After calving, cows go through a period of temporary infertility known as postpartum anestrus. Cows will not experience estrous cycles during this time. Another common term associated with this phe‐ nomenon is postpartum interval, which is the time from calving to the subsequent conception. Post‐ partum interval plays an important role in determining a cow"s calving interval, or the number of days from calving date in one year to calving date the next year. To maintain a 365‐day calving inter‐ val, a cow must have a postpartum interval of 80‐85 days. If a shorter calving interval is desired to move the cow up in the calving cycle, she must have a postpartum interval of less than 80‐85 days.
Several factors can influence the length of the postpartum anestrous period, including uterine invo‐ lution, short cycling, suckling effects, and nutritional status. Uterine involution is the regression of the uterus in both structure and function to a status that is capable of carrying another preg‐ nancy. This entails the uterus returning to a non‐pregnant size, shape, and position, shedding all fe‐ tal membranes, and the repair of uterine tissues. This process is completed in approximately 20‐40 days post‐calving if no complications arise.
The first ovulation postpartum often occurs without visual signs of the cow being in heat, and is of‐ ten followed by abnormal function of the corpus luteum (CL). Normal CL lifespan takes up 14‐18 days of the typical 21‐day estrous cycle of a beef cow. The short estrous cycles experienced by cows overcoming postpartum anestrus are characterized by a CL lifespan of 10 days or less. This is thought to be due to high levels of prostaglandin production and metabolism by the uterus during uterine involution. Prostaglandin is responsible for regression and death of the CL in a normal es‐ trous cycle, but at the elevated levels described, that regression and death of the CL is premature. If fertilization of the egg from this ovulation were to occur, maternal recognition of pregnancy would fail as CL regression would take place too soon, and the embryo would be lost.