Prostaglandin responses in dairy herd breeding programmes

A re-evaluation of results from ten trials conducted in commercial dairy herds between 1974 and 1981 in which lactating dairy cows were injected once or twice with either of two forms of prostaglandin F2alpha (PGF) showed that: the variation in the post-injection interval to oestrus was influenced by the stage of the oestrous cycle at which treatment was administered; this variation was sufficient to reduce pregnancy rates to set-time inseminations; an accurate aid for oestrus detection, such as tail painting, should be used routinely with PGF; if PGF-treated cows were detected in oestrus before being inseminated, the pregnancy rates to first insemination were usually 10% higher than in untreated herd mates. The results show that PGF systems in dairy herds should not be used as au alternative to accurate oestrus detection. These systems can be used to condense the breeding programme and increase pregnancy rates to first insemination. The exploitation of this fertility effect justifies the reassessment of PGF usage in New Zealand dairy herds.