Relationships between protein intake during lactation, LH levels and oestrous activity in first-litter sows

Six pairs of littermate first-litter sows received daily intakes of 60 MJ digestible energy and either 404 or 746 g crude protein (CP) during a lactation period of 22.3 (±1.4) days. Plasma samples were collected via indwelling cannulae at 15-min intervals for 20 h before and 12 h after weaning and then for 8-h periods 24, 72, 120 and 168 h after weaning. Sows which received 404 g CP/day lost more body protein during lactation than sows which received the higher protein intake (1.2 vs 0.4 kg; P < 0.05). Weaning to oestrus intervals ranged from 5 to 28 days and those sows given 404 g CP/day tended to have an extended weaning to oestrus interval (16.0 vs 7.5 days; P<0.10). Between weaning and the day following weaning mean luteinizing hormone (LH) concentration increased (0.44 vs 0.85 ng/ml; P < 0.05), nadir increased (0.38 vs 0.62 ng/ml; P < 0.05), pulse amplitude decreased (1.34 vs 0.68 ng/ml; P<0.05), pulse interval shortened (712 vs 128 min; P<0.05). During the 20-h period prior to weaning mean LH and nadir concentrations were greater (P<0.05) in sows which received higher protein intake during lactation although there was no significant effect of diet on either the frequency or amplitude of the pulses. Mean LH concentration before weaning was also higher in sows which showed an early return to oestrus (≤7 days) than in sows which remained anoestrous for at least 17 days (0.51 vs 0.38; P<0.05). In addition, pulse interval prior to weaning was correlated with the subsequent weaning to oestrus interval (r=+0.63; n=10; P<0.05). It is suggested that the effect of protein intake during lactation on subsequent weaning to oestrus interval of first-litter sows is mediated through effects on the secretion of LH pulses.