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Breeding Performance of Beef Bulls Assigned to Two-Sire Cow Groups on Pasture^1,2,

University of Georgia^,5, Tifton 31793-0748

Abstract

Angus, Polled Hereford and Santa Gertrudis bulls from ages 1 through 5 and 7 yr were assigned to 26 two-sire breeding groups. Each year, straightbred and crossbred cows of these breeds were allotted at random within breed composition, age of dam and calving date to breeding groups on pasture. Sires within each breeding group or pair were the same age at breeding and were two of the three breeds of sires. Neither calving rate nor the proportion of calves born by one vs the other sire in the two-sire breeding groups was affected by sire age among breeding groups. For a given breed, there was no uniformity among the sires in the proportion of calves they sired in their two-sire breeding groups. The proportion of calves bom for the 26 sire pairs averaged .64 vs .36 (SE = .04 for either high or low value) for one vs the other sire in a sire pair with no indication that calving rate was affected by unequal proportions of calves by sires within sire pairs. Cows calved significantly earlier in the calving period (b = –.775 ± .127) as calving rate increased among sire pairs. The number of days from the start of the breeding period to calf birth was affected by differences between sires in sire pairs for 8 of the 26 pairs, but there was no significant differences due to sire pair or breed of sire because of interaction between these two variables.

¹ The authors appreciate the cooperation of the personnel at the Rogers Correctional Institution, Reidsville, GA (of the Georgia Department of Corrections), which made it possible to conduct this research. Also, the authors gratefully acknowledge the assistance of B. G. Mullinix, Jr. for conducting the statistical analyses.

² Supported by State, Hatch and Regional S-10 funds.

³ Dept. of Anim. Sci.

⁴ College of Vet. Med., Univ. of Georgia, Athens.

⁵ Coastal Plain Sta.