Fertility of Lactating Dairy Cows Inseminated after Treatment with Cloprostenol

Abstract

Lactating dairy cows in winter were injected with two doses of cloprostenol 11 days apart to synchronize oestrus and inseminated (AI) 72 and 96 h after the second dose. Others, during late spring were given a single dose and inseminated if they were detected in oestrus; those not so detected were given a second dose 11 days after the first and inseminated (AI) 72 and 96 h later. A detailed study of 16 and 13 cows, randomly selected from those treated in winter and spring respectively, showed that with few exceptions cows at the time of one or other injection (or both) had raised milk progesterone levels, indicative of the presence of corpora lutea, and that marked falls in these levels ensued in the next 8 h and usually 24 h. Responses beyond this initial effect were of two types: those in which induced luteolysis was complete and irreversible and in which there was precise timing of oestrus, ovulation and corpus luteum formation, thus allowing fixed-time AI the possibility of success; and those in which luteolysis may or may not have been complete and in which the subsequent precise timing of ovarian events did not occur, thus precluding the success of fixed-time AI. This latter response was much more prevalent in the herd inseminated in winter. In this herd mean milk progesterone levels before cloprostenol treatment were lower than in the herd treated in late spring, but among individual winter-inseminated cows no relationship between progesterone levels before treatment and the response to this was found.