A sow’s performance during lactation is pre-determined by her genotype. Large, late-maturing animals are traditionally better milkers. In sows, milk yield is determined by litter size. Recently, it has been suggested that sows could compete with high-producing dairy cows in the production of milk energy per unit of body weight if sufficient demand was generated by nursing 14 or more piglets. In order for a sow to achieve a high rate of milk production, the appropriate amount of nutrients must be available. If the dietary supply of nutrients is not sufficient to support milk synthesis, the sow will mobilize her own maternal body tissues, or “milk off her back”. Estimates of the lysine needs of sows nursing ten pigs have already been established, but work continues to determine the lysine requirement of sows at peak milk production.
A recent study at Iowa State University examined the performance of sows nursing 14 piglets. In the experiment, primiparous sows of high (350 – 390 g/d) or low (240 – 280 g/d) capacity for lean growth (LG) were exposed to four levels of dietary lysine (0.58, 0.77, 0.96, or 1.15%) for four weeks. Regardless of genotype, sows were unable to consume the 6.5 kg of feed offered to them daily. Since this level of feed intake was necessary to support maximum milk yield, the sows had to mobilize body tissues to provide additional energy and nutrients. However, the high and low LG sows still managed to achieve peak daily milk energy yields equal to results from high-producing dairy cows.
Total yield, energy and lysine levels in milk were similar between genotypes and increased as daily dietary lysine intake increased. With this increase, less maternal body protein was mobilized in both genotypes as lysine became less limiting for milk synthesis. However, energy supply for milk synthesis became more limiting than amino acids, forcing the sows to mobilize maternal body tissues. The magnitude of response from the high and low LG sows was consistent with differences in their abilities to derive energy from body tissue. Low LG sows were able to produce more milk because of the mobilization of additional energy from maternal fat tissue. However, because the high LG sows had lower body fat stores, they mobilized muscle tissue to meet their energy needs.
Based on these results, the researchers determined that sows require at least 54 g/day digestible lysine to support milk synthesis and minimize maternal protein loss. This is in agreement with the revised recommendations in the 1998 National Research Council’s Nutrient Requirements of Swine, 10th Revised Edition. It is important to note that milk yields peaked earlier in lactation in the high than in the low LG sows, at 14-18 days as opposed to 18-22 days. This is good news for producers who practice early weaning because it means that weaning before 18 days may be favourable for genetically lean sows when energy intake is limiting. Overall, this research clearly shows that the performance of genetically lean sows during lactation hinges on the provision of sufficient dietary energy to fuel maximum milk synthesis. As a result, increasing body stores of fat during gestation may offer improvements in performance during lactation.