However, many of these first estrous periods are not accompanied by strong signs of heat. In fact, only 50 percent of these animals show overt signs of standing heat. In later heats, about 90 percent of these heats in reproductively normal animals are characterized by standing heat. Thus, the major reason for animals not being detected in heat is not lack of estrus by the animal, but inadequate detection by the dairyman. Extremes in temperature cause stress in dairy cattle and lower their reproductive rate, especially in hot weather.
Seasons affect reproduction, too.
In Arkansas, cattle tend to be most fertile in the winter and spring and least fertile in the summer. Younger animals are more fertile than older cows and are less affected by environment. A heifer that is born twin with a bull probably will be a freemartin. Her reproductive organs do not develop normally and she is sterile. Nutritional deficiencies usually are not the primary cause of breeding problems in a herd.