Does dietary supplementation of pregnant mares with a commercial stud feed balancer improve the transfer of passive immunity in their foals? A controlled field trial.

This prospective, controlled field trial aimed to determine the effect of dietary supplementation of mares in late pregnancy with a commercial stud feed balancer on the transfer of passive immunity to their foals. Eighty-two pregnant mares on a single stud farm that were eligible for inclusion were assigned into two groups (Intervention and Control) based primarily on existing social groupings. Between 64-224 days prepartum, all mares received the same forage-based diet but mares in the Intervention group received an in-feed commercial stud feed balancer and mares in the Control group received the stud's home-mix concentrate. Data from 68 mare and foal pairs were analysed according to Intention To Treat (ITT) principles and sensitivity analysis was performed on 57 mare and foal pairs who fulfilled the study protocol. The primary outcome of interest was failure of passive transfer of immunity (FPT), defined as foal IgG <8g/l at 12-36 h after first suckle. Foals of mares in the Intervention group were significantly less likely to develop FPT compared to those in the Control group. Colostral quality (Brix ≥23.0) was also significantly greater in mares in the Intervention compared to the Control group. Group (Intervention vs. Control) and sex of foal were the only variables that were significantly associated with FPT in a multivariable model that explored the effect of other potential risk factors for FPT.