Welcome back, we are continuing in our series on the reproduction cycle of the cow.

In my last post we brought our cow through from just having given birth back to normal conditions to begin cycling again.

Now it’s time to discuss how that works and see what’s going on beside mounting all over the place flattening grass.

There are quite a few names going to be tossed around so here’s a quick reference if you need it:

  • Oestrus-the actual heat event
  • Estradiol-what makes the cow show all the signs of heat
  • Corpus Lutuem (CL)- The yellow body formed after ovulation
  • Corpus Albicans- the white remains of a regressed CL
  • Graafian Follicle- what the egg is stored in from birth to the heat it matures in
  • Prostaglandin (PGF2α)- the hormone that resets the cycle by regressing the CL, found in Estrumate and similar products

Day 0: The cow in heat (Oestrus)

The familiar image of a cow standing to be mounted is the most reliable sign of heat. The accepted landmark for heat is from the first standing mount and we measure time and events from there onwards.

What is happening today is a release of the hormone that governs the signs of heat such as mounting (Estradiol). At this stage the egg hasn’t been released yet but the cascade of hormones that leads to the release of the egg to meet the semen has begun.

The duration of oestrus can vary across the population from as low as 4 hours and upwards of 18 hours, in general oestrus last ~14-15 hours leading to the ‘am-pm’ rule for insemination, where if you see a cow mounted in the morning you call the AI man for that evening.

It is worth mentioning that any sign of blood or red mucus around the vulva is a sure sign she has gone out of heat. At this point you can be from 1-3 days late as the bleeding is a symptom of the drop in hormone levels following a heat.

Ovarian Changes

Day 1-5: Ovulation

After the pulse of Estradiol instigates the visual signs of heat, a cascade of hormones releases the egg (ovum) from the protective cover it has been maturing inside (Graafian follicle).

The egg then travels though the fallopian tube, where it will meet and fuse with the oncoming sperm to be fertilised. This would typically be about 24-32 hours from the onset of heat (Oestrus).

Once the egg has left its follicle, the remnants reform into a different structure (Luteinisation) called a Corpus Luteum (Latin for Yellow body) usually called a ‘CL’, which is filled with yellow material (luteal cells) that are the driving force in maintaining a pregnancy. If the CL goes, the pregnancy ends.

This is achieved by a hormone you may be familiar with, progesterone, which does pretty much the same thing in every mammal; it changes the cow to gestate unless something comes to upset the situation such as not fertilising.

In the background, there is a group of immature eggs (follicles) growing in size to ‘ripen’ but with one dominant follicle growing to be the largest (16-18mm) but only the third wave will be released for ovulation as the previous egg will prevent the waves coming behind it from fully ripening in diameter already in place these will regress back into the ovary (Shown below).

It is these ‘wasted’ eggs in every cycle that lead to very old cows stopping cycling and it is the same mechanism that causes menopause in human women.

Waves of follicles through a cycle


Day 5-16

During this period the CL is increasing in size up to about day 10 where it peaks in progesterone output this occurs alongside her own natural progesterone output raising the hormone level in her body.

This causes the uterus to build up its walls to nourish the embryo and provide anchor points (caruncles) for the placenta to take hold allowing exchange of nutrients from cow to calf.

The cow is not aware whether her embryo is fertilised or not at this stage, so her uterus continues on the assumption she has conceived by building up the walls of her uterus and increasing circulation to the reproductive tract to prepare for gestation.

During this phase the embryo has fused with the sperm forming what will become the calf and placenta (zygote).

It then moves down into the uterus proper to develop into a sphere of cells with an outer shell (blastocyst) which will ‘hatch’ into the uterine wall creating the placenta and the foetal membranes.

The growing placenta will release a hormone (Interferon tau) to the uterus causing it to ignore prostaglandin, which will be crucial to the next step in gestation.

A second wave of follicles is maturing in the background during this time which will also fail to fully mature because of the ovulated follicles is preventing them from fully maturing.

The path of an ovulated egg

path of an ovulated egg

Day 16: Maternal recognition of pregnancy

This is D-Day for a fertilised embryo. If at this point it hasn’t implanted itself and blocked enough hormone receptors(PGF2α) to prevent the CL from being reabsorbed, it’s done for.

The cow’s own progesterone levels will drop, removing support for the developing foetus and allow a following set of follicles(eggs) to mature to release in the next cycle.

What happens here is a pulse of prostaglandin (PGF2α) is released and either the embryo has implanted and grown to a point where it can block the effects of this pulse or it hasn’t and the cycle winds down to ovulate again.

This is the main threshold to a cow being pregnant or not, with some studies claiming 30-50% of all losses of pregnancy occurring before the embryo implants, so it’s not necessarily the bulls fault if your cow didn’t keep.

Day 16-21: CL regression or Uterine regression

In this period there are two paths to be followed: pregnant or not. While it is still too early for us on the outside to tell, the cow’s body knows which is the case, so let’s compare.

If it’s a case where the embryo has implanted, and the cow is aware, she continues with gestation for the next ~270 days so the uterine wall continues to build up and become more complex as the embryo develops.

There is quite a bit more to the pregnancy which we will talk about further down the line, but for now let’s leave it at that.

Now in the opposite case where, for whatever reason, the embryo hasn’t implanted or grown large enough to block the hormone signals, it’s time to get things ready for a new cycle and a fresh new egg.

This entails a breakdown of the CL (Luteolysis) to allow the next wave of follicles to fully mature and a dominant one to be ovulated.

You can see this in our first image above, the progression from follicle to CL to reducing to a Corpus Albicans (white body) as it regresses and finally disappears back into the ovary.

In the uterus the lining will reduce in thickness and just after the heat occurs, it may be visible on the vulva as a red discharge after the period of standing heat as some of the lining (endometrium) is expelled before ovulation.

Hopefully, you have found it interesting to see what’s going on with your cow between heats and appreciate how complex the system nature has come up with to reproduce cattle is.

If you’ve any questions on this topic or suggestions for what you’d like to see discussed contact me: alan@moocall.com or via our Facebook page

All the best,

Alan Horan, Moocall Researcher