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

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Abstract
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 1st 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 > 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 [luliberin] 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.