Ovarian structures and circulating steroids in heifers and lactating cows in summer and lactating and dry cows in winter

Two experiments compared follicular and luteal development and circulating steroid concentrations from induced luteolysis to ovulation in lactating Holstein cows (n = 27; 40.0 +/- 1.5 kg milk/day) vs. nulliparous heifers (n = 28; 11 to 17 mo-old) during summer (Experiment 1), and in lactating (n =. 27; 45.9 +/- 1.4 kg milk/d) vs. dry cows (n = 26) during winter (experiment 2). All females received PGF(2alpha) 6 d after ovulation and were monitored until next ovulation by daily ultrasound and assay of serum progesterone (P-4) and estradiol (E-2). Every female was used two or three times. In Experiment 1, lactating cows had high incidence of multiple ovulation (63.5%) compared with heifers (1.3%). Among single ovulators, there was no difference in maximal size of ovulatory follicles between lactating cows and heifers (15.8 vs. 16.5 mm, respectively). However, lactating cows had lower peak serum E-2 (8.6 vs. 12.1 pg/ml), took longer to ovulate after luteolysis (4.6 vs. 3.8 d), developed more luteal tissue volume (7293.6 vs. 5515.2 mm(3)), and had lower serum P-4 on d 6 after ovulation (2.0 vs. 3.0 ng/ml) than heifers (data included multiple ovulators). In experiment 2, multiple ovulations were similar between lactating and dry cows (17.9 vs. 17.2%, respectively). Peak serum E2 was also similar between lactating and dry cows (7.6 vs. 8.5 pg/ml) although lactating cows had larger ovulatory follicles (18.6 vs. 16.2 0.4 mm). Lactating cows took longer to ovulate (4.8 vs. 4.2 d), developed more luteal tissue (7599 vs. 5139 468 mm(3)), but had similar serum P-4 (2.2 vs. 1.9 ng/ml) compared with dry cows. Therefore, lactating cows had similar or lower circulating steroid concentrations than dry cows or heifers, respectively, despite having larger ovarian structures.