MENDELNET 2016
MULTIPLE PREGNANCY IN MARES
MARIE IMRICHOVA Department of Animal Breeding Mendel University in Brno Zemedelska 1, 613 00 Brno CZECH REPUBLIC [email protected]
Abstract: Mare is in terms of reproduction described as uniparous, it means that she has one foal. More embryos, fetuses or foals represents non – physiological phenomenon and as such it brings a lot of complications. In terms of the etiology is discussed as a predisposing factor mare’s breed, the most common is a higher incidence associated with Thoroughbreds. Although in context of multiple pregnancies often mentioned is mare‘s age. The aim of this study was to evaluate the incidence and results of multiple pregnancy in Thoroughbred and Old Kladruber Horse mares. It was found 323 records of multiple pregnancies in Thoroughbred and 48 in Old Kladruber Horse mares and the result of the multiple pregnancy was in Thoroughbreds in 258 cases twin abortion, in 45 cases parturition of 2 dead foals, 15 records of parturition 1 live and 1 dead foal and in 5 cases it was the parturition of 2 live foals. In Old Kladruber Horse twin abortion was recorded 36 times, in 7 cases 2 dead foals and in 5 cases 2 live foals. Key Words: horse, breeding, reproduction, pregnancy, twins
INTRODUCTION The reproduction standard in horse breeding is the fundamental informative factor affecting its success and profitability. One of the factors that can decide about the reproduction outcome is also multiple pregnancy. Multiple pregnancy basically may occur naturally either by spontaneous division of embryos, or by fertilization of two oocytes after multiple ovulation. Specifically horses in the vast majority of cases have dizygotic twins, thus originating from two separate embryos. In terms of double ovulation, which has the potential to result in a multiple pregnancy, can be distinguished according to the localization of an unilateral (two ovulations in the same ovary) and bilateral (one ovulation per each ovary). In the case of the time determination it may be a synchronous (usually during the 24 hours), and asynchronous ovulation. Multiple pregnancy as such can also be classified as unilateral and bilateral, depending on whether at the stage of fixation one embryo is attached in each uterine horn, or both together in one. All these factors, with maternal and environmental influences together, and especially management of selected twins from the breeder’s side affect the outcome of multiple gestation, and thus how its impact is reflected on mare’s reproduction. The incidence of multiple pregnancy in mares generally, for example, as Bílek et al. say (1957) between 2-2.5% of cases, Hutton and Meacham (1986) at 1.1% and Doležel et al. (2000) from 1.5 to 2.5%. As predisposing factors for multiple gestation are most frequently mentioned – breed, mare’s age, reproductive history and heredity. Górecka and Jezierski (2003), Bresińska et al. (2004), McCue (2009), Sheerin (2014) and others state that a breed with the highest incidence of multiple pregnancy is Thoroughbreds. Specifically, in Thoroughbreds Hutton and Meacham (1968) illustrate the incidence of multiple pregnancies at 2.4%, Doležel et al. (2000) at 2.57%, and Bresińska et al. (2004) and Wolc et al. (2006) at 3.5%. Regarding the age Davies Morel (2012) reported that incidence of multiple pregnancy increases in the older mares. Davies Morel and O'Sullivan (2001), Górecka and Jezierski (2003) and McCue (2009) argues that the mares, in which multiple ovulation or pregnancy already occurred once, have a greater likelihood of recurrence than mares in which this phenomenon has not been recorded. With respect to the inheritance factor from the work of Pawlak et al. (2000), Górecka and Jezierski (2003) and Davies Morel (2008) arises that predisposition for the multiple pregnancy is inheritable.
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In case of occurrence of multiple pregnancy, the most of the authors are united that only in the low percentage it results in one or two foal births. For example, Larson (2011) says that the chance of a mare having full term of two foals and their successful parturition is 1:10 000. Davies Morel (2008) says that 9% of twin pregnancies survives to full term and the parturition results in the death of both foals in 64.5% cases. The aim of this study was to evaluate characteristics of multiple pregnancy occurrence in the Thoroughbred population and for comparison in the Old Kladruber Horse, to find out in how many cases the result of multiple pregnancy is successful parturition of one or two foals and to evaluate the interrelationships between the result of multiple pregnancy and mare’s breed or age.
MATERIAL AND METHODS In the research, selected reproduction characteristics of Thoroughbred and Old Kladruber Horse breed mares were evaluated. Required data were obtained from the Czechoslovak Thoroughbred Studbooks (Volume XI.-XIII.), Czech Thoroughbred Studbooks (Volume 1. to 4.) and the Old Kladruber Horse Studbook. In the Thoroughbred Studbooks records of mares born in the years 1965-2001 and active in breeding in 1985–2008 were found. In the Old Kladruber Horse Studbook they found records of mares born between the years 1978 to 2001 and active in breeding in the years 1981-2009. The Thoroughbred breed was chosen because of increasing incidence of multiple pregnancy within the breed and recorded by many authors and also because of the fact that Thoroughbred can only come of the natural breeding, so the factors like insemination, embryo transport and more cannot influence the evaluation. Moreover, Thoroughbred Studbooks contains exact and comprehensive records about reproduction including evidence of multiple pregnancy for many years. Old Kladruber Horse breed was chosen as a representative of warmblood for comparison with Thoroughbred. Another reason also was that it is possible to find records needed for evaluation in Old Kladruber Horse Studbooks. In the mentioned volumes of Studbooks all the records about reproduction of all the listed mares were reviewed and detailed appraisal was done just in those mares which had the multiple pregnancy during the reproductive life. For evaluating of the founded data and their relationships were used to the statistic programme STATISTICA.12 (© Statsoft, CZ version). The outputs of the programme were rounded to two decimal places or where it was appropriate (eg. an evaluation of the particular year) to integers. Testing took place at a significance level α = 0.05.
RESULTS AND DISCUSSION Table 1 Multiple pregnancy in Thoroughbred mares Variable N valid Average Median Mode Frequency Minimum Maximum The age of the multiple 323 10.21 10 Multiple 35 4 20 pregnancy Table 1 states elementary descriptive statistics about the Thoroughbred mare’s age in the multiple pregnancy age. It was recorded in 323 cases of multiple pregnancies. The average age of the mares in the year of such gravidity was 10 years old. The middle value was also 10 years. The most common value was multiple–exactly the 7 and 9 age with the frequency of 35 times. The minimum age of mares in the multiple pregnancy was 4 years, the maximum 20 years. Table 2 Multiple pregnancy in Old Kladruber Horse mares Variable N valid Average Median Mode Frequency Minimum Maximum The age of the multiple 48 10.65 11 11 8 4 20 pregnancy For the characteristics of the age of Old Kladruber Horse mares in the time of multiple pregnancy 48 records were found. The average age was 11 years and also the median. The most common had the
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multiple pregnancy mares in the 11 years old with the frequency 8 cases. The lowest age was 4 and the highest 20 years old. The observed data regarding the age of the mares do not correspond with the assertion of Reef (1998), which says that the older mares show a multiple pregnancy more likely, since the oldest mare with such pregnancies recorded were twenty years old in both breeds, it can be assumed that the age of seven and nine-year-old in the Thoroughbreds and eleven-year-old in Old Kladruber Horse does not fall into those categories. On the other hand, Bresińska et al. (2004) are indicated as the most predisposed mares between 5 and 10 years of age and Doležel et al. (2000) say the same thing about mares aged 6- 10 years, which would match the data found in the Thoroughbreds. Figure 1 Repeated occurrence of multiple pregnancy in Thoroughbred mares
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Figure 1 shows the frequency of recurrence of multiple pregnancy. First column shows in how many mares from the observed file of multiple pregnancies occurred once in recorded reproductive history (257 mares), i.e. 89.24% frequency phenomenon. Second column describes which of the number of mares were multiple pregnancies reported in this sense, twice, with 29 mares (10.07%) and similarly in category 3 describes it in 2 mares (0.70%). Figure 2 Repeated occurrence of multiple pregnancy in Old Kladruber Horse mares
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As documented in Figure 2, in mares of Old Kladruber Horse breed was found only one variant of repeating multiple pregnancy during the reproductive history of mares, and that in 36 cases (85.71%) and the option of two repetitions in six cases (14.29%). With the results of repeating occurrence of multiple pregnancies in mares generally correspond with McCue’s (2009) assertions that mares which had twins previously have greater probability of their occurrence in future. The phenomenon of recurrence is also confirmed by Bresińska et al. (2004). Figure 3 The result of multiple pregnancy in Thoroughbred mares
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Legend: 1. Column – twin’s abortion, 2. Column – 2 dead foals, 3. Column – 1 live and 1 dead foal, 4. Column – 2 live foals Figure 3 describes the frequency of individual reproductive results of Thoroughbred mares with recorded multiple pregnancies. 258 cases were found (79.88%) when the mare aborted twins, followed by 45 cases (13.93%) births of two dead foals, 15 cases (4.64%) when there was a birth of one living and one dead foal and 5 cases (1.55%) of births of two live foals. Figure 4 The result of multiple pregnancy in Old Kladruber Horse mares
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Legend: 1. Column – twins abortion, 2. Column – 2 dead foals, 3. Column – 1 live and 1 dead foal, 4. Column – 2 live foals The same factors as in the previous Figure were evaluated for reproductive outcomes of Old Kladruber Horse mares in Figure 4. There were 36 cases (75.00%) of abortion of twins, seven cases
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(14.58%) of births of two dead foals and 5 cases (10.42%) of births of two live foals. The records of the birth of one living and one dead foal were not found. Table 3 Relationship between breed and the pregnancy result Variable Chi-square df p Pearson’s chi-square test 14.56158 df = 3 p = 0.00223 Cramér, V 0.1981149 Dependence between the mare’s breed and result of a multiple pregnancy was evaluated by Pearson’s chi-square test, where the null hypothesis was the assumption of independence of assessed characters. Since the p-value < α, the null hypothesis was refuted, therefore between mare’s breed and result of multiple pregnancy exists a statistically significant correlation. However, given the values of Cramér factor described dependence is assessed as weak. Table 4 Relationship between pregnancy result and the age Variable Chi-square df p Pearson’s chi-square test 49.31532 df = 48 p = 0.42035 Cramér, V 0.2104958 Dependence between mare’s age and multiple pregnancy result was also evaluated by using the chi-square test in which the null hypothesis was the assumption of independence of assessed characters. Given that p > α, the null hypothesis cannot be rejected, and therefore between mare’s age and resulting multiple pregnancy there is no statistically significant correlation. Table 5 The proportion of multiple pregnancies from the total number of admitted Thoroughbred and Old Kladruber Horse mares The average of the total number The average of the total number Pregnancy result of mated Old Kladruber Horse of mated mares A1/1 mares Multiple pregnancies 1.53% 0.96% Twins abortion 1.22% 0.72% 1 life and 1 dead foal 0.07% 0.00% 2 dead foals 0.23% 0.15% 2 life foals 0.01% 0.09%
In the Table 5 there are records of multiple pregnancy shown in Thoroughbred and Old Kladruber Horse mares. Data obtained from individual years of reproductive records were evaluated and were filled their average values here. From the total number of admitted mares were recorded incidence of multiple pregnancies in 1.53% of cases in the Thoroughbred. This result is contrary to the research of Hutton and Meacham (1968) which claims the incidence of multiple pregnancies in Thoroughbred mares, specifically at 2.4% and also Doležel et al. (2000) reporting 2.57% frequency. Wolc et al. (2006) also indicate a higher incidence - namely 3.5%. In Old Kladruber Horse was recorded the incidence of multiple pregnancy in 0.96, which approximates to the results of Doležel et al. (2000) who indicates the incidence in mares generally between 1.5 and 2.5%. Přibyl (1952) documents the rate of 1%-almost the same as found. In the case of twin’s abortion specifically it was in the 1.22% and 0.72% of the cases, 1 alive and 1 dead foal recorded at 0.07% of the Thoroughbreds, in Old Kladruber Horse such result was not detected, 2 dead foals at 0.23% and 0.15% of cases and 2 live foals at 0.01% and 0.09%. The average incidence of multiple pregnancy in the assessment of both breeds together was detected in 1.25%.
CONCLUSION Based on the goals of this work the incidence of multiple pregnancy at 1.53% for the Thoroughbred and 0.96% for Old Kladruber Horse mares was evaluated. The successful birth of one or two foals were 20 cases of multiple pregnancies for 323 Thoroughbreds (6.19%) and 5 cases out of 48 in Old Kladruber Horse.
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Based on the information gathered from available literature and my own work, it is possible to argue that multiple pregnancy is a persistent problem in horse breeding. Due to the development of effective reduction techniques already incurred multiparie ceased to be a mare with such a predisposition excluded from reproduction and this may result in an extension of that predisposition in the population. It offers the chance of a multiple pregnancy prevention rather than solving the problem already in place and recommendations on the prudent management of such mares and preferences of their exclusion from breeding, especially because of the probability of transmitting the character to offspring and thus also the indispensability of human intervention in equine reproduction.
REFERENCES Bílek, F., Ambrož, L., Blažek, K., Dušek, J., Hartmann, K., Keil, H., Král, E., Koubek, K., Lerche, F., Michal, V., Munk, Z., Müller, V., Pernička, J., Píša, A., Procházka, V., Přibyl, E., Richter, L., Řechka, J., Sejkora, K., Steinitz, J. 1957. Speciální zootechnika: Chov koní. 2nd edition. Praha: Státní zemědělské nakladatelství. Bresińska, A., Wachowska, L., Szwaczkowski, T. 2004. Genetic and environmental effects on twin pregnancy and lenght of reproduction in Thoroughbred mares. Archiv Tierzucht. 47 (2): 119–127. Davies Morel, M. C. G. 2008. Equine reproductive physiology, breeding and stud management. 3rd edition. Cambridge: CAB International. Davies Morel, M. C. G. University of Wales, Aberystwyth. 2012. Twin pregnancies, a problem in the mare [online]. Available at
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