F. Recent studies indicate that the reproductive performance of cows does not suffer until environmental temperatures are over 90 degrees F. Additionally, the sun's rays can directly have an adverse effect on fertility. Cows with black markings will absorb more heat from the sun's rays, further elevating body temperature. Blood flow may be diverted from internal circulation to peripheral circulation in an attempt to reduce body temperature. The reduction in blood flow to internal organs including the uterus, oviducts and ovaries may reduce available nutrients and increase biochemical waste products at the tissue level. Certainly some cows do become pregnant in very hot weather, but overall fertility rates are low.
Hot weather lowers reproductive efficiency in two ways.
First, cows are harder to detect in estrus and are sometimes classified as anestrus. Second, cows that are bred by artificial insemination have a hard time becoming or staying pregnant. The second problem shows up as a low conception rate or high services per conception. Anestrus Cows may not be detected in estrus due to failure of ovarian function and absence of estrus. More commonly, the frequency of mounting in hot weather is reduced and a producer misses seeing the few mounts which do occur.