Critical Factors Relative to Reproductive Performance

Days in Milk (DIM) at first service. Conception rate (CR) for cows serviced less than 50 DIM will be less than for those cows serviced greater than 50 DIM. Conception rate will generally continue to improve out to 70 DIM. Ovsynch will perform best for those cows serviced at greater than 70 DIM.

Parity has a major influence on Pregnancy Rate (PR). Each additional lactation will cause a decrease in PR of 1% - 2%. Situations where this is not the case suggest some overall problems with certain groups of cows. For example, if the first lactation animals are not performing 1% - 2% better than the 2nd lactation group, something is holding the heifers back.

Anestrus cows will depress PR. These cows are not cycling and will not show heat and do not respond well to synchronization. Poor transition management, metabolic disease, lameness or poor nutrition can all contribute to an increased number of anestrus cows.

Change in body condition score (BCS) will have an influence. Ideally, cows should freshen with a BCS of
3.25 - 3.50 and not decrease in BCS by more than 0.75 BCS at the time of first service; in this case, an ideal BCS of 2.50 - 2.75 at first breeding. Our research has shown that with this management, 75% of the animals can be pregnant by 150 DIM. When more than 40% of the animals fall out of this range, only about 60% will be pregnant by 150 DIM.

Heat stress has a major impact on PR as it causes not only a decrease in estrus activity, but also causes a reduction in conception rate.

Metabolic diseases will have a negative influence on first service PR. Research done in Texas in 1999 by Jordan demonstrated this. First service PR for cows with retained placenta was 10%; that for cows with twins was 10%, for cows with more than one problem, the PR was 17%, for cows with mastitis less than 70 DIM, PR was 23%. For cows with no problems, was 34%.

Ovsynch design. In cows where Ovsynch is initiated at any stage of the estrus cycle, it appears to be best to give the second GnRH injection 48 - 60 hours after the PgF2a injection with insemination following the GnRH in 12 - 20 hours. For presynchronized cows or cows started between the 5th and 13th day of the estrus cycle, a 72 hour cosynch appears to work well. In this case, the GnRH and insemination will occur at 72 hours after the PgF2a injection.

Compliance with the mechanics of and enrollment into a systematic breeding program are essential for its success. All eligible cows must be enrolled. All injections, inseminations must be done in a timely and correct manner.

The time of initiation of the Ovsynch protocol relative to the stage of the estrus cycle can play an important role. Cows started on days 5 through 13 of the estrus cycle perform better than those started at other times of the cycle. This difference will be in the 5% to 10% range. This is the reason for presynchronization with two shots of PgF2a two weeks apart given 12 to 14 days before initiation of Ovsynch with the first GnRH injection. In the case of cows found open at pregnancy examination that will be enrolled in a subsequent Ovsynch protocol, the best time for initiation is 32 - 33 days after the previous insemination. This places most of them in the favorable 5 - 13 day window. This practice will improve conception rate in the 3% to 4% range.