AGE AND PASTURE EFFECTS ON VERNAL TRANSITION IN MARES

The objectives of the present study were to determine if follicular ac tivity was less in old than in young mares during the spring transitio n and if green pasture would hasten onset of the ovulatory season. Exp eriments were conducted over 2 sequential years using young mares (3 t o 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 fo r young mares (5 to 7 yr) and old mares (greater than or equal to 14 y r) 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 d ay-by-age was significant for diameter of the second-largest follicle. Prior to the beginning of the breeding season, the mares were randoml y 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-larg est follicles, being smaller for mares greater than or equal to 20 yr than for younger mares. The interval from development of a follicle gr eater than or equal to 30 mm to ovulation was shorter (P<0.05) for mar es placed on pasture when a greater than or equal to 30 mm follicle de veloped 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 occu rred in old than in young mares during the transitional period, and ma res pastured on green grass ovulated sooner in the spring than mares h oused on dry lot and fed hay. (C) 1997 by Elsevier Science Inc.