Effect of dietary protein intake on energy utilization and feed efficiency of lactating sows.

The objective of the current study was to quantify loss of energy in feces, urine, heat and milk, to evaluate feed efficiency and to evaluate optimal ratio of dietary CP to energy for lactating sows fed increasing dietary CP. A total of 72 sows were included in the experiment from d 2 after parturition until weaning at d 28. Sows were allocated to six dietary treatments formulated to be isocaloric (9.8 MJ NE/kg) and increasing SID CP (11.8, 12.8, 13.4, 14.0, 14.7, and 15.6% SID CP). Sows were weighed and back fat scanned within 2 d after farrowing, at d 18 ± 3 and d 28 ± 3. Litters were standardized to 14 piglets within 2 d after farrowing, and weighed at d 1 or 2 and at d 11, 18, and 28 (within ± 3 d). Feed intake (feed supply minus residue) was registered and milk, urine and fecal samples were collected at d 4, 11 and 18 (within ± 3 d). Sow milk yield was estimated from litter gain and litter size and sow heat production was calculated factorially. On d 4 and 18 (± 3 d) sows were enriched with D2O (deuterated water) to estimate body protein and fat pool size. Overall, sow BW loss, back fat loss, fat and protein mobilization, litter size and piglet performance was not affected by diets, except for sows fed treatment 5, which had lower ADFI and lower milk production, and a tendency to lower piglet ADG compared with the remaining treatment groups (P < 0.01, P = 0.03, P =0.08, respectively). Relative to GE intake, the energy excreted in urine increased from 3.3 to 5.3% (P < 0.001), whereas energy lost as heat increased numerically from 54.5 to 59.0% with increasing dietary CP. The feed efficiency as evaluated by NE corrected for body mobilization peaked when sows were fed at their requirement (treatment 2; 12.8% SID CP; P = 0.01), whereas the feed efficiency was 1% lower for treatment 1, while it was 3 to 6% lower for treatments 3 through 6. In conclusion, energy loss in urine and likely also energy lost as heat increases if the dietary protein to energy ratio is unbalanced and evaluating feed efficiency of lactating sows by correcting for body mobilization seems to be a promising approach to improve sow feeding in the future.