Previous research indicates that the neonatal pig does not alter feed intake in response to changes in the energy density of manufactured liquid diets. Also, the limited response of IGF-I to exogenous porcine ST (pST) previously observed in young pigs may be influenced by the source of dietary energy. Our objectives were to 1) determine the effect of a high-fat (HF; 25% fat and 4,639 kcal/kg ME; DM basis) or low-fat (LF; 2% fat and 3,481 kcal/kg ME; DM basis) manufactured liquid diet on pig

more »

... ormance; and 2) determine whether the limited response to exogenous pST in young pigs depends on the source of dietary energy. Two replicates of 60 pigs (n = 120; barrows and gilts distributed evenly), with an initial BW of 4,207 ± 51 g, were weaned from the sow at 10 d of age and used in a randomized complete block design. Pigs were assigned by BW to one of six pens. Diets were formulated to provide a constant lysine:ME ratio and were fed on a pen basis for a duration of 9 d. On d 5, barrows and gilts within a pen were assigned randomly to receive either 0 or 120 g of pSTؒkg BW −1 ؒd −1 for 4 d. Pigs gained 336 ± 9 g/d, which resulted in an ending BW of 1002 7,228 ± 120 g, regardless of dietary treatment (P > 0.15). Pigs fed the LF diet consumed 17% more DM per pen daily than pigs fed the HF diet (2,777 ± 67 vs. 2,376 ± 67 g/d, P < 0.01), but calculated ME intake did not differ between dietary treatments (P > 0.20). The G:F was 24% greater in HF-than in LF-fed pigs (P < 0.01). Plasma urea N concentrations were higher in the HFfed pigs (11.0 ± 0.6 mg/dL) than in pigs fed the LF diet (6.2 ± 0.6 mg/dL; P < 0.05). Treatment with pST increased circulating IGF-I (P < 0.01) and decreased PUN (P < 0.01) concentration 32 and 25%, respectively, regardless of dietary treatment (P > 0.30). Circulating leptin averaged 1.8 ± 0.1 ng/mL and was not affected by dietary treatment (P > 0.35) or pST (P > 0.40). These results suggest that the ST/IGF axis is responsive in the young pig and the increase in circulating IGF-I and growth is independent of the source of dietary energy. Also, young pigs respond to a lower energy density liquid diet with increased feed intake, without altering growth performance, apparently utilizing a mechanism other than circulating leptin. ing phase. Increased weaning weight decreases postweaning mortality and growth lag, improves nursery performance, and ultimately decreases the age at market weight (Harrell et al., 1993; Kim et al., 2001) . Supplemental feeding strategies of nursing pigs provide evidence that the lactating sow does not optimize baby pig growth (Kelly et al., 1990; Azain et al., 1996) . In addition, results from artificial rearing studies have indicated that sow's milk does not supply an optimal 5