Estrous Synchronization Systems for Managing Reproduction during Periods of Heat Stress

Prolonged periods of hot humid weather adversely affect reproductive performance. As feed intake declines during heat stress the onset of cycling is delayed or some cycling cows may become anestrus. For those cows that continue to cycle, the expression of estrus may be reduced if the thermal stress is not minimized. Thus the heat detection rate is substantially reduced. Lower pregnancy rates should be expected because of higher rates of embryonic mortality. Obviously any management practice that reduces thermal stress and maintains feed intake should help minimize reproductive loss.

Since more dairy producers are adopting estrous synchronization programs to manage reproduction, the question arises which system would be more effective during periods of heat stress. Since estrous behavior is reduced during periods of heat stress a Timed AI program which results in 100 percent submission rate for insemination may be more effective than a synchronization program based upon heat detection. Research at Kansas State University compared the success of a Timed AI program (Ovsynch) with a somewhat similar program that involved detection of heat. Temperature-Humidity index (THI) was calculated daily.

The overall results for all inseminations whether performed during heat stress or non heat stress conditions were as follows. Conception based upon ultrasound diagnosis at 27-30 days post AI was not different between the two programs. Since all cows assigned to the Ovsynch program were inseminated, the pregnancy rate (heat detection rate x conception) was greater than that obtained for the system using heat detection following synchronization, 33.3 and 17.9 percent respectively

The results summarized based upon the heat stress conditions near the time of insemination were as follows: Cows were considered not heat stressed when the THI was < 72. When the THI was > 72 cows were heat stressed. When the THI changed from less than 72 to greater than 72 the pregnancy rate at 27 days post AI for cows on the Oysnch program increased from 29 percent to 38 percent, respectively. Cows on the system using heat detection decreased from 24 to 13 percent. Pregnancy rates at days 40 to 50 post AI for Ovsynch cows inseminated when THI was < 72 and > 72 were 17.6 and 15.2 percent, respectively. For the cows on the other program the values were 17.2 and 8.8 percent, respectively. These results illustrate the initial superiority of the Ovsynch program during periods of heat stress but this advantage is lessened at 40 to 50 days after insemination. Previous research from Florida compared the effectiveness of Ovsynch program with a two injection prostaglandin program during heat stress conditions. The pregnancy rate at 120 days postpartum for the Ovsynch and standard prostaglandin system were 27 and 16.5 percent, respectively.

In conclusion, it appears from these studies that the Ovsynch program timed AI program has an advantage during periods of heat stress. This is due to the fact that all eligible cows are automatically submitted for insemination when expression of estrus is reduced during hot humid weather.

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Michael O'Connor

Dairy and Animal Science Extension