Milk progesterone levels in relation to conception, repeat breeding and factors influencing acyclicity in dairy cows.

Milk samples were collected twice weekly from 535 dairy cows between parturition and the re-establishment of pregnancy to monitor ovarian activity by measurement of milk progesterone levels by radioimmunoassay. The mean progesterone levels after fertile and non-fertile first inseminations were similar from 21 days before until 13 days after insemination, those in non-pregnant animals declined while those in pregnant animals continued to rise until Day 22. Progesterone profiles of 47 repeat breeder cows (i.e. receiving greater than 3 inseminations) demonstrated that a variety of patterns was associated with the poor conception rate. A study of the post-partum interval showed that cows resumed cycles by 24 +/- 0.6 days after calving; the length of this interval varied significantly with the season of calving and non-significantly with the lactation number (i.e. age) of the cow, but was not related to yield. Of the 535 cows studied, 5.2% had not started to cycle within 50 days of calving and a further 5.2% resumed cycles which subsequently ceased. The progesterone profiles of acyclic cows indicated that after treatment with 0.5 mg LH-RH or a progesterone-releasing intravaginal device (PRID) for 14 days 75% of animals apparently ovulated but the calving to conception interval was not significantly altered compared with that of untreated control cows.