The impact of dietary amino acid regimen on the milk nutrient yield of sows differing in genetic capacity for lean tissue growth was determined. Primiparous sows with a high (.77 to .85 lb/day) or low (.53 to .62 lb/day) genetic capacity for lean tissue growth from 40 to 240 pounds body weight were evaluated. During lactation, sows were offered daily 14.3 pounds of one of four fortified corn-soybean meal diets containing .58, .77, .96 and 1.15% lysine (L). Litters were standardized to 14 pigs within eight hours postpartum. Milk yield (via a deuterium oxide dilution technique) and composition were determined over four-day intervals from day 2 to 26 postpartum. Based on net feed consumption and the composition of mobilized body tissues (via a comparative slaughter technique), the daily supplies of lysine and metabolizable energy (ME) available for milk synthesis were calculated.

The bodies of high lean growth (LG) sows possessed more proteinaceous tissues and protein and less fat tissue and lipid than low LG sows at the initiation of lactation. During lactation (days 2 to 28 postpartum), high LG sows (pooled across dietary regimens) consumed more feed, mobilized more body protein, and catabolized less body lipid than low LG sows, whereas milk yields were similar between genotypes. As total daily lysine supplies (from diet and mobilized tissues) increased, daily yields of milk, milk energy, and milk lysine increased, but the magnitude of the responses were less in the high LG sows. At low dietary lysine intakes, total lysine supply was limiting milk synthesis in both genotypes. As dietary lysine intake increased, total ME supply became more limiting than lysine, particularly in the high LG sows because of their inability to provide sufficient energy from mobilized body fat tissues.

Based on these data, milk, milk energy, and lysine yields of high and low lean growth genotype sows are similar when similar total supplies of lysine and energy are available. When energy supplies do not limit milk synthesis, the efficiency of utilization of digestible lysine for milk lysine production is similar between genotypes. Specifically, .85 to .87 Mcal ME and 1.95 to 2.05 grams (g) of digestible lysine are needed per pound of milk produced in sows nursing large litters. If it is assumed that each nursing pig requires daily 1.8 pounds of milk, then lactating sows require about 1.5 to 1.6 Mcal ME and 4.3 to 4.5 g of digestible lysine (5.0 to 5.3 g lysine from a cornsoy diet) for each pig nursed. Because of the limited supplies of mobilizable fat tissues in high lean growth sows, the provision of adequate dietary energy intakes to the sows is more critical in order to allow their maximum lactational capacity to be expressed.