REPRODUCTIVE-PERFORMANCE OF LACTATING DAIRY-COWS FOLLOWING ESTRUS SYNCHRONIZATION WITH PROGESTERONE, ESTRADIOL AND PROSTAGLANDIN

The ability to synchronise onset of oestrus, and hence the time of bre eding and calving, offers potential economic and management benefits t o dairy farmers, especially in herds with seasonally concentrated calv ing patterns. A trial involving 2681 cows in Il seasonal herds was con ducted to evaluate the reproductive performance of lactating dairy cow s following oestrus synchronisation with a combination of progesterone , oestradiol and prostaglandin. Cows were randomly assigned within her ds to synchronised and control groups, balanced for age, date of calvi ng, body condition and breed. Cows in the synchronised group were trea ted with an intravaginal progesterone-releasing device containing 1.9 g of progesterone and a gelatin capsule containing IO mg of oestradiol benzoate IO days prior to the planned start of the breeding season (D ay 0). The device was removed 8 days later on Day -2 and a luteolytic dose of prostaglandin F2 alpha was administered 2 days prior to remova l of the progesterone-releasing device. Returns to service for cows in the synchronised group were synchronised by inserting a previously us ed intravaginal device during Days 16-21 after the start of the breedi ng season. Cows in the control group were left untreated. The percenta ge of cows being inseminated during the first 5 days was 89.0% for the synchronised group compared to 29.7% for the control group. Compared to cows in the control group, those in the synchronised group had a lo wer conception rate to the first insemination (52.9% v. 64.3%, p<0.001 ), a lower conception rate to the second insemination (51.8% v. 62.5%, p<0.001), a higher percentage of empty cows at the end of the breedin g season (7.3% v. 5.1%, p<0.05), and more insemination services per pr egnancy to artificial insemination (2.0 v. 1.6, p<0.001). There was no difference between the synchronised and control groups in the percent age of cows pregnant to artificial insemination (81.8% v. 85.5%, p>0.1 0). The mean day of conception from the start of the breeding season w as advanced (p<0.01) by 1.3 days in synchronised cows (19.9 +/- 0.7 da ys; mean +/- SEM) compared to control cows (21.2 +/- 0.5 days). It is concluded that the oestrus synchronisation regime used in the present study caused a reduction in fertility, which reduced the potential gai ns from using such a programme to increase reproductive efficiency in dairy cows.