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Abstract
The percentages of suckling cows or heifers detected in oestrus during artificial breeding programmes were compared in groups of animals which had or had not been run with vasectomised bulls for 18–21 days before the start of the programmes. The presence of the bulls during this premating period was associated with a significant increase in the percentage of suckling cows subsequently detected in oestrus during a 19-day spring insemination programme (69% of 42 v. 40% of 35). Comparable effects were not detected in three groups of heifers and one group of autumn calving cows.