Experimental and practical approaches to the establishment and maintenance of pregnancy.

A series of three large field trials was carried out to assess the effect of buserelin on fertility in dairy cows. In the first, 10 micrograms buserelin was injected on the day of insemination. There were no significant effects on fertility parameters compared to untreated controls. In the second trial cows were injected on day 12 after insemination. Mean pregnancy rates to first insemination were 53.4 and 65.4% for control and treated cows respectively (P < 0.01). Mean pregnancy rates to repeat inseminations were 52.9 and 59.4% for control and treated cows (NS). Mean calving to conception intervals were 91.4 and 85.3 days (P < 0.01) and the incidence of barren cows was 10.2 and 5.3% (NS). Overall the economic benefit of buserelin injection on day 12 was calculated to be 27.43 pounds per cow treated excluding the cost of the treatment. In trial 3 cows were injected with buserelin either on day 8 or 10 after insemination. There were no significant effects on fertility parameters compared to untreated control cows. In a fourth trial ewes were injected with 4 micrograms buserelin on day 12 after service. There were indications that both pregnancy rate and lambing percentage could be increased by buserelin treatment. Daily blood samples were collected from 5 dairy cows during a control cycle and a cycle in which 10 microgram buserelin was injected on days 11 and 13. Cycle length was unaffected by treatment and the concentration and pattern of progesterone secretion did not differ between control and treated cycles. Plasma oestradiol concentrations were similar in the control and treated cycles before day 11. However from day 12 to 16, equivalent to the time of maternal recognition of pregnancy, the mean concentration of oestradiol was significantly reduced in the treated cycle. As oestradiol stimulates both the development of uterine oxytocin receptors and the secretion of PGF2 alpha we suggest that any improvement in pregnancy rate after buserelin is due to a weakened luteolytic mechanism, resulting from a lower plasma oestradiol concentration.