The effect of exogenous prolactin on lactation performance of first-litter sows given protein-deficient diets during the first pregnancy

Twenty-four sows were used to study the effects of dietary protein restriction during pregnancy and exogenous porcine prolactin (pPRL) during late pregnancy and throughout lactation on lactation performance. Eight sows were given a protein-adequate diet containing 179g crude protein (CP)kg^-1 during their first pregnancy while the remaining 16 sows received the same amount of a diet containing 80g CP kg^-1. Eight of the sows given 80g CP kg^-1 during pregnancy were injected with 15 mg pPRL i.m. twice daily at 08:00 and 20:00 between day (d) 102.1 (±0.3) of pregnancy and weaning after their first lactation. Pregnant sows offered the low protein diet gained significantly less body weight during gestation and tended to eat less in the subsequent lactation than sows given the protein-adequate diet. Dietary protein had no significant effect on birth weight, milk yield, milk composition or growth rate of the litter during lactation. Neither dietary protein intake during pregnancy nor exogenous prolactin affected the concentrations of plasma glucose, serum insulin, urea or non-esterified fatty acid (NEFA) during lactation. The concentration of lactose in plasma during lactation was unaffected by treatment, but at d 105 of pregnancy, plasma lactose levels were greater in sows which had received exogenous prolactin (32.4 vs. 6.2 mg l^-1, P < 0.05). The concentrations of RNA and DNA in mammary tissue biopsies were unaffected by either dietary protein or pPRL. The concentration of RNA and DNA increased between d 70 and 90 from 0.66 to 2.77mg g^-1 and from 0.54 to 1.19mg g^-1, respectively. Thereafter, RNA increased to 4.40 mg g^-1 at d 14 of lactation whilst DNA concentration remained at a similar level of 0.90 mg g^-1. Milk yield of sows between d 5 and 8 and between d 19 and 22 of lactation was reduced from 8.36 to 7.00kg day^-1 and from 10.74 to 8.22kg day^-1, respectively, in sows given pPRL. The protein content of colostrum from sows treated with pPRL was reduced from 164 to 104 g kg^-1 whereas the fat content increased from 47 to 127 g kg^-1. These results indicate that the administration of exogenous pPRL during late pregnancy and throughout lactation initiated lactogenesis prematurely and reduced subsequent milk yield during established lactation.