Understanding the bovine oestrus cycle and the hormones involved, allow us to diagnose reproductive problems and treat them effectively, ultimately improving reproductive efficiency and conception rates.
The cow’s oestrus cycle is 21 days long but can range from 18 to 24 days. Heifers begin to cycle when they reach puberty. The age at which a heifer reaches puberty depends on a number of factors including breed, health status, growth rate and nutritional status.
Day 0 of the oestrus cycle is considered to be the day of oestrus or ‘heat’ and ovulation. A cow will show signs typical of oestrus behaviour: standing to be mounted, restlessness, transient milk drop, and an increased amount of clear vulval mucous. This behaviour can last between 8-12 hours. Oestrus behaviour is due to oestrodiol in the blood stream which is secreted from the ovary. Oestrodiol causes specific changes within the reproductive tract during oestrus such as relaxing the cervix, bulling slime and increase blood flow to the uterus.
Oestrodiol also triggers the release of a hormone called luteinising hormone (LH). LH is secreted into the blood from a gland close to the brain called the pituitary gland, and causes a follicle to ovulate into the oviduct. Ovulation usually occurs approximately 32 hours after a LH surge.
If sperm are present the egg will be fertilised and then travel down the oviduct arriving in the uterus approximately 3-4 days later. Where the egg was released from the ovary, a structure called a corpus haemorrhagicum forms, that over a few days turns into a corpus luteum (CL) which secretes progesterone. Progesterone suppresses the amount of LH in circulation, while a hormone called follicle stimulating hormone (FSH) prepares waves of follicles ready for the next ovulation.
Return to Heat
If fertilisation did not occur the uterus releases prostaglandin (PG) in to the blood stream which removes the CL from the ovary. As a result progesterone levels gradually reduce and LH and oestrodiol levels rise resulting in heat behaviour and ovulation and the cycle continues.
If fertilisation and implantation was successful then the PG is not released from the uterus and the CL continues to secrete progesterone which prevents further heats and ovulations. Progesterone is an essential hormone required to maintain pregnancy. The CL is responsible for progesterone levels up until day 150 of pregnancy; thereafter the placenta also secretes progesterone which maintains pregnancy.
Post calving, the ovaries start to become active again with follicles starting to appear within 5 days of calving. There has to be sufficient oestrodiol to stimulate an LH surge for ovulation to occur, the period when a cow is not cycling and ovulating is called anoestrus. There are several factors that reduce the LH surge, delaying ovulation and prolong anoestrus. Negative energy balance post calving can have a significant effect, so the cow must be in good condition at calving as well as maintaining dry matter intakes pre and post calving.
In suckler cows, the presence of the calf and suckling behaviour reduces LH levels and prolongs anoestrus, so suckler cows often don’t resume normal cyclicity for 4 week to 3 months post calving in comparison to a well-managed dairy cow that can be cycling within 2- 3 weeks. The complex hormone interactions that happen during the oestrus cycle can be manipulated by hormonal drugs to treat cystic ovarian disease, anoestrus and synchronise cows for artificial insemination.