DIETARY-INDUCED SUPPRESSION OF PREOVULATORY PROGESTERONE CONCENTRATIONS IN SUPEROVULATED EWES IMPAIRS THE SUBSEQUENT IN-VIVO AND IN-VITRO DEVELOPMENT OF THEIR OVA

In the first of three experiments, eight ovariectomised Greyface ewes primed with exogenous progesterone were used to provide quantitative data on the effects of two contrasting feeding levels (0.3 vs. 1.4 X maintenance) on plasma progesterone concentrations. Over the 9 day study period, mean ( +/- SEM) daily progesterone concentrations were 4.3 +/- 0.13 and 3.3 +/- 0.17 mu g 1(-1) for the low and high feeding regimens, respectively (P = 0.06), indicating that high feed intake suppressed circulating progesterone levels. The second experiment examined the effect in superovulated Finn-Dorset ewes of a diet supplying either 0.6 (Group L, n = 8) or 2.3 (Group H, n = 8) times their daily energy needs for maintenance, from 1 day before introduction of exogenous progesterone to the time of insemination, on plasma progesterone concentrations and the viability of ova recovered 4 days after insemination. Mean ( +/- SEM) plasma progesterone concentrations were 4.5 +/- 0.17 mu g 1(-1) and 2.8 +/- 0.16 mu g 1(-1) for L and H ewes, respectively, during the 12 day priming period (P < 0.001). Eight hours after progesterone withdrawal, levels had fallen to 0.9 +/- 0.06 mu g 1(-1) and 0.8 +/- 0.07 mu g 1(-1), respectively, then rose to 17.8 +/- 3.01 mu g 1(-1) and 12.9 +/- 2.50 mu g 1(-1) (P > 0.10) at ovum collection. Intervals (mean +/- SEM) to oestrous onset (14.5 +/- 0.38 h) and the luteinising hormone (LH) surge (27.1 +/- 0.98 h) were unaffected by feed intake. Mean (+/- SEM) ovulation rates (8.1 +/- 1.57 vs. 7.8 +/- 1.10) and numbers of ova recovered (5.0 +/- 1.39 vs. 4.8 +/- 1.11) were also similar for each group. However, the proportions of ova considered viable (over 32 cells) at recovery were 0.53 and 0.22 for L and H groups, respectively (P < 0.005). Following 72 h culture (Tissue Culture Medium-199 (M199) + 10% foetal calf serum (FCS)), 0.55 and 0.27, respectively, had developed to blastocysts (P < 0.025). Of ova assessed as viable at recovery, similar proportions (0.86 vs. 0.75) from L and H treatments developed to blastocysts, with corresponding nuclei counts (mean +/- SEM) of 55 +/- 5.2 and 55 +/- 13.2. The third experiment used 12 superovulated Greyface ewes, each offered a different feed level within the range 0.6-2.5 X maintenance, to determine the nature of the relationship between feeding level, pre-ovulatory progesterone concentrations and ovum development at Day 2 following insemination and subsequently during 7 day co-culture (M199 + FCS). Increases in feeding level were accompanied by linear decreases in plasma progesterone (r(2) = 0.79, P < 0.001), the interval to oestrous onset (r(2) = 0.52, P < 0.01) and timing of the LH surge (r(2) = 0.32, P < 0.06). Although undetectable at ovum collection, and somewhat equivocal after 4 day culture, high feeding levels prior to ovulation reduced the proportion of ova (0.16 vs. 0.58) developing to or beyond the expanding blastocyst stage after 7 day culture. Quantitative indices of cell division and protein synthesis confirmed this. In conclusion, excessive feeding during follicular recruitment and oocyte maturation in superovulated ewes imparts a legacy of embryonic loss and developmental retardation.