An estrous cycle can be shortened intentionally by injecting a hormone called prostaglandin, which causes the CL to regress. Prostaglandin injection is one method used in estrous synchronization. Estrus heat is not always accompanied by ovulation, nor ovulation by estrus. Such females will not accept service from a male. There are wide differences between the species of mammals in the various characteristics of the estrous cycle.
Some species have only one heat period each year and are called monoestrus.
The cow is in a group that exhibits heat more than one time per year and is called polyestrus. Even though cows will cycle at any time during the year, recent research has shown that day length may have an effect on cow fertility. Slightly lower fertility has resulted during the short day length period of the year. The hormones of female reproduction Reproduction in the female is controlled by numerous hormones secreted from specialized glands called endocrine glands. These secretions are produced in the glandular cells and pass into the blood and lymph systems for transport to specific parts of the body where they produce their function.