EFFECTS OF INDUCTION OF LOW PLASMA PROGESTERONE CONCENTRATIONS WITH APROGESTERONE-RELEASING INTRAVAGINAL DEVICE ON FOLLICULAR TURNOVER ANDFERTILITY IN CATTLE

The effects of concentration of progesterone in plasma on development and fertility of the first wave dominant follicle were studied in catt le. To identify a source of exogenous progesterone that would permit e xtension of the first wave dominant follicle, nonlactating Holstein co ws (n = 6) received on day 8 of two successive oestrous cycles an inje ction of PGF2alpha (25 mg) and a new (1.9 g of progesterone (Period 1) ) or used (almost-equal-to 1.2 g of progesterone (Period 2)) CIDR-B de vice that was removed on day 17. Control cows (n = 6) received a new C IDR-B device on day 8 that was removed on day 17 and a PGF2alpha injec tion (25 mg) on day 17. Ultrasonography and collection of blood sample s were performed on alternate days throughout the experiment. Plasma c oncentrations of progesterone and oestradiol were different between tr eatments (P < 0.0001 and P < 0.05, respectively). The dominant follicl e was maintained until day 17 and ovulated upon removal of the intrava ginal device in 1 of 6, 6 of 6 and 0 of 6 in new CIDR-B, used CIDR-B a nd control groups, respectively (P < 0.01). The preovulatory dominant follicles were 14.2 +/- 1.6 mm, 20 +/- 1.3 mm and 10 +/- 1.3 mm, respe ctively (P < 0.001) on day 17. There were fewer 5-9 mm follicles in co ws having a persistent dominant follicle (P < 0.01). The interval to o nset of oestrus was negatively correlated with size of the dominant fo llicle on day 17 (P < 0.001). In Expt 2, the fertility of oocytes ovul ated from new (PGF2alpha on day 7; T1; n = 91) and persistent dominant follicles (PGF2alpha on day 7 and a used CIDR-B device inserted on da y 7 and withdrawn on day 16; T2; n = 91) was tested using Holstein hei fers. Size of the dominant follicle and plasma concentrations of proge sterone and oestradiol on days 7 (T1) and 16 (T2) were different betwe en treatments: 11.3 +/- 0.2 versus 16.2 +/- 0.3 mm (P < 0.001); 4.2 +/ - 0.2 versus 2.9 +/- 0.3 ng ml-1 (P < 0.01) and 3.5 +/- 0.3 versus 11. 7 +/- 1.7 pg ml-1 (P < 0.01), respectively. Pregnancy rates at first a rtificial insemination were 64.8% (46 of 71) and 37.1% (26 of 70) for new and persistent dominant follicles, respectively (P < 0.01). Pregna ncy rates at second service were 50% and 52.8%, respectively. Low plas ma concentrations of progesterone, therefore, resulted in persistency of the dominant follicle and temporarily impaired fertility.