Abstract The effects of introducing vasectomised bulls to spring calving beef cows 21 days before joining was studied in an experiment involving mixed age Angus and Friesian × Angus cows. On 20 October, 40 days after the mean calving date and one cycle before entire joining, cows were assigned by breed and age to one of eight replicates (n = 20 or 21). Two vasectomised bulls were joined with each of four replicates (early contact), and a further four replicates had no contact with bulls until the start of entire joining (control). Observations for signs of oestrus were made daily, and in addition, blood samples were collected from all cows before entire joining began to determine plasma progesterone concentrations. A greater proportion of early contact cows resumed ovarian function during the 21-day early contact period as assessed by elevated progesterone concentration (0.20 v. 0.05, P<0.05). More early contact cows had displayed oestrus by the end of the first week of entire joining, but by the end of t...
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