Breeding Success: Mastering the Cow's Estrous Cycle
Brian Karisa, PhD. ©2024
Introduction
The cattle estrous cycle is a recurring physiological process in female cattle (cows and heifers), at puberty between 9 and 15 months of age, that prepares the reproductive system for ovulation, potential fertilization, pregnancy, calving and lactation.
The estrous cycle in cattle lasts on average, 21 days (varies from 18 to 24 days).
A good understanding of the estrous cycle is beneficial to producers, farm managers and veterinarians for breeding management, fertility assessment, pregnancy diagnosis and disease diagnosis.
Breeding Management: Involves timing insemination or natural mating to coincide with ovulation so as to maximize conception rates.
Fertility Assessment: Involves monitoring the estrous cycle to help identify reproductive problems such as cystic ovaries or irregular cycles.
Pregnancy Diagnosis: Knowing the cycle allows accurate prediction of non-pregnancy when heat is observed after breeding.
Disease diagnosis: knowing the estrous cycle helps to be able to identify when the cycle is inconsistent and any underlying medical conditions that might cause these alterations.
Estrous Cycle Phases
The estrous cycle is divided into four main phases: proestrus, estrus, metestrus, and diestrus.
Please note that the terms estrous and estrus are different; Estrous denotes the entire cycle while estrus is just one of the phases in the cycle.
Illustration obtained from https://crinetsupport.blogspot.com/2015/07/
Estrus (Heat)
This phase lasts 12-18 hours, though it can range from 6 to 24 hours in some animals. This phase is associated with high estrogen levels which play a role in the behaviors observed. This is the optimal time for artificial insemination (AI) or exposing the cows to a bull in natural mating systems. Actual ovulation occurs 24–32 hours after the onset of estrus, typically toward the end of this phase.
Although individual animals may differ in how they express heat, generally the following signs may be observed:
- The cow is sexually receptive and will stand to be mounted (also called "standing heat").
- Restlessness or increased activity.
- Swelling and redness of the vulva.
- Clear mucous discharge from the vulva.
- Mounting other cows or being mounted.
- Frequent urination.
Post-heat (Metestrus)
Metestrus lasts 3-5 days. The corpus luteum begins to form and starts to produce a hormone called progesterone. Progresterone levels are still low during this phase. Progestrone is called the 'pregnancy homrone', as the levels increase, it prevents (inhibits) further follicular development and begins to prepare the uterus for a potential pregnancy.
Minor bleeding (metestrus bleeding) may occur in some cows, indicating that estrus has ended. Ovulation is completed early in this phase if it hasn’t occurred during estrus.
Diestrus (Luteal Phase)
The Luteal phase lasts 10-14 days. The corpus luteum (CL) is fully developed thats why its called the Luteal phase. A corpus luteum is a structure formed by the collection of cells that remains after ovulation. It acts as an endocrine gland producing a hormone called Progesterone. If pregnancy occurs, the CL remains functional, continuing to produce progesterone to sustain the pregnancy. If pregnancy does not occur, the uterus releases prostaglandin F2-alpha (PGF2α), which causes regression of the CL (luteolysis), reducing progesterone levels and restarting the cycle.
Proestrus (Pre-heat)
This phase lasts for 2-4 days. The CL from the previous cycle regresses resulting in reduced progesterone production. Other hormones are released by the pituitary gland and the hypothalamus to promote the growth of a new follicle (egg). As the new egg develops, estrogen levels increase and behavioral signs associated with heat may start to appear.
Anestrus
Anestrus refers to the phase when the sexual cycle rests. This is typically a seasonal event and controlled by light exposure through the pineal gland that releases melatonin. Melatonin may repress stimulation of reproduction in long-day breeders and stimulate reproduction in short-day breeders. Melatonin is thought to act by regulating the hypothalamic pulse activity of the gonadotropin-releasing hormone. Anestrus is induced by time of year, pregnancy, lactation, significant illness, chronic energy deficit, and possibly age.