Summary. A PGF-2α analogue (cloprostenol) was injected into 9 cows to synchronize oestrus. The cows were placed with or without a bull in a free-stall in groups of 3 at 46 h after the injection. Sexual behaviour was observed and serum LH concentrations were measured during the next 34 h.
The bull ejaculated with the cows in regular sequence and did not return to cows after an ejaculatory series was completed. The mating behaviour of the bull was closely related to the LH surge of the cows.
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