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Ovarian Structures and Circulating Steroids in Heifers and Lactating Cows in Summer and Lactating and Dry Cows in Winter

R. Sartori^*, G. J. M. Rosa and M. C. Wiltbank^*

^* Dairy Science Department, University of Wisconsin, Madison 53706; and
Department of Animal Science, Michigan State University, East Lansing 48824

Corresponding author:
M. C. Wiltbank; e-mail:
Wiltbank{at}calshp.cals.wisc.edu.

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 PGF₂ 6 d after ovulation and were monitored until next ovulation by daily ultrasound and assay of serum progesterone (P₄) and estradiol (E₂). 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 E₂ (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 mm³), and had lower serum P₄ 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 mm³), but had similar serum P₄ (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.

Key Words: ovary • estradiol • progesterone • dairy cattle

Abbreviation key: CL = corpus luteum or corpora lutea, CR = conception rate, E₂ = estradiol, P₄ = progesterone

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