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Gonadotropin-Releasing Hormone-Induced Luteinizing Hormone Release in Heifers:Effect of Nutrition during Gestation^1,2,

Texas A&M University and Texas Agricultural Experiment Station, College Station 77843

Abstract

The effects of nutrition during the last two trimesters of gestation on GnRH-induced LH release were assessed in crossbred heifers. Heifers (n = 58) were allotted at 90 d gestation to one of three levels of an experimental diet fed at 1, 1.5 or 2% of BW to attain maternal BW loss, BW maintenance or BW gain, respectively, at parturition. Twenty-two heifers were injected (i.m.) once with 100 µg GnRH between d 14 and 1 before parturition, and 32 heifers were injected (i.m.) once with 100 µg GnRH between d 8 and 21 after parturition. Jugular blood samples were collected before and at 30-min intervals after GnRH for 4 h. Least squares means for BW change differed (P < .01) among BW loss (–17.6%), BW maintenance (–6.0%) and BW gain (7.0%) heifers. Basal plasma LH concentration was not influenced by nutritional treatment and was similar before and after parturition for all groups. However, in response to GnRH, peak plasma LH concentration was greater (P < .10) for prepartum than for postpartum heifers. Mean LH peak amplitude in prepartum heifers was approximately twofold greater (P < .10) in the BW loss and maintenance groups compared with the BW gain group. Prepartum LH release was related inversely (r = –.64) to change in heifer BW and increased (P < .01) as BW loss increased during gestation. After parturition, mean LH peak amplitude and area under the response curve averaged 50% less (P < .10) in the BW loss and maintenance groups than in the BW gain group. Nutritional restriction of heifers during gestation dramatically, but differentially, impacts pre- vs post-partum pituitary response to a single injection of GnRH.

¹ Technical article No. 21770, Texas Agric. Exp. Sta. Reference to a company, breed or trade name does not imply approval or endorsement by the Texas Agric. Exp. Sta.

² The assistance of R. E. Knutson, B. Rucker, R. J. Allen, Jr., R. A. May and M. D. Rucker with diet preparation and animal care and of Nancy Turner with typing this manuscript is gratefully acknowledged.

³ Present address: USDA, ARS, R.L.H. Meat Animal Research Center, Clay Center, NE 68933.

⁴ Physiol. of Reprod. Section, Dept. of Anim. Sci.

⁵ Beef Cattle Nutr. and Growth Section, Dept. of Anim. Sci.

⁶ Present address: Resident Director, Texas A&M Univ. Agric. Res. and Ext. Center, Overton, TX 75684.