Reproduction   Beef

Estrous Cycle and Reproductive Hormones of the Cow
L. L. Wilson and C. W. Foley, Animal Sciences Department
This publication is designed as a teaching aid for the intensified cattle reproduction schools conducted by the Indiana Cooperative Extension Service. It is also included in the Beef and Dairy Production Handbooks for County Extension Workers. The purpose of this publication is to help producers more clearly understand the structure, functions, and abnormalities of the reproductive systems of the cow.
Action of glands and hormones
The step-by-step occurrence of the events in the estrous cycle are determined chiefly by hormones. Hormones are chemical substances produced by glands located in different areas of the body. Hormones are usually secreted into the bloodstream and carried with the blood from one part of the body to another. In this way, they serve as chemical messengers to regulate tissue and organ activity.
Introduction
The estrous cycle is the period of time from the beginning of one "heat" period to the beginning of the next. This cycle is controlled by several interlocking mechanisms. The phases of the cycle must occur in proper sequence if the cow is to conceive and produce a calf.
For example, the ovary must release a normal egg near the time the cow is in heat or receptive to mating with the bull. The cow must also prepare the uterus to maintain an environment in which the calf can nourish and develop.
Finally, these events must lead to normal birth of a healthy young animal. The cow must then provide milk for the suckling calf and also be able to start another complete reproductive cycle.
The hormones and glands which regulate the estrous cycle are closely inter-related. Checks and balances operate among the hormones which control the cycle. A simple diagram of an inter-relationship between two glands is illustrated in Figure 1.
Figure 1. Diagram of the hormonal relationships between three different glands.
In this example, Gland A secretes Hormone A. Hormone A then acts on Gland B,
Beef Section • Animal Sciences Department
Cooperative Extension Service, PURDUE UNIVERSITY, Lafayette, Indiana
AS-361 Jan. 1967