Hormonal synchronization protocols are used extensively on dairy farms, and the use of 100 percent timed AI on first breeding is becoming more common. In our practice, most herds using 100 percent timed AI on first service have seem significant increases in pregnancy rates after implementing this practice, especially if the protocol utilizes a second prostaglandin injection prior to breeding.
    Presynch-Ovsynch 10 or 11 seems to work about as well as Double-Ovsynch (DOS) in a 100 percent first breeding TAI program. It is not uncommon to see first service conception rates over 60 percent and 50 percent for first lactation and older cows, respectively, even in herds averaging over 100 pounds of milk. However, the first published field trial of Ovsynch on a commercial dairy farm (Pursely, 1997) reported that conception rates to TAI and Ovsynch were about the same as cows bred on standing heat, and that improvements in pregnancy rates were due entirely to increases in service rates in the TAI program. Interestingly, one of the original developers of Ovsynch, Richard Pursely, states on his website (https://dairycattlereproduction.com), that Ovsynch was developed to increase conception rates by improving timing of insemination. Yet historically, the primary reason most producers used TAI was to increase service rates.
    A recent study (Santos, 2017) looked at the reasons for increases in pregnancy rates in a 100 percent TAI program on first breeding. Early studies compared cows receiving TAI to cows in standing heat, but in those studies cows bred in standing heat had different days in milk than the TAI cows and results were mixed. This new study compared cows in a DOS protocol to cows that were synchronized with a protocol using GNRH and three subsequent prostaglandin shots (EST), and then bred on detected estrus. Cows were inseminated, on average, at 77 DIM in both groups. GNRH was used in the EST group to reduce the rate of anovulation. Heat detection in the EST group was aided by the use of a pedometry system, and a 77.5 percent submission (heat detection) rate was achieved.
    DOS cows had 23 percent more pregnancies per breeding than EST cows when evaluated at 33 days after insemination. DOS cows had a 49 percent conception rate and EST cows had a 38.9 percent conception rate. Progesterone sampling was done on both groups. DOS cows had higher progesterone levels at day 24, which is likely due to the presence of an additional corpus luteum (CL) in the DOS group. Higher progesterone levels at the time of prostaglandin administration have been shown to be associated with higher conception rates in a number of previous studies. The percentage of cows with high progesterone at breeding was not different between groups, suggesting that the percentage of cows in the right stage of the cycle for breeding did not differ between groups. Progesterone levels at seven days post breeding was greater for the EST cows. The authors speculate this is because DOS cows had smaller ovulatory follicles. This happens because GNRH is given earlier during the development of a synchronized follicular wave in an Ovsynch protocol than the GNRH produced by the hypothalamus of the cows in the EST group. The follicles in the EST group would be expected to be older and larger than the follicles in the DOS group at the time of ovulation as a result.
    When evaluating all of the progesterone levels at different stages, the authors concluded that overall 85 percent of cows were synchronized, and the rate did not differ between groups, suggesting that the increase in conception rate of the DOS group was not due to a greater percentage of synchronized cows. Instead the results suggest that inseminating high producing Holstein cows after detected estrus results in lower fertility than insemination following DOS. The exact mechanisms by which DOS increases fertility is still open to debate, and a variety of theories have been proposed. In our experience, DOS may reduce double ovulations and twinning rates as well, and the effect may be, in part, related to the reasons for increased fertility.
    Remember that the submission rate was 77.5 percent for the EST cows, which while very good, was less than the 100 percent submission rate of the DOS cows. As a result the actual number of total pregnancies per enrolled cow in the DOS group was 64 percent higher (33 days) than the EST group, not 23 percent higher, because 22.5 percent of the EST cows were not inseminated. Think about this: using 100 percent TAI resulted in 64 percent more pregnant cows on the first breeding, even in a herd with a relatively high heat detection rate. This is a remarkable improvement in reproductive performance.
    Thus the answer to the question of why Double-Ovsynch works so well is increased fertility and increased submission rate. If you are considering doing this in your herd remember that you need a valid VCPR with your veterinarian and directions from your veterinarian. Also consider that compliance is extremely important to success with these programs; missing or improperly administering a few shots can result in horrible results, so be sure to do it right. Done right though, 100 percent TAI first service programs can dramatically increase pregnancy rates.
    Jim Bennett is a dairy veterinarian at Northern Valley Dairy Production Medicine Center in Plainview, Minn. He and his wife, Pam, have four children. Jim can be reached at bennettnvac@gmail.com with comments or questions.