Periparturient characteristics of mares and their foals on a New Zealand Thoroughbred stud farm

To describe selected periparturient variables in a sample of Thoroughbred mares and their foals on a commercial stud farm in New Zealand.

Data were collected for 969 foaling records, from 592 mares, by foaling attendants from one Thoroughbred stud farm in the Waikato region of New Zealand for the breeding seasons 2008-2013, inclusive, and collated retrospectively. The associations between mare- and foal-level variables and gestation length, time of birth (expressed as the number of hours after midday), fetal membrane retention time, time for the foal to stand and suckle after birth, and foal mortality were quantified using linear and logistic regression models.

Mean gestation length for mares in this study was 350 (min 296, max 429) days, and was shorter for filly foals (348 (min 296, max 429) days) compared with colt foals (350 (min 310, max 404) days) (p=0.001). Overall, 517/968 (53%) foalings occurred between 19:00 and 01:00 and 212/968 (22%) foalings occurred during daylight. Overall, 815/855 (95%) expelled their fetal membranes within 4 hours of parturition. The mean interval from birth to standing and to suckling was shorter (p<0.001) for filly foals than colts by 13 (95% CI=9-17) and 17 (95% CI=8-26) minutes, respectively. Dystocia was reported for 81/968 (8.4%) foalings with relative fetal oversize being the most common cause (36/81 (44%) cases). Perinatal foal death was recorded for 26/966 (2.7%) foalings. The odds of foal mortality decreased with increasing length of gestation (p=0.027), and were greater for colt than filly foals (p=0.042) and when dystocia was reported (p<0.001).

Gestation length was influenced by the sex of the foal, as was the time taken for the foal to stand and suckle after birth. One fifth of mares foaled during daylight hours. The majority of mares in this study population expelled their fetal membranes within 4 hours of foaling.

Accurate measures for periparturient behaviour can be important in the management of mares and newborn foals. Clinically relevant differences appear to exist between mares and foals in New Zealand and those in the Northern Hemisphere for some key parameters, likely as a result of differences in management and environment. The results of this study suggest that mares should be considered to have retained their membranes if not passed within 4 hours of parturition.