Effects of maturity of the potential ovulatory follicle on induction of oestrus and ovulation in cattle with oestradiol benzoate

The effect of maturity of the dominant follicle (DF) on the capacity of oestradiol benzoate (ODB) to induce oestrus and ovulation was examined in cattle. In experiment 1, 31 prepubertal heifers each received an intravaginal progesterone insert (IPI) and 1 mg ODB i.m./500 kg BW (ODB1). Daily ovarian ultrasonography detected emergence of a new follicular wave 3.1±0.1 days after ODB1. The IPI was removed when newly emerged DF were “young” (1.3±0.1 days after emergence; YDF; n=15) or “mature” (4.2±0.1 days; MDF; n=16), and 24 h later, heifers received 0.75 mg ODB/500 kg BW (ODB2; n=16) or no further treatment (NoODB2; n=15). Most of the heifers receiving ODB2 were observed in oestrus (15/16) and ovulated (12/16), as compared to 0/15 and 1/15 in the NoODB2 group, respectively (P<0.01). In experiment 2, 32 heifers received ODB1 on day 6 of the oestrous cycle, and new follicular wave emergence was detected 3.2±0.1 days later. Heifers received an injection of prostaglandin-F2α (PGF) when the DF was young (1.1±0.1 days after emergence; YDF; n=16) or mature (4 days; MDF; n=16), and then ODB2 24 h later or no further treatment (NoODB2). The interval from PGF to oestrus was greater (P<0.01) in the YDF–NoODB2 (70±3.9 h) as compared to MDF–NoODB2 group (57±1.8 h). Inclusion of ODB2 reduced (P<0.01) this interval to 47.0±0.7 h without regard to the maturity of the DF (maturity×ODB2, P<0.05) and also reduced (P<0.05) the interval to ovulation. In experiment 3, 21 suckling anoestrous cows received an IPI and ODB1 at 29.3±1.7 days postpartum. The IPI were removed either 1 day (YDF; n=9) or 3.9±0.1 days (MDF; n=9) after emergence of a new follicular wave and every cow received ODB2. Oestrus was subsequently detected in all but one animal. Ovulation of the newly emerged DF was detected within 48 h of ODB2 in nine of nine cows of the MDF group, and in four of nine of the YDF group (P<0.05). During the subsequent ovulatory cycle, luteal size and plasma concentrations of progesterone were greater (P<0.01) in the MDF group compared to the YDF group. We conclude that behavioural oestrus is readily induced by 0.75 mg ODB i.m./500 kg BW. Maturity of the DF appeared to have little influence on the ability of the DF to ovulate in heifers. In contrast, young DF in lactating anoestrous cows were less likely to respond to the ovulatory cue provided, and luteal development was compromised in those that did ovulate.