The high temperature in Altus, Oklahoma on April 1st was 98 degrees F. (No April Fool’s joke intended! It was just 93 degrees in Stillwater.) As the breeding season for spring calving herds is getting closer, understanding heat stress in cattle takes on increased importance. Producers that choose to synchronize and then artificially inseminate replacement heifers will begin the process in the next few weeks. If the hot weather continues or returns during the AI breeding season, some management and breeding alterations may be helpful.
For years, producers that bred artificially upon detected standing estrus (heat), would wait 12 hours before breeding the female in heat. If she was first observed in standing heat in the morning she would be inseminated that evening. If she was first observed in standing heat in the evening she would be inseminated the following morning. (This was called the AM/PM rule of artificial insemination.) New research at Oklahoma State University on the internal temperature of heat stressed cattle may cause us to re-think handling cattle in the evening.
Research with rumen temperature boluses has shown that the core body temperature of beef cows peaks at 2 to 5 hours after the highest daytime temperature. On a hot spring/summer day the highest daytime temperature is often late afternoon. Therefore the peak body temperature of cattle will occur at 6 PM to 11 PM. Elevated core body temperatures have been implicated from other research in reduced pregnancy rates in heat stressed cattle. Inseminating all cattle in the morning hours would avoid the heat stress of evening breeding. Some would be bred at first standing heat, others would be bred at the conventional 12 hours after standing heat. If the temperatures cool down during April and May, then using the conventional AM/PM program should be best. (Source: Pye, Boehmer, and Wettemann. 2011 Page 104; Abstract 285 http://adsa.asas.org/midwest/2011/Midwest_Abstracts_2011-revised.pdf.)
Source: Glenn Selk, Oklahoma State University Emeritus Extension Animal Scientist