Nitrogen retention and lysine oxidation were measured in growing pigs given diets which supplied 0, 0·2 or 0·8 of the lysine requirement, with other amino acids in relative excess. Eight groups of three female littermate pigs were used: one of each group was given each of the three diets. In half the pigs (four groups) N retention was measured at body weights (W) of approximately 25,35 and 45 kg. The other four littermate groups of three pigs were given the same three diets; when they reached 35 kg W they were given a continuous (6h) primed infusion of L-[6-3H]lysine. Lysine oxidation was estimated from the production of tritiated water. Rates of both N retention and lysine oxidation increased significantly with lysine intake; mean values (g/kg W0·75 per d) for the three diets respectively were for N retention, 0·00, 0·32 and 1·22, and for lysine oxidation 0·051, 0·058 and 0·078. From the N balance results (assuming a constant lysine concentration in body protein) the efficiency of utilization of absorbed lysine was estimated to be 0·85; from the oxidation results (assuming lysine absorbed but not retained is oxidized) the estimate was 0·95.

Two growth experiments were conducted to determine the effect of heat on the utilization of ileal-digestible lysine from field peas (Pisum sativum cultivar Dundale) fed to growing pigs. Five lysine-deficient diets (0.36 g ileal-digestible lysine/MJ digestible energy (DE)) were formulated using raw field peas, and field peas heated to either 110°, 135°, 150°, or 165° for 15 min respectively in a forced-air dehydrator. Additional diets were formulated with supplements of free lysine to verify that lysine was limiting in the diets containing the raw peas, and peas heated to 150° or 165°. The growth performance and retention of ileal-digestible lysine by pigs given the diets was determined over the 20–45 kg growth phase. Heat had a significant quadratic effect (P < 0.01) on growth rate, with responses declining from 543 g/d with pigs given the raw peas, to 407 g/d for those given the peas heated to 165°. Similarly, crude protein deposition declined in a quadratic manner (P < 0.001) from 76 to 36 g/d for pigs fed on raw peas and peas heated to 165° respectively. Retention of ileal-digestible lysine was 0.85 in the pigs given the raw field peas and declined in a quadratic manner (P < 0.001) with the application of heat to 0.48 in those pigs given the peas heated to 165°. Pigs fed on field peas heated to 165° had increased (P < 0.05) liver weights. The results indicate that heat applied to protein concentrates, even at mild temperatures, renders lysine in a form that is apparently absorbed but inefficiently utilized by the growing pig. Consequently, ileal digestibility values for lysine in heat-processed meals are unsuitable for diet formulations.

Two experiments were conducted to determine the utilization of ileal digestible lysine by pigs. In the first, the apparent ileal digestibility of amino acids in cottonseed meal, meat-and-bone meal and soya-bean meal was determined in pigs fitted with‘T'-shaped cannulas. In the second experiment, three lysine-deficient diets were formulated to 0.36 g ileal digestible lysine/MJ digestible energy (DE), with lysine contributed from the three protein concentrates as the only source of lysine in sugar-based diets. An additional three diets were formulated with supplements of lysine to verify that lysine was limiting in the first three diets. The growth performance and retention of lysine by pigs given the six diets over the 20–45 kg growth phase were then determined. The apparent ileal digestibility of lysine in the three protein concentrates (proportion of total) was: cottonseed meal 0.74, meat-and-bone meal 0.78, soya-bean meal 0.89. Growth rates (g/d) of the pigs given the three diets formulated to 0.36 g ileal digestible lysine/MJ DE were significantly different (P < 0.001): cottonseed meal 377, meat-and-bone meal 492, soya-bean meal 541. The response of pigs to the addition of lysine confirmed that lysine was limiting in these diets. Crude protein (nitrogen× 6.25) deposited by the pigs was significantly higher (P < 0.001) for those given soya-bean meal (77 g/d), relative to meat-and-bone meal (66 g/d) and cottonseed meal (38 g/d). The proportion of ileal digestible lysine retained by pigs given the three protein concentrates was: cottonseed meal 0.36, meat-and-bone meal 0.60, soya-bean meal 0.75. The results indicate that values for the ileal digestibility of lysine in protein concentrates are unsuitable in dietary formulations as the assay does not reflect the proportion of lysine that can be utilized by the pig. It appears that, with heat-processed meals, a considerable proportion of the lysine is absorbed in a form(s) that is (are) inefficiently utilized.