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Serving Capacity of Crossbred Yearling Beef Bulls. I. Single-Sire Mating Behavior and Fertility during Average and Heavy Mating Loads at Pasture^1,2,

Kansas State University^,3, Manhattan 66506 and U.S. Department of Agriculture^,5, Clay Center, NE 68933

Abstract

Eighty crossbred, virgin, yearling beef bulls were subjected to three serving capacity (SC) tests. Ten low SC (LSC) and 10 high SC (HSC) bulls similar in weight (avg wt = 452 kg), testicular size and seminal traits but differing in average services were selected. Objectives were to evaluate the mating behavior and fertility of LSC and HSC bulls at pasture when exposed to an average and a heavy mating load and to determine the effect of sexual activity on body weight and testicular characteristics. One LSC and one HSC bull per block were exposed single-sire to 25 naturally cyclic (N) cows for 3 d and to 9 estrus-synchronized (S) cows for 1 d in a randomized complete block design consisting of 10, 4-d blocks. Bulls were fitted with pedometers to record distance travelled. Behavioral data were collected by periodic visual observation. during the N cow treatment, LSC bulls had fewer (P < .05) services per cow, total services and a higher mount to service ratio than HSC bulls. during the S cow treatment, LSC bulls showed increased sexual activity, though they achieved fewer (P < .09) services per cow. Pregnancy rates for LSG and HSC bulls did not differ (P > .20) and across SC groups averaged 53.6% for N cows and 31.9% for S cows. Distance travelled also was similar (P > .20) between SC groups for both treatments. Sexual activity during pasture exposure had no effect (P > .20) on testicular characteristics but resulted in an average decline in body weight of 25.7 kg across SC groups during the 4-d period. Post-exposure SC tests found that LSC bulls had increased (P < .05) services, decreased mounts and mount to service ratio compared to pre-exposure measurements, but LSC were still lower (P = .06) than HSC bulls for average services. These results suggest that LSC virgin, yearling bulls should be offered sexual experience and retested before their inherent SC can be determined.

¹ Contribution No. 88-288-J from the Kansas Agric. Exp. Sta., Manhattan 66506.

² The authors gratefully acknowledge the technical assistance of Al Kruger, Ed Mohney, Steve Furman and Tom Garvin in recording behavioral data, measurements, semen collection and evaluation, and handling of animals.

³ Dept. of Anim. Sci. and Ind.

⁴ Present address: Dept. of Anim. Sci., Colorado State Univ., Fort Collins 80523.

⁵ Reprod. Res. Unit, Roman L. Hruska U.S. Meat Anim. Res. Center, ARS.