Pregnancy rates in dairy cows can be drastically reduced when temperatures rise in the summer. Dairy cows are highly susceptible to heat stress due to the metabolic heat produced during lactation. Producers use a variety of management strategies to minimize the loss of pregnancy rates.
In the Midwest, lower estrus detection and conception rates occur from June through October. This results in lower pregnancy rates. During times of heat stress, reduced pregnancy rates can be caused by a number of factors including reduced expression of estrus, impaired egg quality and abnormal follicular function, including cystic and anovular cows.
Early embryonic death is significantly higher when cows are heat stressed due to an altered uterine environment, reduced blood flow to the placenta, and sensitivity of the embryo to elevated temperatures during the first three days after breeding.
One strategy employed by many producers in the southern states is to concentrate on getting cows bred before the heat, making an extra effort to ensure that any cow past the voluntary waiting period and not confirmed pregnant is inseminated. Heat stress reduces the length and intensity of estrus behavior and results in fewer cows receiving artificial insemination (A.I.). Utilizing a synchronization program in heat stressed cattle will maximize service rate, thus enhancing pregnancy rate.
Heat abatement is essential in the weeks preceding breeding, as well as the first week after breeding. The time of greatest susceptibility is immediately after the onset of estrus and early post-breeding. Providing adequate shade and water to cows on pasture can help keep them cool, resulting in increased embryo survival. In the barn and holding areas, the use of fans and sprinklers will help to cool the cow and the air around her.
Make sure that the sprinklers are set to the appropriate time interval and that the droplet size is large enough to soak the cow to the skin – not a mist, which will sit on top of the hair and insulate the cow. Sprinklers should be running for between one to three minutes in every 10 to 15 minutes. It is critical that the sprinklers be turned off and fans are running for evaporation to occur, resulting in the cows feeling cooler because some of their body heat is used to evaporate the water. Avoid overcrowding pens and keep up on fly control to prevent bunching.
Although bulls may be used to help compensate for cows not showing estrus, they cannot improve the altered uterine environment, blood flow to the placenta or embryo sensitivity. Besides, bulls are also affected by heat stress. They are less active, breed fewer cows, and have lower quality semen when they are hot. In addition, just two to three days of exposure to temperatures over 85°F can reduce semen quality for the following eight weeks.