A comparative study of the effect of 2 hormonal treatment protocols on the reproductive performance of previously anoestrous dairy cows.

The objective of this study was to compare the reproductive performance of anoestrous dairy cows treated just prior to the mating start date (MSD) with a Controlled Internal Drug Releasing Device (CIDR) which was placed intravaginaly for either 6 or 8 days, and a combination of oestradiol benzoate. Lactating dairy cows (n = 926) that had been diagnosed with anovulatory anoestrus were divided into two sub-groups. A hormonal treatment protocol that involved the use of a CIDR device containing 1.9 g of progesterone was inserted into the vagina of each cow and left intravaginally for either 6 (6-day group, n = 441) or 8 days (8-day group, n = 485). Every cow in the trial was inseminated after being detected in oestrus from Day -2 onwards (where Day 0, was a herd's MSD), using an appropriate detection aid according to the herds' preference. Cows that had been seen in oestrus and were inseminated by Day 2 were selected for re-synchrony. The standardised re-synchrony involved the re-insertion of a previously used CIDR device into the vagina of each cow on Day 14, together with an injection of 1.0 mg oestradiolbenzoate i.m. This CIDR device was removed on Day 22 and each of these cows injected with 1.0 mg oestradiol benzoate i.m. on Day 23. Each re-synchronised cow that was detected in oestrus was re-inseminated. Treatment with an 8-day CIDR increased the proportion of cows submitted for insemination within the first 3 days of the MSD, compared with the 6-day group (83.7 % vs 71.2%, respectively, P < 0.001), as well as the proportion of cows conceiving within the first 3 days of MSD (36.2% vs 27.7%, P = 0.02), but reduced both the interval from MSD to the first service (4.5 +/- 0.5 vs 6.8 +/- 0.7, P = 0.01), and the interval from MSD to conception (28.1 +/- 1.5 vs 34.0 +/- 1.8, P = 0.009). A greater percentage of the cows in the 6-day group that were not pregnant to the first insemination were submitted for a second insemination by Day 28 compared with the 8-day group (81.1% vs 68.3%, P < 0.001). Conception rates for cows submitted for this second insemination by Day 28 of the MSD were also higher in the 6-day than in the 8-day group (48.4% vs 33.9%, P = 0.009). The percentage of cows pregnant at the end of a herd's AI period of 6 weeks did not differ (57.1% vs 54.8% for 8-day and 6-day groups, respectively, P = 0.42); neither did the proportion of cows pregnant at the end of the a herd's combined AI and natural mating period of 21 weeks (81.4% vs 79.2%, for 8-day and 6-day groups respectively P = 0.36). Treatment of anovulatory anoestrous dairy cows with a combination of an 8-day CIDR and oestradiol benzoate before the MSD improved their reproductive performance by increasing the portion of cows submitted for insemination within the first 3-days of the MSD by reducing the interval from MSD to first service and by increasing the conception rate to the first insemination to collectively reduce the average interval from MSD to conception.