Plasma gonadotrophin concentrations during the oestrous cycle and after ovariectomy in two breeds of sheep with low and high fecundity

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Abstract
LH [luteinizing hormone] and FSH concentrations were measured during the estrous cycle in 2 local Moroccan breeds of sheep with low (Timahdite: 1 CL[corpus luteum]/cycle) and high (D''man: 3 CL/cycle) ovulation rates. Twenty ewes were used from each breed and blood was collected at 3 or 6 h intervals from 5-4 days before estrus up to Day 14 of the new cycle, when 4 D''man and 4 Timahdite ewes were ovariectomized. After surgery, blood sampling was continued at 6-h intervals for 2 wk. The mean basal concentration of LH, the maximum value of the preovulatory LH surge and the area under the curve were significantly higher (P < 0.05) in Timahdite than in D''man ewes. The pattern of FSH in each breed showed no clear basal level but a periodic succession of peaks with variable amplitudes. The 1st and highest peak corresponded with the preovulatory LH surge. The 2nd peak followed immediately after the 1st peak and reached its maximum 24-30 h later. The 3rd peak was flatter and occurred around Day 6 of the cycle. The 4th peak was observed around Day 10 of the cycle and showed the lowest amplitude. The 5th and last peak occurred 66-87 h before the next preovulatory surge. FSH concentrations were higher in the prolific D''man than in Timahdite ewes around the time of estrus (pro-estrous peak, preovulatory surge and the 2nd peak). The drop in FSH concentrations observed in D''man ewes before the preovulatory surge was more pronounced and started later. After ovariectomy there was a rapid increase in LH and FSH concentrations. The maximum values reached after ovariectomy were similar in the 2 breeds.

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