Two least squares methods of estimating nutrient requirements from growth data were compared. One method involved fitting a broken line by the method of least squares. The requirement was taken as the abscissa of the breakpoint in the curve. The other method involved fitting an appropriate exponential function to the growth data and estimating the requirement as the abscissa of the point on the fitted curve whose ordinate was 95% of the upper asymptote. For the nine sets of data studied, the broken line provided adequate fits for only six. The nonlinear models provided adequate fits for all the data studied. When both the broken line and the chosen nonlinear model provided adequate fits, the estimated requirements were nearly the same. However, the consistently good fits obtained with the nonlinear models suggest that this approach may generally be more useful.

The bioavailability of zinc-methionine (ZnMET) was compared to that of feed-grade ZnSO4.H2O using three different diets: purified (crystalline amino acid [AA]), semipurified (soy isolate), and complex (corn-soybean [C-SBM]) diet. With the Zn-deficient purified or semipurified diet, weight gain and tibia Zn responded linearly to both ZnSO4.H2O and ZnMET supplementation. Common-intercept, multiple linear regression indicated differences in Zn bioavailability between ZnMET and ZnSO4.H2O for both diets as indicated by bone Zn. With the ZnSO4.H2O standard set at 100%, bioavailability of Zn from ZnMET was 117% (P less than .05) in the AA diet and 177% (P less than .01) in the soy isolate diet. The ZnMET was also compared to ZnSO4.H2O in a C-SBM diet containing 117 mg of Zn/kg. When high levels of Zn were added to this diet (0, 250, 500, and 750 mg/kg of supplemental Zn), consistent tissue Zn responses did not occur beyond the first increment. Addition of lower levels of supplemental Zn (0, 5, 10, 20, 30, 40 and 50 mg/kg) to a Zn-unsupplemented C-SBM basal diet (45 mg/kg of Zn), however, resulted in a broken-line, two-slope response in tibia Zn for both ZnMET and ZnSO4.H2O. Inflection points occurred at 60 and 54 mg of Zn/kg of diet for ZnSO4.H2O and ZnMET, respectively. The ratio of slopes (ZnMET:ZnSO4.H2O) below the inflection points was 206% (P less than .01), indicating that Zn was considerably more bioavailable in ZnMET than in ZnSO4.H2O for chicks consuming C-SBM diets. When feed-grade ZnO was compared to feed-grade ZnSO4.H2O in chicks consuming C-SBM diets, bone Zn slopes below the respective inflection points indicated that Zn was 61% bioavailable in ZnO relative to ZnSO4.H2O.

The ideal protein concept has allowed progress in defining requirements as well as the limiting order of amino acids in corn, soybean meal, and a corn-soybean meal mixture for growth of young chicks. Recent evidence suggests that glycine (or serine) is a key limiting amino acid in reduced protein [23% crude protein (CP) reduced to 16% CP] corn-soybean meal diets for broiler chicks. Research with sulfur amino acids has revealed that small excesses of cysteine are growth depressing in chicks fed methionine-deficient diets. Moreover, high ratios of cysteine:methionine impair utilization of the hydroxy analog of methionine, but not of methionine itself. A high level of dietary L: -cysteine (2.5% or higher) is lethal for young chicks, but a similar level of DL: -methionine, L: -cystine or N-acetyl-L: -cysteine causes no mortality. A supplemental dietary level of 3.0% L: -cysteine (7x requirement) causes acute metabolic acidosis that is characterized by a striking increase in plasma sulfate and decrease in plasma bicarbonate. S-Methylmethionine, an analog of S-adenosylmethionine, has been shown to have choline-sparing activity, but it only spares methionine when diets are deficient in choline and(or) betaine. Creatine, or its precursor guanidinoacetic acid, can spare dietary arginine in chicks.

The emerging view is that reduced feed intake, lean muscle accretion, and growth in immunologically challenged pigs is the result of increased cytokine activity, but this has not been directly tested. To begin addressing this issue, 72 crossbred barrows and gilts (11.55 +/- .19 kg BW) were not fed for 12 h and then injected i.p. with 0, .5, or 5 micrograms/kg of Escherichia coli lipopolysaccharide (LPS). Blood was collected by jugular puncture at 0, 2, 4, 8, 12, and 24 h after injection. Plasma levels of tumor necrosis factor-alpha (TNF-alpha), interleukin-6 (IL-6), cortisol, plasma urea nitrogen (PUN), NEFA, and triglycerides were determined. Immunological stress was induced by LPS as indicated by increased secretion of TNF-alpha, IL-6, and cortisol. In pigs receiving 5 micrograms/kg of LPS, plasma TNF-alpha was increased 10-fold at 2 h after injection and was still elevated (P < .01) at 4 h. In these same pigs, plasma concentration of IL-6 was increased at 2 h and peaked at 4 h with levels exceeding baseline values by 200-fold (P < .01). Cortisol was elevated at 2, 4, and 8 h after injection (P < .01). The increased secretion of cytokines and cortisol in pigs injected with 5 micrograms/kg of LPS was followed by an increase in protein degradation, as evidenced by PUN values that were increased two- and threefold at 8 and 12 h after injection, respectively. However, unlike previous reports in laboratory animal species, plasma glucose, NEFA, and triglycerides were not altered by LPS. Nonetheless, as the period of feed deprivation progressed from 12 to 36 h, plasma NEFA and triglycerides increased (P < .05) and plasma glucose tended to decrease. We believe that immunological challenge induces cytokine synthesis and secretion in swine which, in turn, may induce protein catabolism.