| Four experiments were conducted to investigate the effects of protein on endogenous ileal amino acid flows and methods of determining endogenous flow. (1) The optimal guanidination conditions for synthetic lysine and soybean protein HP310 were investigated. The optimal molar ratio of O-methylisourea (OMIU) to lysine was 1.5 : 1 in the guanidination reaction of synthetic lysine, and under this ratio the transformation rate of lysine was 99.46%. For soybean protein HP310, the appropriate guanidination conditions were 0.6 M of OMIU concentration, 10.8 of pH and 20°C of temperature, and 144 h incubation period, and the pH 3.5 of washing water for precipitating protein. Under these conditions, the lysine conversion of 88.4% was obtained. (2) An experiment was conducted to investigate the effects of graded levels of the dietary crude protein on endogenous ileal lysine loss, apparent ileal amino acid digestibility, standardised true ileal amino acid digestibility calculated using the protein-free (PF) method, and real ileal amino acid digestibility calculated using the homoarginine (HA) method in growing swine. At the same time, a comparison was conducted among HA technique, PF method and regression method for determining endogenous ileal amino acid losses. Six barrows, with an initial body weight of 37.4 ± 1.3 kg, were surgically fitted with simple T-cannulae at the distal ileum and fed six cornstarch-based diets according to a 6 × 6 Latin-square design. The diets included a PF diet (0% CP) and five diets formulated with varying levels of CP (5, 10, 15, 20, and 25% ) by including graded levels of a protein source (HP310). The guanidination technique, PF method and regression method were used in order to estimate endogenous amino acid flows and real (or standardized true) ileal amino acid digestibility. Each experimental period lasted 8 days. Endogenous ileal lysine losses for pigs fed the PF, 5, 10, 15, 20 and 25% CP diets appeared to follow a sigmoid curve starting at about 370 mg/kg dry matter intake of endogenous ileal lysine flow for pigs fed the PF diet and continuing asymptotically to about 750 mg/kg dry matter intake when the CP level was increased above 10%. The endogenous ileal lysine flow for pigs given the PF diet was similar to that of pigs given the 5% CP diet (P > 0.05), and it increased sharply (P < 0.05) with increasing dietary CP content from 5 to 10%. Thereafter, it became relatively constant (P > 0.05). The estimated values of endogenous ileal amino acids using HA technique, PF method and regression method were apparently different. An underestimation can be found by using PF and regression method comparing with HA technique. We also found in the study that the endogenous ileal losses of most amino acids calculated using regression method were lower (P < 0.1) than that of PF method. With an increase in protein content, there was a quadratic increase (P < 0.05) in apparent ileal digestibilities for the majority of amino acids. The standardised true ileal amino acid digestibility decreased (P < 0.05) with an increased dietary protein level. However, the real ileal amino acid digestibility was not influenced by dietary protein levels between 5 and 20%. (3) To investigate the effects of balance status of amino acids in protein-containing diets on endogenous ileal lysine loss, six barrows of 65.4 ±3.1 kg fitted with simple T-cannulae at the distal ileum were used and fed two cornstarch-zein-based diets with balanced or imbalanced amino acids according to a cross-over design. Each experimental period lasted 6 days including 5 days adjusting time, and on day 6, a 24 h continuous collection of digesta was conducted. The balance status of amino acids in protein-containing diets had no influence on endogenous ileal lysine loss (P = 0 24). (4) Three methods were tested for their ability to measure endogenous ileal lysine flow and amino acid digestibilities in pigs fed zein-based diets. The methods tested included blood perfusion of lysine and oral administration of either homoarginine or lysine. Six... |
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