The current study investigated effects of dietary amino acid (AA) availability on lactational body condition loss and metabolic status, in relation to reproductive parameters after weaning up to Day 8 post-ovulation. Primiparous sows (n = 35) were allocated to one of two lactation diets containing either low crude protein (CP, 140 g/kg) with a low percentage (8%) of slow protein in total protein (LL, n = 18) or high CP (180 g/kg) with a high (16%) percentage of slow protein (HH, n = 17). The HH diet was expected to improve AA utilization by supplying more AA, in a more gradual fashion. The diets did not affect sow body condition loss during lactation, while the HH diet tended to increase litter weight gain during the week 3 of lactation (Delta = 1.3 kg, P = 0.09). On Day 14 post-farrowing, HH diet led to higher plasma urea both pre-feeding and post-feeding (Delta = 2.3 mmol/L, P < 0.01, Delta = 2.4 mmol/L, P < 0.01, respectively), whilst plasma creatinine, NEFA and IGF-1 were similar. No dietary effects on reproductive parameters were found, however several relationships were found between body condition and reproductive parameters. Sows with higher body weight on Day 1 or Day 21 post-farrowing had greater follicle size on Day 3 post-weaning (beta = 0.03 mm/kg, P < 0.01, beta = 0.04 mm/kg, P < 0.01, respectively). At Day 8 post-ovulation, plasma progesterone concentration was negatively related to loin muscle loss (beta = -0.67 ng/ml <middle dot> mm(-1), P = 0.02), backfat loss (beta = -2.33 ng/ml <middle dot> mm(-1), P = 0.02), and estimated body fat loss (beta = -0.67 ng/ml <middle dot> mm(-1), P = 0.02). Both plasma progesterone and the number of corpora lutea were positively related to the energy balance during lactation (beta = 0.03 ng/ml <middle dot> ME MJ(-1), P = 0.01, beta = 0.01 CL/ME MJ, P = 0.02, respectively). The conceptus size at Day 8 post-ovulation was negatively related to body weight loss (beta = -0.01 mm/kg, P = 0.01), estimated body fat loss (beta = -0.02 mm/kg, P = 0.03) and estimated body protein loss (beta = -0.06 mm/kg, P = 0.04), and was positively related to the energy balance during lactation (beta = 5.2*10(-4) mm/ME MJ, P = 0.01). In conclusion, body protein and fat losses during lactation reduced subsequent plasma progesterone concentration and conceptus development at Day 8 post-ovulation.
