Role Follicle Size Breeding Time In Thoroughbred Mares Biology Essay

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Ireland has long been associated with the breeding of thoroughbred horses. The ideal landscape provides a perfect backdrop to the success that has been achieved in breeding and on the racetrack all over the world. Ireland holds bases for many of the world's largest breeding operations while also containing over 700 successful indigenous stud farms. Employment form the thoroughbred industry is huge, according to the Irish Thoroughbred Marketing report, 16,000 people are employed directly along with 9-10,000 being indirectly employed. In order to maintain the level of success of this industry in Ireland key reproductive practices must be looked at and investigated so as to further increase the likelihood of future success.

There are many factors that affect the reproductive performance of the thoroughbred mare and these factors hold major significance for breeders both financially and in relation to breeding practices used. Work done on this subject has been limited number and hence factors which may affect follicle size and its importance and significance have yet to be researched.

The pre-ovulatory follicle size is of major significance to the reproductive performance of the mare (Watson et al 2002). According to Perry et al (2005) when cows were compared at pregnance at day 25, it was found that the smaller pre ovularoty follicle size was less likely to be represented by pregnancy than a larger size. In the mare, follicular growth occurs in a pattern that occurs in a wave like fashion, this wave like pattern ultimately ends with one dominant follicle and one subordinate follicle.(Ginther 2000). Dominant follicles regularly grow until they are >22.5mm in diameter. They then separate away from the subordinate follicles, they do this through the realise of inhibin which has the feedback effect of reducing follicle stimulating hormone (FSH) which halts the subordinate follicular growth (Donadeu et al 2002) and interfollicular insulin like growth factor (IGF). The dominant follicle is then able to respond to the FSH and continues to grow (Ginther 2000). This allows the follicle to continue its dominance while the subordinate follicles halt there growth pattern.The dominant pre-ovulatory follicle in the horse can reach up to 40 mm diameter before ovulation with the next nearest subordinate follicle normally being at least 15 mm smaller (Palmer et al 1980). Follicles need to be >20 mm in order for oocyte to reach meiotic competence. (Hinrichs et al 2000).

Time within the Breeding season

Fillies begin oestrous cycles at puberty which is usually at 18 months. They are seasonally polyoestrus (Pilliner et al 2004) which means that they will come into oestrus many times during the breeding season. In the mare this season is spring through summer to autumn. In the winter they are anoestrus (Raz et al 2009) , which means there is no cyclical activity and thus no ovulation takes place. This winter anoestrus is then followed by a period of change over to regular cyclical activity or seasons. During this transitional phase the oestrous periods may be irregular or very long. During the first oestrus after the winter period, there is often a poor correlation between sexual behaviour and ovarian activity (Pilliner et al 2004). These heats often do not show palpable follicles when the mare is investigated.

The oestrous cycle has two phases, that of oestrus, where ovulation takes place and that of dioestrus (between oestrus periods) when the mare is unreceptive. The cycle usually lasts 21 days in the mare, oestrus lasting for approximately 5 days and dioestrus for 15-16 days (Watson et al 2002).

Follicle size varies throughout the breeding season. It can be expected that pre ovulotary follicle size would increase/decrease with reproductive activity as the mare experiences transition periods (Raz et al 2009), however this is not conclusive as it has been found that maximum pre-ovulatory follicle size has been found at the start of a season (Feb 1st-14th) and the smallest at the end of the season (Aug 15th-31st), (Davies-Morel et al 2010). Table 1.

Follicular development occurs in a wave like fashion during the equine oestrus cycle, pregnancy, and seasonal transition. Follicular waves have been categorized as major( primary and secondary) and minor waves depending on whether the largest follicle of a wave reaches ≥ 30 mm in association with follicle selection (major wave) or < 30 mm without selection (minor wave) during the oestrus cycle (Samper 2009). Both types of wave patterns follow a similar growth pattern at the start of a wave but only major waves exhibit a dominance phase after selection of the largest follicle (dominant) and selection against the next largest (subordinates) at the end of the common growth phase (Davies-Morel et al 2010) Subordinate follicles will decrease in diameter and dominant follicles will increase until it ovulates or regresses depending on the time of the season (Samper 2009).

Hence it is important that the timing of breeding be considered as Davies-Morel et al (2010) found that especially with older mares if mated towards the end of the breeding season and especially if they have more than one large follicle, are significantly more likely to ovulate from smaller follicles. If, as suggested by evidence largely from other mammals, follicle size is related to reproductive success, small pre-ovulatory follicle size may account, at least in part, for the poorer reproductive performance of such mares and so justify their increased monitoring and careful reproductive management ( Davies-Morel et al 2010)

Table one: The effect of season on preovulatory follicle (F1) diameter. (Morel et al 2010)



Average dominant diameter (mm)


95% Confidence interval


Feb 1st-14th





Feb 15th-28th





March 1st-14th





March 15th-31st





April 1st-14th





April 15th-30th





May 1st-14th





May 15th-31st





June 1st-14th





June 15th-30th





June 15th-30th





July 15th-31st





Aug 1st-14th





Aug 15th-31st





Sept 1st-14th





Sept 15th-30th





Feb 1st-Sept 30th




The effect of mare age on pre follicular diameter

Mare age is one of the main factors reported to have an affect on the reproductive performance of mares. Older mares generally become less reproductively competent, largely due to decreasing uterine health and oocyte viability (Carnevale et al 1997). If a link between decreasing follicle size, decreasing oocyte viability, and hence poorer reproductive performance does exist it might be expected that pre-ovulatory follicle size decreases with age.

In a study by Davies-Morel et al (2010) mare age did have a significant negative effect on pre-ovulatory follicle diameter. The smallest pre-ovulatory follicles (33.30 ± 4.66 mm) were observed in mares aged over 19 yrs and the largest pre-ovulatory follicle diameter (38.95 ± 5.61 mm)were observed in mares aged 2 to 4 yrs. Recent work by Ginther et al (2004) agreed with Davies-Morel et al (2010) and additionally indicated that older mares had slightly (1 day) longer oestrous cycles. This may indicate a slower final growth rate of the pre-ovulatory follicle or a longer time interval from the achievement of maximum pre-ovulatory follicle diameter to ovulation. This is in contrast to the findings of Carnevale at al (1997)who found that for pregnant mares, differences between age groups were not significant for the diameter of the largest follicle, cross-sectional area of the corpus luteum, growth profile of the embryonic vesicle or embryo mobility characteristics. The results of this study did confirm that old age was associated with increased endometrial inflammation, reduced pregnancy rate and increased embryo-loss rate. The results also indicated that uterine contractility and uterine tone were reduced and the fixation of the embryonic vesicle occurred later in old than in young mares.

Thus there is conflicting views about what exactly mare age effects, whether it be follicle size and hence the subsequent poor reproductive performance or other factors affecting the poor reproductive performance of older mares.

The effect of multiple ovulations (MO) on pre-ovulatory follicular diameter

The presence of more than one pre-ovulatory follicle is not of rare occurrence in thoroughbred mares, especially older mares (Davies-Morel 2001). Little work has been done to investigate the effect of the presence of more than one large follicle on pre-ovulatory follicle size in the mare. However, that which has been done in mares (Pierson et al 1985) agrees with Davies-Morel et al (2001) which found an incidence of 24.56% of multiple ovulations. Davies-Morel et al (2001) also indicates that this scenario results in a decrease in pre-ovulatory follicle diameter from 37.44 ± 4.84 mm in SO mares to 35.45 ± 4.53 mm in MO mares.

The result obtained in Davies-Morel et al (2001) is unsurprising and might be explained by a disruption in the normal pre-ovulatory follicle selection and dominance mechanism. This disruption may alter the normal process of follicle selection (Ginther et al 2003) allowing the dominance of two follicles in the same wave. Why this disruption should occur is unclear, though MO is considered to be a particular problem in older mares which have a decreased pool of follicles, resulting in a decreased inhibin production which in turn leads to elevated FSH. This might allow the continued growth of the largest subordinate follicle in addition to the continued development of the dominant follicle [46] Y. Nambo, H. Kaneko, S. Nagata, M. Oikawa, T. Yoshihara, N. Nagamine, G. Watanabe and K. Taya, Effect of passive immunization against inhibin on FSH secretion, folliculogenesis and ovulation rate during the follicular phase of the oestrus cycle in mares, Theriogenology 50 (1998), pp. 545-557. Article | C:\Users\Quinn\Desktop\Dans Files\College\FYP\Journals!\science_files\icon_pdf.gifPDF (991 K) | View Record in Scopus | Cited By in Scopus (42). The increased size of the previous subordinate follicle along with the failure of the dominant follicle to maintain its dominance may lead to reciprocal inhibition between the two and so decrease pre-ovulatory follicle size. It is surely of significance that the number of embryos conceived either after flushing or at early pregnancy diagnosis per ovulation is considerably lower from multiple than from single ovulations (Newcombe 2004)

Whatever the cause, the smaller pre-ovulatory follicle size observed in MO may in part account for the reduced pregnancy rate per ovulation especially observed in older MO mares

The timing of breeding in regards to ovulation

The correct time of breeding maybe be the ideal, helping to reduce the number of coverings without negatively impacting on conception rates. Clement et al (1998) studied the effects of artificial insemination in mares when they were scanned and showed a preovulatory follicle >30 mm. The mares where inseminated when the follicle was observed and then every 48 hours thereafter. Semen samples used in insemination were tested for semen quality and numbers. The study achieved a per cycle conception rate of 45% and showed that more frequent insemination was not significantly better when compared to a single well timed insemination per cycle. This leaves the question, how to judge the timing of a covering or insemination. This is of critical importance to breeding managers as it is often desired to have as least as possible matings per served oestrus cycle.

Breeding managers must have good knowledge of the hormones which effect the reproductive system. Figure 2 below illustrates the important hormones and there function which will influence follicle growth and development.


(Pilliner et al 2006)

Palpation and teasing are the two most common management tools used in the detection of heat. Rectal palpation and ultrasonography will help define the time of ovulation, and thus aid in mating management. Parameters of follicular size, follicular consistency, cervical size and consistency, and uterine tone can be monitored through rectal palpation. A mare with a large, very soft follicle that has an open cervix is a prime candidate for breeding. On the other hand, a mare that has no or very small turgid follicles and a closed cervix would be a poor candidate for breeding. Most managers use a combination of teasing and palpation for estrous detection and breeding determination. With real-time ultrasonography, veterinarians can determine follicular size, early ovulation, uterine changes characteristic of estrous, and abnormalities of the reproductive tract. Ultrasonography is a significant contributor to reduction of the number of breedings or inseminations required per estrus. (Pilliner et al 2006)

The Effect of Pharmacological Intervention on pre-ovulatory follicles

It appears that the most prevailing issue on pharmacological intervention on pre- ovulatory follicles is the rate of twinning obtained from such practices. Veronesi et al (2003) compared the results between 324 mares that received no treatment (control) with 356 mares that were treated during the oestrus cycle in which they became pregnant. Treatments consisted of an injection of Estumate (R) on detection of a Corpus Luteum, an injection of hCG before mating or a combination of both. Mares treated had a higher rate of twinning than control mares (16.5% versus 6.5% respectively). Mckinnon (2008) also showed differences between marres that were treated and those that were not. 19.7% of those treated with a pharmacological substance became pregnant with twins.

Hence, pharmacological intervention can have the desired effect in inducing ovulation and managing the oestrus cycle but great care must be taken in scans post mating to identify and deal with any resulting twin pregnancies. This may be due to the drugs ability to bring on a wave where the dominant and subordinate follicle do not have a separation point and continue on the same growth path causing twin pregnancy.

Aims and objectives

The aim of this project is to investigate the role follicle size plays at breeding time in mares and its subsequent influence on reproductive performance in the thoroughbred mare. It will be set out to investigate different factors which effect follicle size and why they affect it. This will then be applied to the sample undertaken in this project and observed to see how the statistical analysis of the sample used in this study can relate to the different factors which affect follicle size and it will be investigated whether these factors had an effect on the Mares subsequent reproductive performance.

The objective of this project is to provide more information about the role of the follicle in thoroughbred reproductive performance. This area had not been extensively researched and allot of questions are still unanswered as to when is the perfect time for breeding in relation to follicle size and also possible reason as to why follicle size changes with such variables as multiple ovulations, age of the mare, breeding management, and status of the mare. The objective therefore is to help and inform the breeding manager or any other mare owner of the possible importance of monitoring follicle size and the variables that may effect it.

Methods and Materials

Data was obtained from Castleton Lyons Stud Farm, Lexington, Kentucky.

The Data obtained contains health records from 100 mares over a period of 5 years per mare meaning a sample size of 500 records.

Data will be entered into excel and cleaned into a readable format containing essential information.

Data will then be input into SPSS.

It will be checked for distribution of data to see if it is normally distributed.

Analysis will then be run on the data.

The results obtained will be summarised in graph form

The results will then be interpreted.

Key Stages

The compiling of all relevant data.

Sourcing relevant literature on the subject matter.

The inputting of data into Excel.

The cleaning up of data and choosing which format to put the data in.

Deciding appropriate statistical tests to run on the data.

Running the analysis of the data

Interpreting Results

Drawing conclusions from the project.

Possible difficulties and how to overcome them

The sample size; The sample size of 500 records is slightly small but it is not importantly too small as many of the studies already done on this subject matter have not used large sample sizes possibly due to the difficulty in obtaining records of this nature from Stud farms. So in comparison to other available studies the sample size is of sufficient quantity.

The narrow range of follicle size; The follicle size given in the health records of the mare increase or decrease in increments of 5 mm. This is when the mare is palpated and the follicle is judged to be at this size.

The only assurance to this is that the Veterinary surgeon who took these measurements is fully qualified with many years of practical experience.

Availability of Materials

Data has been sent in electronic format

Computers are readily available in the College with software such as excel and SPSS available on the. SPSS and excel are also available on my laptop.

Literature relevant to my topic is available from the Glucksman Library, University of Limerick. It is also available from sources such as Science direct and Pubmed and general information is available freely on Internet explorer.