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University of Melbourne, Werribee 3030, Australia
Abstract
An experiment was conducted to determine the effect of serving capacity of bulls on herd fertility. Four groups of three bulls were mated to 114 heifers each for a 6-week period. Two high serving capacity groups HI and H2 comprised bulls which completed a mean 10.3 and 9.3 services, respectively, while two medium serving capacity groups Ml and M2, consisted of bulls achieving a mean 3.0 and 2.3 services, respectively, in a 7.5-hr pasture mating period.
In the first 3 weeks of mating, 95.1% of heifers exhibited estrus. Each bull mating group, irrespective of its serving capacity, was highly effective in detecting estrous heifers (96.2% to 100%). Observations in the mating pastures revealed that estrous heifers concentrated themselves into a sexually active group which roamed extensively about the pasture and which, through its mobility and the mounting activity within it, attracted pro-estrous heifers into it. To detect estrous heifers, bulls merely had to locate the sexually active group, which they appeared to do by visual rather than olfactory cues.
The first oestrus conception rates of heifers mated to bull groups HI, H2, Ml and M2 were 81.0%, 73.6%, 60.7% and 56.7%, respectively. After 6 weeks of mating the highly significant 18.5% difference in conception rate between groups HI and H2 and groups Ml and M2 was reduced to a 2.3% difference in pregnancy rate (90.5% and 88.2%, respectively). However heifers mated to high serving capacity bulls conceived (P<.01) earlier in the mating period than heifers mated to bulls of medium serving capacity.
1 This study was supported by a grant from the Australian Meat Research Committee. The author is indebted to Dr. D. B. Galloway for advice and assistance and to J. Norman, A. Lade, M. and J. Halls, L. Cummins, J. Graham, P. Guthrie, G. Saul and G. Fruin for technical assistance. Mr. G. Walsgott generously made facilities, land and animals available for this study.
2 Present address: Pastoral Research Institute, Department of Agriculture Victoria, Hamilton 3300, Australia.
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