FEEDING LACTATING PRIMIPAROUS SOWS TO ESTABLISH 3 DIVERGENT METABOLICSTATES - II - EFFECT ON NITROGEN PARTITIONING AND SKELETAL-MUSCLE COMPOSITION

We established an experimental model to study nitrogen (N) partitionin g in lactating primiparous sows alimented to three levels of nutrient intake. Thirty-six sows fitted with a gastric cannula and fed a 15.4 M J DE/kg and 18.6% CP diet were allocated to one of three treatments af ter farrowing: 1) ad libitum-fed; 2) restricted-fed to 55% of the ad l ibitum feed intake; and 3) superalimented to at least 125% of the ad l ibitum feed intake. These feed intakes were successfully achieved thro ughout lactation. Nitrogen balance was studied for three 5-d periods s tarting on d 2, 11, and 19 of lactation, and a triceps muscle biopsy w as taken on d 26. For all treatments, N intake increased, milk N produ ction increased, urinary N losses decreased, but fecal N losses increa sed as the 28-d lactation progressed. Restricted-fed sows had the lowe st fecal N and urinary losses and mobilized the most maternal protein (-23.0 vs -7.4 +/- 6.5 g N/d for ad libitum-fed sows) during lactation . As a consequence of these economies, and extensive protein mobilizat ion, restricted-fed sows were able to maintain milk N production simil ar to that of sows on the other treatments. Superalimented sows did no t mobilize protein, had the poorest protein digestibility, directed th e least digestible N toward milk (40.1 vs 78.3% in restricted-fed sows ), and produced amounts of milk N similar to those produced by sows on the other treatments. The treatment differences in N retention measur ed by N balance were reflected in differences in skeletal muscle varia bles and urinary creatinine. Skeletal muscle cell size (protein:DNA ra tio) and protein synthetic capacity (RNA:DNA ratio) increased in respo nse to feed intake. The protein:DNA ratio increased (P < .01) linearly and the RNA:DNA ratio increased (P < .05) in a curvilinear manner. Th ese data suggest that primiparous sows partition additional retained N toward their maternal reserves rather than milk N. They also suggest that sows fed inadequate N intakes maintain milk production by mobiliz ing maternal protein reserves. Such sows also conserve maternal N duri ng lactation, possibly by reducing muscle protein synthesis.