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University of Wyoming, Laramie 82071, Livestock and Range Research Station, US Department of Agriculture and and Montana Agricultural Experiment Station, Miles City 59301, Cooperating
Abstract
An experiment was designed to determine the effects of dietary monensin on age and weight at puberty in Bos taurus crossbred beef heifers. One hundred and forty heifers were allotted to two weight classes: heavy (H) and light (L) groups, consisting of heifers above and below the average weaning weight, respectively. Heifers within each weight class were assigned to one of three diets during a 203-d winter feeding period: (1) R, 80% roughage: 20% concentrate; (2) Ml, R diet plus monensin (200 mg/head daily) with feed intake restricted to produce average daily gains (ADG) similar to R and (3) M2, R diet plus monensin (200 mg/head daily). ADG was higher (P<.001) for M2 heifers than for R and Ml heifers, which performed similarly. Age and weight at puberty were similar across treatments in L heifers. However, in the H group, HM1 and HM2 heifers were younger (P<.07) at puberty than HR heifers and this difference was not removed by covariate adjustment of either ADG or body weight. Weight at puberty was greater (P<.01) for HM2 heifers than for HM1 or HR heifers; however, this difference was removed by covariate adjustment of either ADG or body weight. Ninety-one percent of LM2 heifers and 100% of all other groups had reached puberty before the breeding season. Pregnancy rates did not differ significantly between treatment groups. Supplementation of a high roughage diet with monensin decreased age at puberty of heifers in the heavy weight class and this decrease was not due to increased body weight or ADG. Perhaps heifers with above average weaning weight possess a greater inherent growth potential than heifers below the average weaning weight and hence are more capable of utilizing monensin to an advantage.
1 This study was a contribution to Western Regional Research Project W-112, "Reproductive Performance in Beef Cattle." Publication has been approved by the Director of the Montana Agr. Exp. Sta., Journal Series No. 1184.
2 Present address: The Upjohn Co., Exp. Anim. Sci., Kalamazoo, MI 49001.
3 Division of Anim. Sci., Univ. of Wyoming, Laramie 82071.
4 Livestock and Range Res. Sta., USDA-ARS, Miles City, MT 59301.
5 The authors gratefully acknowledge Mr. J. McAskill and the Eli Lilly and Co. for their technical assistance and financial support.
6 Mention of trade names or companies does not constitute an implied warranty or endorsement by the authors or by the USDA.
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