A NOTE ON THE INTERACTIVE INFLUENCE OF DIETARY LYSINE AND THREONINE ON PLASMA UREA LEVELS IN THE GROWING PIG

A low protein diet (120 g CP kg-1) was divided and adjusted at two lys ine levels, 8.0 and 10.0 g kg-1. At each of these levels L-threonine w as included at four rates, producing diets with threonine contents of 4.4, 4.9, 5.4 and 5.9 g kg-1. Each of these eight test diets were give n to eight growing pigs in a change-over experiment, which was designe d as a 8 X 8 Latin square. After 3 days of adaptation to each diet the uraemic response to a morning meal was observed for 6 h. Over this ti me the mean urea plasma responses ranged between 148 and 123 mg urea l -1. These two extreme values were statistically different (P < 0.01) a nd were associated with Diet A (4.4 g threonine and 8.0 g lysine kg-1) and Diet B (4.9 g threonine and 8.0 g lysine kg-1), respectively. Als o on the 10.0 g lysine level the addition of 4.9 g kg-1 threonine of t otal diet (Diet F) appeared sufficient to produce maximum suppression of plasma urea. Since Diets B and F supplied very different threonine: lysine ratios, it is concluded that the relation between lysine and t hreonine was not the main determinant of plasma urea levels.