Body composition at farrowing and nutrition during lactation affect the performance of primiparous sows: II. Milk composition, milk yield, and pig growth.

Sows that were either fat or lean at farrowing (340 or 280 g of body fat/kg BW, respectively) were offered either a low-protein (LP; 7.9% CP and 15.5 MJ DE/kg as fed) or a high-protein (HP; 19.0% CP and 15.6 MJ DE/kg as fed) diet on an ad libitum basis throughout a 4-wk lactation to test the hypothesis that the amount of milk and its composition are responsive to the supply of endogenous (body reserves) and exogenous (diet) substrates. Pigs were weighed at birth and weekly during lactation, milk yield was estimated using deuterium oxide in early (d 4 to 8) and late lactation (d 24 to 28), and milk samples were collected to determine composition in early (d 4 to 6) and late lactation (d 25 to 27). Throughout lactation, milk yield and composition were mainly associated with differences in litter size. Milk yield was about 15% higher in lean than in fat sows and in sows fed HP rather than LP, but large CV (17 to 32%) prevented these differences from attaining significance (P > .273). The responses in milk yield were reflected in pig growth. Differences in milk composition between treatment groups were not significant; however, during early lactation there was a tendency for fat sows to produce milk with a fat content 21% higher, and a protein content 12% lower, than that of lean sows. Changes in the protein:energy ratio of milk during the course of lactation and small changes in milk yield composition collectively suggested that in early lactation, sow body composition affected milk production but, as lactation progressed, the dietary supply of precursors for milk synthesis became more important.