The breeding season is, or soon will be underway for those herds that have a spring calving program. The most important factors that determine if, and when, a cow returns to cycling activity were studied by Kansas State University reproductive scientists. Over a period of 7 years, Kansas State scientists used more than 3,200 beef cows in estrous synchronization studies. As a part of these studies they determined which cows were cycling before the start of the breeding season both before and after synchronization treatments. They then looked at the previous data about each cow and determined the major factors that influenced the likelihood that she would have returned to heat by the start of the breeding season. The research indicated that three main factors were the most important determinants as to whether the cow would recycle before the breeding season began. Body condition, age of the cow, and the number of days since calving were the biggest influences on incidence of cycling activity before breeding.
Body condition: Cows ranged in body condition score from 1 (extremely emaciated) to 7 (very fleshy). As body condition score increased the percentage of cows cycling increased in a linear fashion. The Kansas data reported that there was an 18% increase in percentage cycling for every 1 full condition score improvement.
Age of the cow: The percentage of first calf two-year-olds cycling was about 10% less than mature cows that were having at least their second calf. The extra nutrient requirement for growth clearly limits the cycling activity at the beginning of the breeding season of two-year-olds. Also two-year-olds are in the stage of life where the baby teeth are being replaced by permanent teeth. Some of these young cows have problems consuming roughage similar to “broken-mouth” older cows. This explains why many producers choose to breed replacement heifers ahead of the cow herd and therefore give them more days before the breeding season begins for mature cows.
Numbers of days since calving: Cycling activity was also influenced by the number of days since calving. For every 10 day interval since calving (from less than 50 days to 70 days) the percentage cycling increased by 7.5%. A short calving season is important because it allows a higher percentage of cows to be cycling by the start of the breeding season.