ENERGY-BALANCE AND OVARIAN FOLLICLE DEVELOPMENT PRIOR TO THE FIRST OVULATION POSTPARTUM IN DAIRY-COWS RECEIVING 3 LEVELS OF DIETARY-FAT

The objectives of this study were to characterize early postpartum (PP ) follicular development in dairy cows and to evaluate the influence o f energy balance (EB) and the level of dietary fat on dominant follicl e development and function. Forty-five multiparous Holstein cows were fed either low (LF, 3.3%), moderate (MF, 5.2%), or high (HF, 7.1%) tot al dietary fat beginning at parturition, and daily EB was determined f or all cows. Follicular development was monitored by ultrasonography, and blood plasma was analyzed for metabolites, metabolic hormones, pro gesterone, estradiol, and FSH. After an increase (p < 0.01) in mean pl asma FSH during Days 1-5, all cows experienced a wave of follicular de velopment during the second week PP regardless of diet or EB. Feeding the MF (p < 0.05) or HF (p < 0.06) diet resulted in a greater number o f class IV (> 15 mm) follicles on Day 14 PP as compared to the LF diet . Cows fed the MF diet had higher (p < 0.05) peak plasma estradiol dur ing the first follicular wave and a shorter (p < 0.05) interval to fir st ovulation than both LF and HF cows. Follicular development prior to first ovulation was characterized by either ovulation of the first do minant follicle (OV, n = 19), one or more waves of nonovulatory domina nt follicles (NOV, n = 18), or the formation of a follicular cyst (CYS T, n = 8). Ovulation failure during the first follicular wave PP was a ccompanied by lower (p < 0.001) peak plasma estradiol (0.96 +/- 0.36 v s. 5.0 +/- 0.35 pg/ml), a smaller (p < 0.01) maximum follicle diameter (16.0 +/- 0.9 vs. 20.0 +/- 1.0 mm), lower (p < 0.09) levers of plasma insulin-like growth factor-I (ICF-I; 73.5 +/- 10.1 vs. 102.9 +/- 9.9 ng/ml), and a longer (p < 0.02) interval to the EB nadir (14.6 +/- 1.9 vs. 6.4 +/- 1.9 days). The number of days to first ovulation was posi tively correlated with days to the EB nadir (r = 0.55), with dominant follicles that emerged after the EB nadir exhibiting enhanced (p < 0.0 1) production of estradiol and greater (p < 0.01) ovulatory success. T he development of large (> 10 mm) dominant follicles was not a limitin g factor in PP reproductive recovery, and moderate dietary fat shorten ed the interval to first ovulation. Follicular competence early PP was associated with higher plasma IGF-I and a shorter interval to the EB nadir.