BACKGROUND Horses produce only one foal from an eleven-month gestation period, making the maintenance of high reproductive rates essential. Genetic bottlenecks and inbreeding can increase the frequency of deleterious variants, resulting in reduced reproductive levels in a population. In this study we examined the influence of inbreeding levels on foaling rate, gestation length and secondary sex ratio in Australian Thoroughbred mares. We also investigated the genetic change in these traits throughout the history of the breed. Phenotypic data were obtained from 27,262 breeding records of Thoroughbred mares provided by three Australian stud farms. Inbreeding was estimated using the pedigree of each individual dating back to the foundation of the breed in the eighteenth century. RESULTS While both gestation length and foaling rate were heritable, no measurable effect of inbreeding on either trait was found. However, we did find that the genetic value for both traits had decreased within recent generations. A number of environmental factors also had significant effects on foaling rate and gestation length. Secondary sex ratio had only an extremely small paternal heritable effect and was not susceptible to environmental influences. CONCLUSIONS In contrast to racing performance, inbreeding had no measurable effect on foaling rate or gestation length in Australian Thoroughbred horses. This could be because the level of inbreeding in the population examined is not high enough to show a discernible effect on reproductive traits. Populations that experience higher levels of inbreeding due to use of artificial reproductive technologies or extremely small population sizes may show a more pronounced reduction in natural foaling rate or gestation length. It is also possible that the intensive management techniques used in the Thoroughbred population masks any negative effects of inbreeding. The decrease in the genetic value of foaling rate is likely to be because horses with unfavourable genetic potential have not yet been selected out of the population. The change in genetic value of gestation length may be due to selective breeding favouring horses with shorter pregnancies. We also found that prioritising the mating of older mares, and avoiding out of season mating could lead to an increased breeding success.

中文翻译：

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近交对澳大利亚纯种马的成功率，妊娠长度和驹性别比的影响。

背景技术从十一个月的妊娠期开始，马只产生一只小马驹，因此维持高繁殖率至关重要。遗传瓶颈和近交会增加有害变异的频率，从而导致人口繁殖水平降低。在这项研究中，我们研究了近交水平对澳大利亚纯种母马的产驹率，妊娠期和次生性别比的影响。我们还研究了整个犬种历史中这些性状的遗传变化。表型数据来自于三个澳大利亚种马场提供的27,262匹纯种母马的繁殖记录。近亲繁殖是根据每个人的血统书追溯到18世纪。结果虽然妊娠期和产驹率都是可遗传的，没有发现近交对这两个性状有可测量的影响。但是，我们确实发现这两个性状的遗传价值在最近几代人中均有所下降。许多环境因素对产驹率和妊娠期长度也有重要影响。次生性别比仅具有极小的父系遗传效应，并且不受环境影响。结论与比赛表现相反，近交对澳大利亚纯种马的产驹率或妊娠期没有可测量的影响。这可能是因为所调查人群的近交程度不够高，无法对生殖性状表现出明显的影响。由于使用人工生殖技术或近乎很小的种群规模而进行近交繁殖的种群，其自然产驹率或妊娠期可能会显着减少。纯种种群中使用的集约化管理技术也有可能掩盖近亲繁殖的任何负面影响。产驹率的遗传价值下降很可能是由于尚未从种群中选出具有不利遗传潜力的马。妊娠期遗传价值的变化可能是由于选择性育种有利于妊娠较短的马。我们还发现，优先安排较老的母马交配，避免交配过时，可以提高育种成功率。纯种种群中使用的集约化管理技术也有可能掩盖近亲繁殖的任何负面影响。产驹率的遗传价值下降很可能是由于尚未从种群中选出具有不利遗传潜力的马。妊娠期遗传价值的变化可能是由于选择性育种有利于妊娠较短的马。我们还发现，优先安排较老的母马交配，避免交配过时，可以提高育种成功率。纯种种群中使用的集约化管理技术也有可能掩盖近亲繁殖的任何负面影响。产驹率的遗传价值下降很可能是由于尚未从种群中选出具有不利遗传潜力的马。妊娠期遗传价值的变化可能是由于选择性育种有利于妊娠较短的马。我们还发现，优先安排较老的母马交配，避免交配过时，可以提高育种成功率。产驹率的遗传价值下降很可能是由于尚未从种群中选出具有不利遗传潜力的马。妊娠期遗传价值的变化可能是由于选择性育种有利于妊娠较短的马。我们还发现，优先安排较老的母马交配，避免交配过时，可以提高育种成功率。产驹率的遗传价值下降很可能是由于尚未从种群中选出具有不利遗传潜力的马。妊娠期遗传价值的变化可能是由于选择性育种有利于妊娠较短的马。我们还发现，优先安排较老的母马交配，避免交配过时，可以提高育种成功率。