Reproductive performance of 714 Holstein Friesian dairy cows was monitored between October 1995 and June 1998 using thrice weekly milk progesterone determinations. Defined endocrine parameters such as interval topost-partumcommencement of luteal activity, inter-ovulatory interval and length of luteal and inter-luteal intervals were used with a number of traditional measures of reproductive performance to investigate the current status of fertility in a sample of United Kingdom dairy herds. A comparison of the results of the 1995 to 1998 trial with those of a previous (1975 to 1982) milk progesterone database, which included 2503 lactations in British Friesian cows monitored using a similar milk sampling protocol, revealed a decline infertility between these periods.Between 1975-1982 and 1995-1998, pregnancy rate to first service declined from 55·6% to 39·7% (P< 0·001), at a derived average rate approaching 1% per year. This decline was associated with an increase (P< 0·001) in the proportion of animals with one or more atypical ovarian hormone patterns from 32% to 44%. There was a significant (P< 0·001) increase in the incidence of delayed luteolysis during the first cyclepost partum(delayed luteolysis type I; 7·3% to 18·2%) and during subsequent cycles (delayed luteolysis type II; 6·4% to 16·8%), although the incidence of prolonged anovulation post partům (delayed ovulation type I; 10·9% to 12·9%) and prolonged inter-luteal intervals (delayed ovulation type II; 12·9% to 10·6%) did not alter significantly. These changes resulted in an increase in mean luteal phase length from 12·9 (s.e. 0·09) to 14·8 (s.e. 0·17) days and an increase in inter-ovulatory interval from 20·2 (s.e. 0·1) to 22·3 (s.e. 0·2) days. The decline infertility was also reflected in traditional measures of fertility since although interval to first service remained relatively unchanged (74·0 (s.e. 0·4) to 77·6 (s.e. 1·1) days) calving interval lengthened from 370 (s.e. 2·2) to 390 (s.e. 2·5) days. Collectively these changes may have contributed to the decline in pregnancy rates observed over the last 20 years.

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 ofP 4 levels >3 ng/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 an 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.

Summary. Milk

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The effects of administration of progesterone and oestradiol on ovine endometrial oxytocin receptor concentrations and plasma concentrations of 13,14-dihydro-15-keto prostaglandin F-2 alpha (PGFM) after oxytocin treatment were determined in ovariectomized ewes. Ewes received progestagen pre-treatment, progesterone and/or oestradiol in 11 different treatment schedules. Progestagen pre-treatment decreased oxytocin receptor concentrations in endometrium from ewes treated subsequently with either progesterone for 5 days or progesterone for 5 days plus oestradiol on Days 4 and 5 of progesterone treatment. Oestradiol increased endometrial oxytocin receptor concentrations when administered on Days 4 and 5 of 5 days progesterone treatment. Progestagen pre-treatment followed by progesterone treatment for 12 days caused a large increase in oxytocin receptors and no further increase occurred when ewes were given oestradiol on Days 11 and 12, or when progesterone was withdrawn on Days 11 and 12, or these two treatments were combined. Oxytocin administration caused an increase in plasma PGFM concentrations in ewes which did not receive progestagen pre-treatment, and subsequently received progesterone treatment for 5 days and oestradiol treatment on Days 4 and 5 of progesterone treatment. Similarly treated ewes which received progestagen pre-treatment did not respond to oxytocin. Oxytocin administration also increased plasma PGFM concentrations in ewes which received progestagen pre-treatment followed by progesterone treatment for 12 days, progesterone treatment for 12 days plus oestradiol on Day 11 and 12 of progesterone treatment, progesterone withdrawal on Day 11 and 12, or progesterone withdrawal and oestradiol treatment combined. The results indicate that (1) progesterone pre-treatment affects oxytocin receptor concentrations in the endometrium and uterine responsiveness to oxytocin and (2) progesterone treatment alone for 12 days after a treatment which mimics a previous luteal phase and oestrus is sufficient to induce oxytocin receptors and increase oxytocin-induced PGF release. These results emphasize the importance of progesterone and provide information which can be used to form an hypothesis for control of luteolysis and oestrous cycle length in the ewe.