Saturday, March 30, 2024

Understanding oestrus

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Cows in oestrus, cows in anoestrus, cycling, non-cycling – what does it all mean? Your vet will use these words, and yourartificial breeding technician, but getting to grips with the terminology surrounding your cows’ ability to get in calf is not that easy, especially because there are a few more complicated words to understand. You might have covered this in biology at high school and after exams promptly forgot it. Hopefully you were paying some attention. First the basics. Cows have a 21-day hormonal cycle (compared with 17 for sheep and 20 for deer) but it can be anywhere between 18 and 24 days. That means about every three weeks after it reaches sexual maturity, unless it is pregnant or following calving, a healthy cow will come on heat. This is also known as cycling or in oestrus. In the paddock it means she will be ridden by other cattle. Anyone winter-grazing yearling heifers will know the problems that causes.
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The oestrus cycle is controlled by four main hormones – oestrogen, luteinising hormone, progesterone and prostaglandin.

Oestrogen is produced by follicles on the ovary surface. It enters the bloodstream and circulates around the cow’s body, affecting its brain and nervous system.

It causes the cow to be restless, seek out bulls, ride other cows and stand to be ridden. Usually it lasts about 24 hours. 

The oestrogen surge also causes luteinising hormone (LH) to be released by the anterior pituitary gland (at the base of a cow’s brain), which causes the follicle on the ovary to swell and release an egg – ovulation.

This bit is important – there is a gap between the oestrogen surge and the LH surge, which means ovulation occurs late in oestrus.

So don’t panic and think you need the AB technician straight away when you see a cow starting to ride other cows as it comes into heat. It’s okay to inseminate at the end of the heat, instead of at the start.

Luteinising hormone is so-named because that’s what it does – it changes the follicle that has released the egg into a corpus luteum (which is simply Latin for “yellow body”, which is what it looks like on the ovary).

The corpus luteum takes about four to five days to form (this time is called metoestrus) and then it starts producing progesterone.

During metoestrus you might see blood on the tail of cows, especially heifers. This is because the oestrogen surge can cause increased blood flow and bleeding in the uterine area. It is not a sign of conception or failure of conception.

A variety of heatmount detectors.

More importantly, you might hear people talk about silent heats. This is when a cow is ovulating but not showing any signs of it in the paddock – it isn’t standing to be ridden.

It’s a problem because although they could possibly get in-calf at this time, it is impossible for farmers to identify these animals.

They won’t cause a heatmount detector like a KAMAR to go off, or have rubbed tail paint. They won’t even stand for a bull to ride them.

A silent heat is caused because there aren’t enough of the right hormones. For the cow’s nervous system to recognise the oestrus hormone surge it first has to be exposed to high progesterone levels.

But you don’t get oestrogen without progesterone, which is why silent heats commonly happen after calving when progesterone levels are low.

One way to help prevent silent heats is to put the cows that have gone up for AB back into the herd straight away. They will still be bulling and will bring other cows on.

Intervention methods which start or interrupt a cow’s oestrus cycle use either prostaglandin or progesterone. These are used to synchronise cows, so they can all be mated at once, or to bring non-cycling cows into heat.

Intravaginal CIDRs (controlled internal drug releaser) release progesterone continuously for about seven days and are then removed, with cows coming into oestrus three days later. Cows are usually injected with prostaglandin when the CIDRs are removed.

Injections of prostaglandin are also used without CIDRs but as prostaglandin on its own will only affect the oestrus cycle if the animal is in dioestrus two injections are needed 10 to 12 days apart.

If you are having problems with cows not cycling, make sure you are feeding them well.

Otherwise animals are mated as they come into heat for about seven days, then those that haven’t been inseminated are given an injection of prostaglandin that will cause them to come into heat in the following three to four days.

Both intervention methods use drugs that are only available through vets. If you are planning to use these treatments contact your vet well in advance and they will advise you of the programme that best suits you.

Also let your AB technician know so they are ready to inseminate the animals.

With both of these intervention methods cows might not show signs of heat. Don’t panic and follow your vet’s advice.

Also remember the cows that don’t become pregnant following prostaglandin and-or progesterone treatment will all cycle together about 21 days later – if not using AB you will need to have enough bull power to cover it.

With increasing knowledge of the oestrus cycle and improvements in hormone drugs and their delivery, these programmes change from year to year. Get advice from your vet each year and follow it.

If you are having problems with cows not cycling, make sure you are feeding them well.

Studies have shown underfed yearlings will reach puberty later and might not be ready for mating, and low condition score heifers and cows are more likely to be noncyclers or have silent heats.

An animal with lifting condition is ideal. If they are fat and go into a feed pinch you will have trouble getting them to cycle.

If they calved late you might be asking them to cycle and reach peak lactation at the same time, which is difficult to do, especially if the weather turns bad.

Recent research also shows well-fed cows have healthier follicles and eggs and so are more likely to become pregnant.

The oestrus cycle

Oestrus (day 1) – The 24 hours when the cow is on heat, which is caused by an oestrogen surge produced by the follicle on the ovary.

Metoestrus (day 2-4) – The egg is released from the follicle on the ovary and the remnants of the follicle become a corpus luteum that begins secreting progesterone.

Dioestrus (day 5-17) – The corpus luteum secretes progesterone.

Prooestrus (day 18-21) – In response to fertilisation of the egg not happening, the uterus secretes prostaglandin that destroys the corpus luteum allowing a new follicle on the ovary to form which produces oestrogen.

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