Objective To compare fixed-time artificial insemination (FTAI) conception rates and serum progesterone concentrations at the time of FTAI for cows treated with the original Ovsynch program (OV) with those treated with a modified Ovsynch (MO) program.Design This was a randomised clinical trial. MethodsThe study used five split-calving, pasture-based dairy herds in Southwest Victoria, Australia. Controls (n = 851) received the OV program: day 0 gonadotropin-releasing hormone, day 7 prostaglandin F 2α (PGF), day 9 gonadotropin-releasing hormone and FTAI at day 10. The treatment group (n = 852) received a MO program with an additional prostaglandin injection on day 8. Subsets of cows from each group were sampled for blood progesterone at the time of FTAI. ResultsThe treatment group demonstrated FTAI conception rates that were 7% (95% confidence interval 2%-12%) greater than the control group. After adjusting for the effect of age, days in milk at Mating Start Date and herd, the odds of conception using FTAI was 1.36 (95% confidence interval 1.12-1.66) times greater for treatment group cows compared with control group cows. The variability of serum progesterone concentrations at the time of FTAI was significantly less for treatment group cows compared with control group cows.Conclusion For Holstein-Friesian and Holstein-Friesian crossbred cows managed in pasture-based dairy herds in southern Australia, a MO protocol, including a second injection of prostaglandin F 2α on day 8, increased FTAI conception rates compared with cows receiving the OV protocol.

In recent years the fertility of the dairy cow has been decreasing and there has been some speculation that the current Ovsynch programs on the market have not been used at the optimum time. Ovsynch is an oestrus synchrony program where hormone injections are used to bring cows into oestrus so that fixed time artificial insemination (FTAI) can be used. The original program involves an injection of gonadotropin-releasing hormone (GnRH) on day 0, prostaglandin (PGF2α) on day 7, GnRH on day 9 and FTAI on day 10. Based on studies looking at progesterone levels with the current Ovsynch program, a new program was proposed incorporating a second injection of PGF2α on day 8 to increase the probability of atresia of the corpus luteum (CL). The efficacy of the program will be established through early pregnancy diagnosis. A subset of animals in each group was submitted for progesterone testing on two separate occasions as an indication of whether ovulation occurred at AI.