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Tuesday 7 July 2015

The hormonal reproductive cycle in female animals is known by a few different names. For clarification I will only refer to it in this document as the Oestrus Cycle (OC). Other names include the estrus cycle and oestrous cycle along with the reference of coming ‘on heat’ or ‘bulling’ – they all refer to the same thing.
Let’s start from the beginning… when a female cow (heifer) is born the stockman will aim for this cow to give birth to its own calf at 24 months of age – the gestation period for cows is 9 months – so the heifer must get pregnant at 15 months of age. In order to get pregnant the heifer must be at the correct stage of her OC; this stage is when the cow is most fertile and most sexually active. During this stage of the cycle many changes are taking place within the cow’s reproductive organs (the ovaries).

Heifers will begin cycling at the onset of puberty, which is determined mostly by liveweight. Generally speaking, a stockman should aim for a heifer of 12 months of age to be 50% of its expected adult liveweight (e.g. if it is expected to be a 600kg adult cow, it should be 300kg at 12 months) and following this, should gain 10% of its adult weight in the 3 months leading up to its mating with a bull (e.g. at 15 months should be 360kg in weight). Puberty can also be affected by breed, nutrition and disease. A heifer will stop cycling when they get pregnant and resume cycling after calving, usually around 20-30 days after giving birth. However, this time can be variable and accounts for considerable losses, especially when a heifer has not cycled before her first mating.

However, it is important and beneficial to the heifer to undergo 2 or 3 cycles before she actually mates with the bull. By allowing the heifer to cycle, her chances of conceiving are increased for the immediate future and in the long term – rushing a heifer into getting pregnant can have detrimental economic and physiological effects. In light of this, the heifer should be having her first normal oestrus cycle starting at 12 months of age… but what is a normal oestrus cycle?

What is an Oestrus Cycle?

An oestrus cycle is a 21 day period consisting of hormonal changes of oestrogen, progesterone and others, which eventually leads to a climatic ‘heat’ period in which the cow is most sexually active and viable for conception.
21 days is the average time span of one cycle. Some may last longer than others and it may vary between cows. Generally the range is between 18 and 24 days.
The OC is governed by the complex interactions of various hormones that are produced in the brain and ovaries. Two of the most important are progesterone and oestrogen.
In short, the follicle (the egg) grows throughout the cycle and ovulation (releasing off the egg) occurs when the progesterone levels drop and oestrogen levels rise.
A structure called the corpus luteum then forms on the ovary, which then produces progesterone again, restarting the cycle.

The best way to understand the OC is by viewing the changes in hormones in relation to changes in behaviour when ‘on heat’, as shown in the following diagram…

Oestrus, as shown above by the graph labelled as ‘Heat’, is defined as the period of maximal sexual activity. The average duration is thought to only be around 8 hours for the modern dairy cow; however it can range from 2 – 30 hours.

The Biology of the Oestrus Cycle
Many oestrus cycle explanations use the two hormones Oestrogen and Progesterone to explain the changes in behaviour and ovary function over the 21 day cycle – however, this is a very shallow description and in reality, many more hormones take part. It is important to know the functions and changes in all the hormones throughout the cycle, in order to get a full and thorough understanding of what is happening and why each hormone is significant.
The best way to understand the changes in the hormones and ovarian activity is to start off simply. The following graph shows a normal oestrus cycle duration – lasting 21 days and experiencing ‘heat’ at the end of this period, followed by the restart back to day 1 of the next cycle.

Ovulation is the rupturing of the follicle that releases the ovum (egg) and the beginning of the cycle. The same hormones control the timing of behavioural heat and ovulation. This is important to know because the bull (or artificial insemination technician) must know the best time to deposit the semen for optimum fertility.

The tissue of the newly ruptured follicle does not just go away – it continues to have an important function by turning into the corpus luteum (CL) by approximately day 5 of the cycle.

The main role of the CL is to produce progesterone, a steroid hormone that must be in the blood to establish and maintain pregnancy.

A timing mechanism is in place to remove (lyse) the CL if a pregnancy does not result from the previous ovulation (i.e. on the graph, if the first ovulation does not lead to a pregnancy, the progesterone levels at day 15 will purposefully decline) – this timing mechanism is housed in the uterus. It is necessary for this to happen in order to restart the cycle.
The uterus produces another hormone, prostaglandin F2a (PG), around day 17 of the cycle. The PG ruptures the CL and causes it to regress in about 3 – 5 days.

 PG is always released from the uterus, whether or not the cow is pregnant. However, in the event of a pregnancy, the effects of PG must somehow be cancelled out, in order to maintain the CL; the embryo either releases a hormone which blocks the PG or a hormone which will stop the release of PG from the uterus (this pathway is not fully understood) – but in either case, the CL is prevented from being lysed and it continues to release progesterone. This means that cycling ends (the next ovulation and oestrus are blocked) and the pregnancy is maintained. In a normal healthy animal the embryo will develop into a foetus; which will then be born about 283 days after the egg was fertilized.
Another important issue is the growth of the follicles before ovulation. This involves more hormones, which are produced by a gland located under the brain (pituitary gland). These hormones travel through the blood to the ovary in order to direct follicular growth. They are follicle stimulating hormone (FSH) and luteinizing hormone (LH).

When FSH is released, it stimulates a group of follicles to grow (recruitment). These follicles continue to grow, although some eventually die off and regress, until only one remains and mature enough to ovulate. However, if the other hormones are not at the correct concentration, the follicle fails to ovulate and regresses. When this happens a new group of follicles is recruited and the process starts again.
After recruiting of a group of follicles, LH is responsible for keeping them growing. As previously mentioned, many of the follicles die off until only one is left to mature. If progesterone is not gone from the blood when that one follicle is ready to ovulate, then it also regresses and lets a new group of follicles be recruited (i.e. if progesterone is gone from the blood, the dominant follicle will ovulate).

LH is also the hormone that stimulates ovulation. If progesterone is gone from the blood (the CL has been lysed), LH is allowed to reach a greater concentration in the blood and triggers ovulation. Gonadotropin releasing hormone (GnRH) controls the release of both FSH and LH, which is why an injection of GNRH can lead to ovulation.

Usually, 2 or 3 groups of follicles are recruited before the CL is gone and ovulation occurs.

On the graph above, the first recruitment of follicles leads to one dominant, mature follicle around day 7 – this follicle regresses due to the levels of progesterone (red line) being high at this time. The same can be said for the second follicular wave.
Something different happens for the third follicular wave, at day 20, when the dominant follicle is mature, the levels of progesterone have declined and so LH has been able to stimulate ovulation.
The biological theory behind these hormonal changes and ovulation is fairly simple to understand. At the start of the cycle, when oestrus occurs and ovulation follows, if the egg becomes fertilised by a sperm cell, the cow becomes pregnant. Throughout the cycle the progesterone levels will consequently stay high and so will not allow ovulation to occur anymore, as there is no need, an egg has already been fertilised. However if the cow is not pregnant, the oestrus cycle is essentially a time for the reproductive organs to replenish and reorganise themselves ready for the next ovulation.
It is useful to view the oestrus cycle in 2 ways depending on what happens at the start of the cycle when ovulation occurs.
If ovulation at the start of the cycle results in pregnancy (i.e. the egg is fertilised) then the rest of the cycle aims to maintain this pregnancy.
If ovulation at the start of the cycle does not result in pregnancy, then the rest of the cycle aims to get ready for the next ovulation in 21 days’ time.

Another important hormone to mention is Estrogen/Oestrogen. This hormone is produced by the follicle, travels to the brain and stimulates the final rise in LH that triggers ovulation. The rise in oestrogen concentration is also what causes the cow to become sexually receptive. She stands to be mounted by another cow or heifer and stands to be serviced by a bull.
The blood concentration patterns of some major hormones of the oestrus cycle appear below. Progesterone is dominant during the middle of the cycle but is removed towards the end (CL is lysed) by PG. this allows oestrogen to increase and trigger oestrus and ovulation.

The next image shows the hormones, relative ovarian structures and the days of the cycle.

Yellow circles = follicle growth, purple line = oestrogen, red line = luteinizing hormone, blue line = progesterone, purple star = ovulation
·         Day 0: defined as day of oestrus. During this period, a cow will stand to be mounted. Heat usually lasts for an average of 8 – 12 hours but can sometimes be shorter or longer. During this time the follicle structure on the ovary is present. Oestrogen from the follicle triggers GnRH release.
When GnRH is released it will also cause the release of LH, which in turn induces ovulation and releases the egg. The LH surge occurs at the onset of oestrus, ovulation occurs approximately 32 hours after the LH pulse. If sperm are present when the egg is released, fertilization will occur.
·         Day 1 – 5: the follicle “luteinizes” to form the CL which secretes progesterone.
·         Day 9 – 10: the CL reaches maximum size and progesterone output.
·         Day 16 – 18: if a cow does not become pregnant PG will be released by the uterus. As progesterone levels decrease and an increase in GnRH takes place, release of LH increases and a preovulatory estrogenic follicle develops. If the cow is pregnant, the embryo blocks the release PG and progesterone continues to be secreted by the CL.
·         Day 20 – 21: high oestrogen levels cause heat behaviour which precedes ovulation.

What is Oestrus/Heat?

As previously mentioned, being on heat means the cow is the most sexually active that it will ever be throughout that particular cycle. Oestrus is accompanied by behavioural and physiological changes which make detecting its present fairly simple.
There are various signs of oestrus and some animals will express these to varying degrees.

When a cow is on heat she will be mainly focussing on trying to mate. In this mind set, whether the mating partner is female or male, it doesn’t matter to the cow. This is why a common observation seen is cows mounting each other.
The cow that is on heat will mount another cow, jumping up on her hind quarters just as a bull would to serve a female cow. This indicates that the cow that is jumping up is on heat. 
But also, if the cow that is being jumped on is not on heat she will resent this action and run away/struggle to get the other cow off her back – however, if the cow being mounted stands still and accepts the mount, it is likely that she is also on heat as she wants to mate too (regardless of the fact that they are both females).

Due to the above reasons, rub marks/sores on the tail head and pin bones are seen where cows have been mounted many times, meaning they are likely on heat.

Sometimes it is not as severe as this photo. In white coated animals it is sometimes seen as if their coat has been worn away slightly on their hind.
When two or more cows are on heat at the same time they tend to stay together in a small group and will follow one another throughout the day. Isolating one or taking one away may cause the others to become increasingly inquisitive or bellow persistently.

For example the cows above were all on heat and stayed with each other all day in a small group, intermittently mounting each other.
Just before mounting, cows will often stand with their chin rested upon the pin bones or just behind the other cow – watch out for this as the cows will often follow this action by mounting.
Increased restlessness and activity is also sometimes seen. Along with decreased feed and intake and milk yield. Other less common signs are slight increase in body temp (0.1 degree), clear vulval mucus (bulling string) and odd behaviour such as standing alone or becoming erratic.

Improving Heat Detection
For good heat detection there must be…
·         Clear identification of cows (e.g. by freeze branding or easy to read ear tags)
·         Adequate light to ensure cows can be seen in heat and identified
·         Regular allotted oestrus observation times. Not just at milking or feeding. 20-30 minutes every day to observe the cows naturally. Most mounting occur between 6am-6pm.
·         Good recording system so that those in heat can be recorded and any abnormalities or patterns identified. This will also help with pregnancy planning.
·         Adequate loafing areas with non-slip floors to allow cows to exhibit normal behavior.
Recently there have also been new ways to aid heat detection, such as…
·         Heat mount detectors – these are stuck on the cows’ backs, just above the tail head and are triggered by the pressure of another cow mounting them, leading to a colour change on the detector. Popular products for this are Kamars™, Bovine Beacons™ and Estrotect™.
·         Tail paint – this works in a similar way to the heat detectors. The paint is rubbed off the backs of the tail head when the cow is mounted. This needs to be reapplied once it becomes dry.
·         Motion detectors/pedometers – are attached to either the neck or the leg bands and any increase in walking activity are remotely detected and recorded on a computer. These can be very useful but care must be taken to interpret the results in light of other aspects.
·         Milk progesterone assays – regular samples of milk can detect the fall in progesterone prior to oestrus. These are available as on farm kits but will only become practical on a large scale once in-line detectors become available.
·         Hormone treatment – groups of cows can be synchronised with hormone treatment to allow fixed AI times.