛的标准断奶后生长性能测试中，断奶时生长激素（GH）对生长激素释放激素（GHRH）刺激的反应可指示平均日增重（ADG）。在本研究中，我们检验了以下假设：断奶前GH对GHRH的反应可预测断奶后ADG。在3年期间内，相对于同代牛，对GHRH反应最高和最低的公牛被用作种公牛，以便通过选择来研究GH对GHRH反应的持续性。本研究中所选的犊牛由四头基于对GHRH的GH反应挑选出的安格斯种公牛之一所生（高反应组，n = 2；低反应组，n = 2）。49头安格斯犊牛（公牛，n = 24；母牛，n = 25）在大约60日龄、105日龄、150日龄和断奶时（219日龄；标准差 = 25）接受GHRH刺激。在静脉注射4.5微克GHRH/100千克体重的清除剂量之前及之后10分钟采集血样，2小时后，在注射1.5或4.5微克GHRH/100千克体重的刺激剂量之前及之后10分钟再次采集血样。2小时后，重复该程序，每头犊牛接受另一种刺激剂量。每28天测量一次体重，并在140天的生长性能测试中计算ADG（母牛和公牛分开饲养）。数据进行对数转换用于统计分析。在所选的公牛和母牛中，60日龄和105日龄时GH对1.5微克GHRH/100千克体重的反应与断奶后ADG呈正相关（P < 0.05）。105日龄和断奶时GH对4.5微克GHRH/100千克体重的反应与断奶后ADG呈正相关（P < 0.01）。在分析中纳入种公牛因素后，对于GH反应高的种公牛所生犊牛，GH反应与ADG之间的关系从R2 = 0.18（P = 0.01）改善至R2 = 0.33（P = 0.02）。当将未选犊牛断奶时对GHRH的GH反应数据添加到所选动物的数据中进行分析时，公牛的GH反应与断奶后ADG相关（R2 = 0.09；P = 0.04）。总之，早在60日龄时GH对GHRH的反应可指示肉牛断奶后ADG。此外，通过选择对GHRH有更大GH反应，GH对GHRH的反应与断奶后ADG之间的关系得到改善。
