SOME EFFECTS OF PREMATURELY ELEVATED CONCENTRATIONS OF PROGESTERONE ON LUTEAL AND FOLLICULAR CHARACTERISTICS DURING THE ESTROUS-CYCLE IN HEIFERS

The aim of this study was to characterise the luteal and follicular response to artificially elevated concentrations of progesterone during the metoestrous phase of the oestrous cycle of heifers. An intravaginal controlled internal drug releasing (CIDR) device containing 1.9 g progesterone was inserted at either Day 1 of the cycle (oestrus designated Day 0) for 4 days (T1; n = 12), or on Day 4 for 5 days (T4; n = 11). A third group of heifers (CTRL; n = 13) remained untreated. The diameters of the corpus luteum (CL) and all follicles of at least 5 mm were recorded daily in ovaries of eight heifers from each group by transrectal ultrasonography throughout the cycle. A blood sample was collected daily from every heifer to determine the concentrations of progesterone and luteinising hormone (LH) in sera. Two of the heifers with elevated progesterone levels from Day 1 had 'short' cycles which were characterised by ovulation of the first dominant follicle following the premature demise of the CL. These data are considered separately from the general analysis. Progesterone concentrations of heifers in both treatment groups were elevated (P<0.05) during the period of CIDR insertion, but were not different at the mid-cycle phase compared with untreated contemporaries. A sustained decline to basal concentrations of progesterone occurred earlier (P<0.05) in heifers treated from Day 1. Elevated progesterone concentrations were associated with decreased (P<0.05) mean concentrations of LH on Days 4 and 5 in heifers of the TI group, but only on Day 5 in the T4 group. The average diameter of the corpus luteum (CL) of treated heifers from Days 8 to 18 was less than in untreated heifers (Days 11-13 and 16-18, P<0.05). Heifers in the T1 group had either one or two waves of follicle turnover, with a mean inter-oestrus interval of 8 days and 18 days, respectively. In contrast, heifers in other groups had some two-wave but mostly three-wave cycles and mean inter-oestrus intervals of about 21 days. Premature elevation of progesterone reduced (P<0.05) the size of the first dominant follicle in both treated groups of heifers. Administration of progesterone during the early and late metoestrous phase of the oestrous cycle in heifers reduced the diameters of the CL and the first dominant follicle. Elevation of progesterone from Days 1 to 5, but not from Days 4 to 9, reduced the lifespan of the CL to produce 'short' and 'shortened' cycles, with either one or two follicle waves, respectively.