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Estrous Control and Calving Performance in Beef Cattle Fed 6-Methyl-17-Acetoxy-Progesterone Under Ranch Conditions

Montana State University, Bozeman

Abstract

Two trials, one with 47 heifers and one with 149 cows of various ages and parities, were conducted to determine the effect of feeding 6-methyl-17-acetoxy-progesterone (MAP) on estrous synchronization and calving performance.

Concentrate to provide 180 mg. of MAP per animal per day was offered to 130 animals for 18 days, at one or two feedings per day. Sixty-six cows served as controls.

MAP suppressed estrus during the feeding period and 51 % of the females were in estrus between 42 and 66 hr. after withdrawal of MAP. The proportion in estrus between 18 and 78 hr. after withdrawal of MAP was 87% in Trial I and 55% in Trial II. The number of daily feedings of MAP had no significant effect on the proportion of cows in estrus within 84 hr. after last MAP.

The calving rate to breeding during first estrus was 33% in 119 treated animals, not significantly different from 37%, among 60 controls. The calving rate to breeding during three estrous cycles was 97% for treated animals and 83% for controls which is significantly (P<.01) different.

The conception rates in groups inseminated once, twice, or three times were 25%, 38%, and 21% respectively. These differences in conception rate due to number of inseminations were not significant (P<.05).

¹ Present address: Animal Science Department, Animal Genetics, University of Illinois, Urbana, Illinois,61801.

² Department of Medicine, University of Oregon Medical School, Portland, Oregon.

³ The authors acknowledge the assistance of P. J. Dziuk and H. W. Norton in preparation of manuscript and of R. Gessert, Upjohn Company, Kalamazoo, Michigan who provided the MAP.

⁴ Approved for publication as Journal Article No.713 by Director of Montana Agricultural Experiment Station, Bozeman, Montana.