OVARIAN FOLLICLES, OVULATIONS AND PROGESTERONE CONCENTRATIONS IN AGEDVERSUS YOUNG MARES

The objectives of this study were: 1) to document age-related ovulatio n failure in mares and 2) to contrast the number of ovarian follicles, occurrence of ovulations, and postovulatory concentrations of progest erone in aged versus young mares. In Experiment 1, 4 of 10 aged (25- t o 33-years-old) mares were anovulatory between July 1 and September 1, 1 989. In Experiment 2, two of 25 aged (20- to 30-years-old) and none of 21 young (3- to 12-years-old) mares were anovulatory between Febru ary 1 and June 30, 1 990. The average (+/- SEM) day of the first ovula tion was later (P<0.05) for aged versus young mares (May 9 +/- 7.1 vs April 25 +/- 7.4 days, respectively). There tended (P<0.10) to be fewe r 11- to 20-mm ovarian follicles in aged versus young mares (2.8 +/- 0 .2 vs 5.3 +/- 0.1, respectively), but there was no difference (P>0.10) in the total number of ovarian follicles in aged versus young mares ( 21.0 +/- 0.3 vs 26.1 +/- 0.2, respectively) during the pooled periovul atory period of the first and second (single) ovulations. The number o f ovulatory cycles during the study period was less (P=0.01) for aged versus young mares (2.2 +/- 0.3 vs 3.2 +/- 0.3). Plasma progesterone c oncentrations on Days 10 and 15 of the first ovulatory cycle were high er (P<0.05) in aged versus young mares.