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Serving Capacity of Crossbred Yearling Beef Bulls. II. Summer Grazing Activity and Body Temperature Patterns during Average and Heavy Mating Loads at Pasture^1,2,3,

Kansas State University^,4, Manhattan, KS 66506 and U.S. Department of Agriculture^,6, Clay Center, NE 68933

Abstract

Ten low (LSC) and 10 high (HSC) serving capacity yearling bulls were exposed individually to 25 naturally cyclic (N) cows for 3 d (average mating load) and subsequently to 9 estrus-synchronized (S) cows for 1 d (heavy mating load) in a randomized complete block design consisting of 10, 4-d blocks. Bulls were fitted with vibracorders and temperature acquisition modules to record grazing activity and body temperature (BT), respectively. during the N cow treatment, LSC bulls had fewer (P < .05) services per cow and a higher mount to service ratio than HSC bulls, and LSC bulls tended (P = .12) to graze less total time than HSC bulls (7.8 vs 9.0 h/d, respectively). However, both groups of bulls exhibited similar diurnal grazing patterns with two major daily grazing periods; the first (0400 to 1300) peaked early in the morning (0600) and the second (1700 to 2200) occurred in late afternoon and evening. during the S cow treatment, LSC and HSC bulls did not differ (P = .60) in grazing time or pattern, but similar mating activity was exhibited by both SC groups while exposed to S cows. Bulls grazed little during the hottest part of the day (1300 to 1700), and LSC and HSC bulls spent only 2.2 and 2.4 h, respectively, grazing during dark, cooler periods near midnight. Body temperature did not differ between SC groups and averaged 39.3°C during N cow treatment and 39.4°C during S cow treatment. Body temperature had a distinct, monophasic diurnal pattern in bulls exposed to an average mating load. during exposure to S cows, BT pattern changed dramatically, and BT was .7 to 1.2°C higher (P < .05) when bulls were engaged in heavy mating activity. We conclude that LSC and HSC bulls did not differ in grazing activity or BT pattern, but that patterns of grazing activity and BT may be interrelated and that BT pattern can be influenced by heaving mating activity.

¹ Contribution No. 88-289-J from the Kansas Agric. Exp. Sta., Manhattan 66506.

² The authors gratefully acknowledge the loan of vibracorders from Don C. Adams, USDA, Miles City, MT.

³ The authors wish to thank Lynn Gose for his technical assistance with the temperature modules and Al Kruger, Ed Mohney, Steve Furman and Tom Garvin for the technical support they provided in recording behavioral data, obtaining measurements and handling of animals.

⁴ Dept. of Anim. Sci. and Ind.

⁵ Present address: Dept. of Anim. Sci., Colorado State Univ., Fort Collins 80523.

⁶ Roman L. Hruska U.S. Meat Animal Research Center, ARS.