Herd of cows eating in stanchions.

Cows in Heat: The Bovine Oestrous Cycle

What is the oestrous cycle?

The oestrous cycle is a series of changes that a female animal’s body goes through to prepare for fertilisation and pregnancy. These changes, both behavioural and physiological, are driven by hormonal fluctuations. Understanding how cows go into and out of heat is vital for the modern dairy farmer.

What does it mean when a cow is in heat?

Cows, like the majority of other mammals, have an oestrous cycle. Theirs is a relatively short cycle and lasts between 17 and 25 days, with an average of around 21 days. Most of this time, cows are not fertile, but during a short period known as oestrus the cow will be receptive to mating and can get pregnant. This is known as being in heat. Cows often show behavioural changes which, along with other cues like scents and pheromones, help to let bulls know that they are ready to accept mating.

Hormonally, when a cow isn’t in heat, her progesterone levels are high and her oestradiol (an oestrogen) is low. When she starts to move into oestrus, a combination of other hormones causes a follicle containing an egg to grow on her ovary and her progesterone level starts to drop. Progesterone is very low for around 3 days and towards the end of this, the egg is released (ovulation). The period of time when a cow’s progesterone is low is oestrus or heat.

Why do we need to know?

With the move towards industrialisation of dairy farming across the world, more producers are mating their cows using Artificial Insemination (AI)1 instead of putting them to a bull. As a result, it is crucial to identify when cows are in heat so that the AI is successful. A mistimed AI can incur financial impacts from wasted straws and technicians fees, to more days open and increased calving intervals. If an already pregnant cow is incorrectly AI’d it can cause her to lose the calf or introduce infections into her system.

How can you tell if a cow is in heat?

Standing heat is the primary behavioural sign of heat. This is when a cow will stand to be mated and not move away from any other animal mounting her. Not all cows do this, however, and it can be difficult to spot this behaviour if the cow isn’t under constant observation. Standing heat does not last for long, usually only 8 – 12 hours, but other behavioural signs can sometimes be seen before active oestrous starts.

Some cows vocalise more when they are in heat.

Other signs of heat might be:

  • Increased activity
  • Mounting other cows (bulling)
  • Licking or sniffing other cows’ vulvas
  • Curling her upper lip (flehmen response)2
  • Vaginal discharge
  • Slight increase in temperature
  • Increased vocalisations

Activity monitors such as pedometers and tail paint can help to spot these behaviours. While this can increase heat detection rate, these have their own limitations and can falsely indicate heat.

The only consistent indicator of heat is a drop in progesterone levels. Of course, this is not something that can be observed by looking at a cow, but progesterone levels can be measured in a cow’s blood, or more readily, her milk. This can provide reliable and accurate information about a cow’s fertility. A cow will usually have low progesterone for approximately three days, with highest rates of AI success occurring on the second day3, and then her progesterone levels will begin to rise again.

After oestrus, if a cow is not fertilised, she will return to cycling and the same process will repeat itself in 17 – 24 days. All healthy cows continue to cycle for their entire lifespan, only stopping during pregnancy and the early post-partum period, or if there is a problem.

  1. Morrell, J. M. Artif. Insemin. Farm Anim. (2011)
  2. Houpt et al., Theriogenology (1989).
  3. Garcia-Ispierto, I. & López-Gatius, F. J. Reprod. Dev. (2014).