Age and pasture effects on vernal transition in mares

The objectives of the present study were to determine if follicular activity was less in old than in young mares during the spring transition and if green pasture would hasten onset of the ovulatory season. Experiments were conducted over 2 sequential years using young mares (3 to 7 yr) and old mares (greater than or equal to 14 yr). In Experiment 1, growth of the largest and second-largest follicles were compared for young mares (5 to 7 yr) and old mares (greater than or equal to 14 yr) for 21 d prior to the first ovulation of the year. More follicular activity was noted in young than in old mares. Main effect of age was significant for diameter of the largest follicle, and interaction of day-by-age was significant for diameter of the second-largest follicle. Prior to the beginning of the breeding season, the mares were randomly divided into dry-lot and pasture groups. The interval from May 2 to ovulation was shorter (P<0.005) for mares put on pasture on May 2 than for mares kept in dry lot (means +/- SEM, 14.5 +/- 2.7 and 21.3 +/- 3.2 d, respectively). In Experiment 2, follicular activity was compared among 3 age groups (3 to 7, 17 to 19, and greater than or equal to 20 yr). The total number of follicles greater than or equal to 10 mm was higher (P<0.05) for young mares and lower (P<0.05) for old mares than for mares of an intermediate age. Main effect of age and interaction of day-by-age were significant for diameter of largest and second-largest follicles, being smaller for mares greater than or equal to 20 yr than for younger mares. The interval from development of a follicle greater than or equal to 30 mm to ovulation was shorter (P<0.05) for mares placed on pasture when a greater than or equal to 30 mm follicle developed than the interval for mares kept in dry lot (5.7 +/- 0.7 and 8.2 +/- 0.9 d, respectively). In summary, less follicular activity occurred in old than in young mares during the transitional period, and mares pastured on green grass ovulated sooner in the spring than mares housed on dry lot and fed hay. (C) 1997 by Elsevier Science Inc.