The phenotypic association between the interval to post-partum ovulation and traditional measures of fertility in dairy cattle

The relationship between the interval to post-partum (PP) commencement of luteal activity (CLA) and traditional measures of fertility was studied in 1733 lactations of British Friesian cows. Milk progesterone (P-4) concentrations were used to determine the interval to PP ovulation and to confirm pregnancy. Milk samples were taken thrice weekly from cows in 20 commercial herds and daily in the University of Nottingham research herd. Commencement of luteal activity was defined as the day of first PP occurrence of P-4 levels > 3 mu g/l which was then maintained for at least two consecutive readings in the commercial herds and for 4 days in the university herd. Records from animals with retained placenta, uterine infection or those treated with reproductive hormone preparations for reproductive disorders were excluded for this analysis from the original database of cows during 2503 lactations. The interval to PP CLA averaged 27.0 (s.d. 12.1) days (no. = 1733), interval to first service 71.2 (s.d. 19.9) days (no. = 1646), interval to conception 87.2 (s.d. 35.0) days (no. = 1510), number of services per conception 1.50 (s.d. 0.83, no. = 1510) and conception to first service was 0.65. Linear models were then fitted to subsets of the data with complete information to assess the influences of lactation number, season and herd (at least 1243 observations in all analyses). The interval to PP CLA was favourably correlated with measures of fertility such that for every day delay in the interval to CLA there was art average delay of 0.24 and 0.41 days (P < 0.001) in the interval to first service and conception, respectively. The number of services per conception was reduced by 0.11 for each additional 21 days in the interval between PP CLA and first service (P < 0.001). Furthermore, conception rate was influenced by the interval from the onset of luteal activity to first insemination (P < 0.01) such that each additional 21 days progressively reduced the probability of failure at a given insemination number to 0.89 of its previous value. There were significant effects of parity and season (P < 0.05) on the interval to first PP service while the variation among herds in the interval to PP service and days open was significant (P < 0.01). The regressions of measures of fertility on milk yield were not significant. This extensive database on the hormone profiles of PP cows shows that the early re-establishment of PP ovarian activity is an important prerequisite for high fertility.