Resumption of follicular waves in beef cows is not associated with periparturient changes in follicle-stimulating hormone heterogeneity despite major changes in steroid and luteinizing hormone concentrations.

To test the hypothesis that emergence of follicle waves postpartum is associated with a change in circulating FSH isoform distribution, 10 Limousin-cross suckler cows were blood sampled daily from 5 wk prepartum until first ovulation postpartum for FSH, LH, estradiol (E2), and progesterone assay. Follicular growth was monitored daily by ultrasonography from Days 5 to 10 postpartum until first ovulation. Distributions of circulating FSH isoforms were characterized (n = 4 per group) by chromatofocusing at 1) 18-33 days prepartum, 2) 3-5 days prepartum, 3) the first postpartum FSH rise responsible for emergence of the first follicle wave, and 4) the FSH rise that stimulated the ovulatory follicle wave. The interval to detection of the first postpartum dominant follicle (DF) was 9.6 +/- 0.58 days. The number of DF before first ovulation was 2.1 +/- 0.18, and first ovulation occurred at 28.6 +/- 1.54 days postpartum. Serum E2 concentrations were higher (p = 0.0001) in cows during the 5-wk period prepartum (53.8 +/- 6.29 pg/ml) than in the postpartum period up to first ovulation (1.5 +/- 0.15 pg/ml). In late pregnancy, there was an absence of recurrent FSH rises and LH concentrations were decreased (p < 0.0001) compared with those in the postpartum period. The emergence of each follicle wave postpartum was preceded by a 2- to 4-day rise in FSH concentrations. The pattern of FSH isoform distribution did not differ (p > or = 0.75) between the pre- and postpartum periods.