Delayed conception in standardbred mares

Abstract

Background: Nutrient modelling of Thoroughbred mares managed under commercial management conditions has identified a potential negative energy balance later in the breeding season that could extend the interval from parturition to conception. Anecdotally, Standardbred mares are believed to have a greater feed conversion efficiency than Thoroughbred mares and are managed with lower levels of feed (pasture DM on offer) during breeding season. At present, there is no published data on the association between foaling date and delayed conception in Standardbred mares. Objective: To investigate the association between increased foaling date (days after August 1) and delayed conception in a cohort of New Zealand Standardbred mares. Methods: Official breeding records were collected from 558 Standardbred broodmares managed on two farms during the seasons 2016-2017 and 2018-2019 breeding seasons. Horse and breeding record data were obtained as an electronic extract from HRNZ. These records were collated and summarized in Microsoft Excel. Data were examined using simple descriptive statistics and then examined according to mare age, breeding status and foaling date. Pasture growth (kg DM/ha/day) were obtained from official records. Kaplan Meier Survival curves were used to examine the interval from parturition to conception, with and without mares that carried over. Results: The mean foaling date was ~4 November for mares inseminated as dry mares and 14 November in wet mares. The mean conception date of the Standardbred mares was 27 November in dry mares and 13 December in wet mares. Mares that foaled earlier in season had the longest interval between foaling and conception, smallest percentage of mares bred on foal heat and smallest percentage of mares that carried over. The group of mares that foaled latest in season had the shortest interval, greatest percentage of mares bred on foal heat and greatest percentage of mares that carried over. Stocking density was relatively low and pasture growth was mostly sufficient during the year. Conclusion: Foaling late in the season was not associated with a delay in conception in Standardbred mares. These findings indicate that they did not experience a negative energy balance. Further research is required to investigate to what extent farm management influences mare reproductivity.