Diminishing returns in weight, nitrogen, and lysine gain of pigs fed six levels of lysine from three supplemental sources

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Abstract
Diminishing returns responses to increments of lysine intake were evaluated in 82 crossbred barrows (10.2 ± SE = .31 kg) fed one of six concentrations of lysine expressed as 55, 80, 90, 100, 115, or 140% of a concentration (10.9 g/kg of diet) that supported maximum weight gain. Supplemental lysine was provided from either soybean meal (SBM), SBM + synthetic lysine (SBM + L), or SBM + corn gluten meal (SBM + CGM) additions to a basal diet consisting of corn, minerals, and vitamins. A logistic equation was used to describe weight (kilograms/day), nitrogen (grams/day) and lysine (grams/day) gain as a function of lysine intake from each diet. The parameter Rmax, asymptotic maximum response at infinite intake, was shared for SBM and SBM + L (.70 ± .02, 17.6 ± .4 and 7.6 ± .5, respectively) but was different (P < .05) for SBM + CGM (.62 ± .02, 15.7 ± .4 and 6.4 ± .4, respectively) diets. A plateau in weight gain response was observed at approximately 100% of our estimate of the lysine requirement (9.8 g/kg, based on analyzed values), but nitrogen and lysine gain responses did not approach a plateau until 120 or 145% of the requirement. No differences were detected among pigs fed the SBM and SBM + L sources; however, the efficiency of lysine use by pigs fed the SBM + CGM diet was lower. Maximum marginal efficiency (dr/dI) occurred at 45, 40, and 35% of Rmax for weight, nitrogen, and lysine gain, respectively. The maximum marginal efficiency of lysine gain was 81% for pigs fed the SBM and SBM + L diets but only 68% for pigs fed the SBM + CGM diet. Consequently, diminishing returns were apparent for at least the upper 55 to 65% of the response curve. Whole-body lysine content increased (P < .03) from 5.5 to 6.0 g/16 g of N and glycine (9.6 to 7.9) and proline (6.4 to 5.4) content decreased (P < .001) as lysine intake increased. No changes were detected in body threonine content (pooled average = 2.8).

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