Abstract
This experiment compared the reproductive performance of synchronised anoestrous dairy cows that were treated initially with a combination of progesterone and oestradiol benzoate and then with either gonadotrophin-releasing hormone (GnRH) or oestradiol benzoate to resynchronise returns to service. It was hypothesised that injecting anoestrous dairy cows with GnRH 12-15 days after insemination and coinciding with the time of insertion of a controlled intravaginal progesterone-releasing (CIDR) device would increase conception rates to the preceding 1st insemination compared with oestradiol benzoate treated cows; both GnRH and oestradiol benzoate would resynchronising the returns to service of those cows that did not conceive to the preceding insemination. Groups of cows in 11 herds were presented for a veterinary examination after they had not been seen in oestrus postpartum. Those cows diagnosed with anovulatory anoestrus (n = 1112) by manual rectal palpation and/or ultrasonography were enrolled in the trial. Each enrolled cow was injected with 2 mg oestradiol benzoate i.m. on Day -10, (where Day 0 was the 1st day of the planned insemination) concurrently with vaginal insertion of a CIDR device. The device inserted was withdrawn on Day -2 and then each cow injected i.m. with 1 mg of oestradiol benzoate on Day -1 unless it was in oestrus. Observation for oestrus preceded each insemination. Every cow that had been inseminated on Days -1,0,1 or 2 was presented for treatment for resynchrony on Day 14 (n = 891). They were divided into 2 groups; those with an even number were each injected i.m. with 250 microg of a GnRH agonist (Treatment group n = 477); each of the cows with an odd number injected i.m. with 1 mg of oestradiol benzoate (control group, n = 414). Each GnRH or oestradiol benzoate injection preceded reinsertion of a CIDR device previously inserted from Days -10 to -2. It was withdrawn on Day 22, 24 hours before injecting 1 mg oestradiol benzoate. Cows observed in oestrus were submitted for a 2nd insemination. Every enrolled cow still present in the herd was pregnancy tested by palpation of uterine contents per rectum about 6 weeks later and again at the end of a herd's seasonal breeding programme. The alternative use of GnRH instead of oestradiol benzoate did not affect the percentage of cows conceiving within 3 days of the mating start date (MSD) (35.6 % vs 35.3 %, P = 0.90), resubmission rates for a 2nd insemination among cows not pregnant to the 1st insemination (81.6 % vs 83.5 %, P = 0.41), 6-week pregnancy rate (59.3 % vs 60.6 %, P = 0.65), 21-weekpregnancy rate (86.6 vs 85.0, P = 0.36), mean interval from MSD to conception (32.5 +/- 1.8 days vs 29.9 +/- 1.8 days, P = 0.26) or conception rate of cows reinseminated by Day 28 (43.3 % vs 38.8 %, P = 0.39). When GnRH was compared with oestradiol benzoate, it did not increase conception rates to the 1st service; it was as effective as oestradiol benzoate in synchronising returns to service in previously treated anoestrous cows that did not conceive to the 1st service. Its use affected neither conception rates to the preceding 1st inseminations nor to the following 2nd inseminations.
Highlights
Seasonal calving dairy production system is a system where breeding and calving are concentrated within a limited period of the year
There were no differences in the age at calving (5.03 ± 0.122 vs 5.05 ± 0.11 years for oestradiol benzoate and gonadotrophin-releasing hormone (GnRH), respectively, P = 0.90), body condition score at initial examination, on a 1 to 8 scale (4.83 ± 0.02 vs 4.82 ± 0.02 for oestradiol benzoate and GnRH groups, respectively, P = 0.76), or interval from calving to mating start date (MSD) (62.4 ± 1.2 vs 62.0 ± 1.2 days for oestradiol and GnRH groups, respectively, P = 0.80) for the cows in the 2 treatment groups which were included in the final analysis
The percentage of cows that did not conceive to the inseminations done from Day –1 to Day 2 and were submitted for a 2nd insemination between Days 22 to 28 after MSD did not differ between the 2 treatment groups (83.5 % vs 81.5 % of cows for oestradiol benzoate and GnRH groups, respectively, P = 0.41, Fig. 3, Table 1)
Summary
Seasonal calving dairy production system is a system where breeding and calving are concentrated within a limited period of the year. This system is widely practiced in Australia and New Zealand. In order to maintain the target intercalving interval of 365 days optimum reproduction efficiency of cows must be maintained so as to achieve at least 87 % submission rates within the 1st 3 weeks of the MSD16. This can be achieved through controlled breeding programmes as well as timely identification and treatment of reproductive disorders such as anovulatory anoestrus. Delayed conception affects calving patterns[16] and cows conceiving late are more likely to be induced to calve prematurely[28]
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