The pattern of LH secretion and the ovarian response to the 'ram effect' in the anoestrous ewe is influenced by body condition but not by short-term nutritional supplementation
Résumé
In sheep, the ‘ram effect’ induces out-of-season fertility and good nutrition increases prolificacy. This experiment determined if fatness or short-term nutritional supplementation modified the response to the ‘ram effect’. A group of 48 Île-de-France ewes were fed diets that produced groups with body-condition scores (BCS) of >3.0 and <2.0. Within each BCS group animals were supplemented daily with 500 g of lupins from Day –5 to Day 0 (ram introduction) resulting in four groups: low BCS, supplemented (n = 7) and non-supplemented (n = 8) and high BCS, supplemented (n = 12) and non-supplemented (n = 11). The blood concentrations of glucose and insulin and the LH response to gonadotrophin-releasing hormone (GnRH) were determined. After the ‘ram effect’ the pattern of LH pulsatility, the LH surge and ovarian responses were analysed. Low BCS ewes had lower glucose and insulin (P < 0.001) and supplementation increased both (P ≤ 0.001). The increase in LH induced by GnRH was reduced in low BCS ewes (P = 0.015) but it was not affected by supplementation. Similarly, LH pulsatility was reduced in low BCS ewes (P < 0.05). The LH surge and ovarian cyclicity were not affected but the follow-up cycle was delayed (P = 0.034) and progesterone was reduced (P = 0.029) in low BCS ewes. There was an effect of BCS on ovulation rate (P < 0.05). These results show that the BCS can modify the response to the ‘ram effect’ and that supplementation has little effect on this response.