The use of hormonal treatments in the reproductive cycle of the cow - Veterinary Practice
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The use of hormonal treatments in the reproductive cycle of the cow

As the margins in farming are constantly reducing, the reproductive health of the herd is becoming more and more important

The profitability of both beef and dairy cattle farms depends on the reproductive effectiveness of their stock. This article will detail the stresses of producing a calf promptly, the bovine oestrus cycle, how the different hormones influence the oestrus cycle, and the ways different hormones and hormonal treatments can help farmers breed their cows at the right time.

Herd of cows

Cattle production outputs

Do not underestimate the pressures the average farmer is under. The main output of a suckler cow or heifer is the calf produced; a suckler that does not produce a calf within a year is, therefore, less productive. With an average gestation period of about 280 days, there is effectively less than three months available to get a cow pregnant again when the aim is to have a newborn calf every 365 days.

The ideal calving interval in dairy cattle is more variable than in beef cattle – to keep up milk production, a dairy cow needs to produce a calf at a regular interval. In the United Kingdom, if a dairy cow produces a high continuous supply of milk, an average calving interval of around 380 days can be profitable. It is unusual that dairy farms are optimal; calving intervals are often greater than this.

As the margins in farming are constantly reducing, the reproductive health of the herd is becoming more and more important

As the margins in farming are constantly reducing, the reproductive health of the herd is becoming more and more important. Many farmers struggle to get their cows pregnant within the desired timeframe.

Reproduction in cattle

Normally, cows have oestrus cycles all year round, and the average length of an oestrus cycle is about 21 days. After the calf is born, time is needed for the uterus to involute and to be clean and ready for a new pregnancy. It also takes a few weeks for a cow to return to cyclicity.

How quickly a cow is ready to get pregnant again depends on many factors, including post-calving illnesses and a low body condition during early lactation – both of which can delay oestrus cycles and the ability to conceive. Cows with uterine infections or metabolic illnesses take longer to return to normal cyclicity and find it more difficult to hold. Low conception rates and a lack of oestrus detection are the most common reasons for infertility in beef and dairy cattle bred with artificial insemination (AI).

Even when cows have normal reproductive cycles, it can be difficult to see them in heat. (A previous article on heat detection in dairy cattle provides more in-depth information on how to solve this problem.)

In the last few decades, different hormonal treatments have been used to synchronise reproductive cycles and time artificial insemination. By timing the moment a cow is standing to be serviced as closely as possible, it becomes easier to see it. Fixed-time AI can also be carried out when a protocol is very accurate. At the time of writing, the products licensed in the UK for reproductive treatments in cattle are synthetic gonadotropin-releasing hormone (GnRH), prostaglandin F2α (PGF2α), progesterone and chorionic gonadotropin.

The oestrus cycle

The oestrus cycle consists of two main phases: the follicular and luteal phases. The follicular phase is the period from the regression of a corpus luteum (CL) to ovulation. During the follicular phase, the main ovarian structures are preovulatory follicles, which produce oestradiol. The luteal phase is the period from ovulation until CL regression. The dominant ovarian structure is the CL, and the primary reproductive hormone is progesterone. Even though the luteal phase is dominated by progesterone from the CL, follicles continue to grow and regress during this phase (Senger, 1999).

The oestrus cycle can be divided into four stages: pro-oestrus, oestrus, metoestrus and dioestrus. Each of these stages is a subdivision of the follicular and luteal phases of the cycle. The follicular phase consists of pro-oestrus and oestrus, and the luteal phase of metoestrus and dioestrus (Senger, 1999).

The oestrus of a cow (day zero) is, on average, 18 hours. A sharp increase in luteinising hormone (LH) takes place about six hours after the onset of oestrus. Ovulation (day one) tends to take place, on average, 12 hours after oestrus and 24 hours after the LH peak. From days four to five, a CL that produces progesterone is present (Pieterse, 2008). When there is no embryo in the uterus to ensure maternal recognition, the endometrium will start producing PGF2α, and due to its production, the CL will go into regression. This then reduces the production of progesterone. Subsequently, GnRH production increases, stimulating the hypothalamus to produce more follicle-stimulating hormone (FSH) and LH. The increase in FSH stimulates the growth of follicles that produce oestrogen, which, in turn, stimulates oestrus behaviour (Pieterse, 2008).

Vet checking on cows

Follicular waves

Follicular waves take place during the entire oestrus cycle; typically, between two to three waves occur in cattle. During these follicular waves, a group of follicles is recruited. The goal of follicle recruitment is the production of a dominant follicle that will ovulate. Even though this process occurs repeatedly during the different stages, a dominant follicle will only develop to its final stage and ovulate when the right endocrine conditions are met.

All the other follicles that are recruited will undergo atresia, the regression of a follicle. It is important to remember that most recruited follicles will undergo atresia in the ovaries. The occurrence of a dominant follicle and ovulation is a process that only takes place after luteolysis and the subsequent decline in progesterone (Senger, 1999).

Reproductive programming

In order to able to control the reproductive cycle in the cow, it is essential to first control follicular recruitment and, second, to establish luteolysis.

The first programmes for oestrus synchronisation in cattle were focused on luteolysis and removal of the CL only. PGF2α was administered to establish luteolysis. In many of these simple programmes, two injections of PGF2α were administered 9 to 11 days apart. One of the negative aspects of this programme is that the time between injection of prostaglandins and ovulation is difficult to predict. This is because the timing of the follicular waves is not controlled in any way (Cockcroft, 2015).

Therefore, hormonal treatment programmes that include both PGF2α and GnRH were subsequently developed. The GnRH does three things:

  1. Firstly, it removes the dominant follicle that could be present at the start of the programme
  2. Secondly, it inhibits ovulation until the new dominant follicle has developed
  3. Thirdly, it helps to precisely control the time of ovulation of the new dominant follicle

Removal of the dominant follicle can take place in two ways. In the past, oestradiol esters were used to regress the follicle. Some time ago, they were taken off the market in Europe for use in cattle, which means that, nowadays, GnRH is used to ovulate the present dominant follicle (Cockcroft, 2015). For a successful synchronisation programme that includes only GnRH and PGF2α, the cow must be cycling at the time the programme commences. Prediction of ovulation time works best if the programme either pushes (by forced luteolysis by using PGF2α before GnRH) the cow into a timed oestrus cycle or if the programme is instigated at the right time of the cycle.

For a successful synchronisation programme that includes only GnRH and PGF2α, the cow must be cycling at the time the programme commences

Cows that are not cycling or have issues with their LH peak (for example, due to a problem in follicular recruitment or atresia) can be helped by the inclusion of an intravaginal progesterone-releasing insert in a synchronisation programme (Cockcroft, 2015). Also, the inclusion of a progesterone device aids an anoestrus cow to resume cyclicity.

Good, science-based synchronisation protocols for hormonal treatments are freely available and are listed by the Beef Cattle Reproduction Leadership Team and the Dairy Cattle Reproduction Council. (Both are American organisations that have a cross-section of various experts in bovine reproduction working together in creating and promoting the most successful synchronisation programmes.)

Final thoughts

Protocols for beef and dairy cows tend to differ slightly. One reason for this is that the numbers of animals with two- versus three-wave cycles are more or less equally distributed in beef cattle, whereas more two-wave cycles have been reported in lactating dairy cattle. There appears to be no clear breed- or age-specific preference for one follicular wave pattern over the other, nor is there any apparent difference in fertility. In addition, beef cattle protocols tend to have fewer treatments as it tends to be less problematic to handle dairy cattle frequently than beef cattle (Colazo and Mapletoft, 2014).

Maiden heifers and cows do not always respond in the same way to identical protocols for hormonal treatments

As a last note, it is important to point out that maiden heifers and cows do not always respond in the same way to identical protocols for hormonal treatments. Therefore, it is important to compare the differences and to stick to protocols that are recommended for either beef or dairy cattle, and either heifers or cows.

References

Cockcroft, P. D.

2015

Bovine Medicine, 3rd edn. Wiley and Sons Ltd, Chichester

Colazo, M. G. and Mapletoft, R. J.

2014

A review of current timed-AI (TAI) programs for beef and dairy cattle. Canadian Veterinary Journal, 55, 772-780

Pieterse, M.

2008

Rund; Praktische tips Fertiliteit, 3rd edn. MSD Animal Health

Senger, P. L.

1999

Pathways to Pregnancy and Parturition, 1st revised edn. The Mack Printing Group-Science, Ephrata

Lies Beekhuis

Lies Beekhuis, DVM, DipECBHM, MRCVS, is a European and RCVS specialist in cattle. She originates from The Netherlands, where she graduated in 2006. Lies has worked at universities and in private practice, and since 2022, she owns Tivy Vets, a large animal veterinary practice.


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