EFFECTS OF DIETARY-PROTEIN CONCENTRATION (CORN-SOYBEAN MEAL RATIO) AND BODY-WEIGHT ON NITROGEN-BALANCE OF GROWING BOARS, BARROWS, AND GILTS- MATHEMATICAL DESCRIPTIONS

The effects of dietary CP level (actually corn:soybean meal ratio) and BW on N intake, N digestibility, the efficiency of utilization of N ( apparent biological value), and N retention were measured in 36 pigs: 12 boars (five collection periods), 12 barrows (four collection period s), and 12 gilts (nine collection periods). Initial and final BW of th e boars, barrows, and gilts were 19, 24, and 27 kg and 80, 69, and 110 kg, respectively. Diets were based on corn and soybean meal and conta ined 11 to 23% CP. Averaged over all collection periods, N retention w as 24.2, 20.0, and 19.8 g/d for boars, barrows, and gilts, respectivel y. The response of N retention to CP levels was quadratic (P < .15) an d cubic (P < .01) for boars, linear (P < .01) and quadratic (P < .05) for barrows, and linear (P < .15) for gilts. Rates of N retention resp onded quadratically (P < .001) for all sexes as pigs gained weight, re aching a maximum when pigs weighed 55 to 65 kg. Multiple-regression an alyses were conducted and equations were developed that described the effects of CP level (or lysine intake) and BW on N retention for each sex. The results indicated that N balance was a function of both CP le vel (or lysine intake) and BW and that the nature of the response diff ered for each sex.