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 d ata on the effects of two contrasting feeding levels (0.3 vs. 1.4 X ma intenance) 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 regime ns, respectively (P = 0.06), indicating that high feed intake suppress ed 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 n eeds for maintenance, from 1 day before introduction of exogenous prog esterone to the time of insemination, on plasma progesterone concentra tions and the viability of ova recovered 4 days after insemination. Me an ( +/- 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, du ring the 12 day priming period (P < 0.001). Eight hours after progeste rone 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. Interva ls (mean +/- SEM) to oestrous onset (14.5 +/- 0.38 h) and the luteinis ing hormone (LH) surge (27.1 +/- 0.98 h) were unaffected by feed intak e. 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 sim ilar 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, re spectively (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 treatmen ts developed to blastocysts, with corresponding nuclei counts (mean +/ - SEM) of 55 +/- 5.2 and 55 +/- 13.2. The third experiment used 12 sup erovulated Greyface ewes, each offered a different feed level within t he range 0.6-2.5 X maintenance, to determine the nature of the relatio nship 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 synt hesis confirmed this. In conclusion, excessive feeding during follicul ar recruitment and oocyte maturation in superovulated ewes imparts a l egacy of embryonic loss and developmental retardation.