With the start of winter feeding around the corner, vet Roger Blowey explained the basic biology of getting a cow in calf on Drew McConnell’s dairy farm near Omagh, Co Tyrone.

At birth a heifer calf’s ovaries contain all the eggs she will need for her reproductive life, with around 75,000 present in each ovary. These are seen as grey dots under a microscope which swell up into a fluid-filled sac called a follicle before a cycle begins, Blowey told farmers.

The egg is contained within the follicle on the surface of the ovary. The follicle produces the hormone oestrogen which causes the signs of heat such as mounting behaviour, enlargement of the vulva and passing of mucous from the vulva.

Egg release

The follicle bursts about 12 hours after the end of standing heat and the egg is released into the oviduct and passes down to the junction of the oviduct and uterus where fertilisation may take place if semen is present.

“The burst follicle leaves a granular tissue which then forms a corpus luteum. This stays in the ovary over the main part of the cycle and produces the hormone progesterone,” Blowey said.

If the cow conceives, the corpus luteum stays for the duration of the cycle. The hormone progesterone is produced by the corpus luteum to prepare the uterus to accept the fertilised egg. If the cow is not pregnant, the corpus luteum decreases in size from day 16-18 due to the production of the hormone prostaglandin, which will allow for the development of a follicle and the start of a new cycle.

“It is important to minimise stress to cows around breeding time. Stress is like loud noise and if the cow is pregnant the signal from the uterus to the brain is not heard as it is drowned out by a range of other signals caused by stress on the cow,” Blowey said. If signals to the brain from the reproductive system are then mixed up, prostaglandin can be produced even if the egg has been fertilised, meaning the embryo is not implanted and the cow does not go in calf.

Blowey listed a range of potential stress factors that could cause this to happen such as lameness, mastitis, ketosis, change of diet, overcrowding and changes to groupings of cows.

Blowey recommended that farmers AI cows earlier rather than later if an AI technician comes to the yard once a day. He pointed out that the egg will be viable for 12 to 24 hours once released from the ovary, whereas semen can survive for at least 36 hours in the cow.

Semen has to go through a period of capacitation for six hours in the uterus before it is ready to fertilise an egg and it also has to swim from the cervix to the end of the oviduct to fertilise the egg.

“For these reasons it is best to have semen ready and waiting on an egg. Personally, if I see a cow on standing heat today I would AI her today as you don’t know if she is at the start or end of heat so putting the straw in early is the best bet. If she is still standing the second day then maybe give her a second straw,” he said.

Farmer AI

Blowey added that for farmers doing their own AI work which can allow flexibility in timing, the optimum time to AI is after the end of standing heat.

“When artificially inseminating a cow put the gun into the cow at an angle; if it goes in straight it will hit the bladder. When a cow is bulling there will be plenty of mucous around the cervix. Rock the AI gun over the ridges in the cervix until the end is through the other side,” he said.

Blowey said the horns of the uterus divide soon after the cervix so the semen should be released from the AI gun once it passes through the cervix. If the gun is pushed in further, then the possibility of semen being left in only one horn of the uterus increases. This means that the egg will not be fertilised if the cow releases the egg from the opposite ovary.

Infertile heifer

An example of a placenta with two young foetuses was also shown to farmers at the event by Blowey. He said the smaller foetus would have most likely died as it was substantially smaller than the other foetus present.

He reminded farmers than when cows have twins that are a bull and a heifer then 95% of heifers will be infertile. Blowey said that the cause of some single heifer calves being infertile for no apparent reason can often be that originally a male foetus was also present in the placenta but later died.

He said that the movement of hormones from the male to female foetus causes the infertility in heifers and that this can happen at a very early stage in pregnancy meaning, if the male foetus later dies without being noticed, the effects of the hormone transfer still impact the heifer’s fertility.