Factors affecting pregnancy rates in the German Thoroughbred Breed

Objective: The aim of this study was to evaluate factors affecting pregnancy rates in the German Thoroughbred Breed with particular emphasis on optimisation of fertility rates for breeding stallions of older ages. Material and methods: Data from the studbooks of the German Thoroughbred Breeding Association from 1996 to 2009 analysed. This analysis included the records of 319 stallions and 6622 brood mares, resulting in 21 372 pregnancies at the end of the season. Results: Pregnancy rates were significantly affected by the age of the stallion and mare as well as the season (month in the breeding season in which covering occurred). Furthermore, significant interactions were found between stallion age and book (number of mares covered per stallion), and between stallion age and season. Pregnancy rates decreased with increasing age of the stallion and brood mare as well as with the progression of the breeding season. However, stallions older than 16 years did show higher pregnancy rates when booked for more than 12 brood mares per season. Pregnancy rates for the older stallions were in particular decreased for later months in the breeding season. Conclusion and clinical relevance: For breeding with older stallions mares should be bred as early as possible in the breeding season and a sufficiently large number of mares should be covered.