Effects of dietary fat level and lysine:energy ratio on energy utilization and tissue synthesis by broiler chicks.

An experiment was designed to estimate the effect of dietary fat (20 or 80 g/kg) on the responses of broiler chicks to changes in the ratios of bioavailable lysine (LB) to true metabolizable energy corrected to zero nitrogen balance (TMEn) in their diets. A comparative slaughter experiment used an initial slaughter group of 44 10-day-old male chicks; an additional 120 chicks from the same population were used in the 14-day experiment. Dietary treatments comprised two basal diets differing in fat content and formulated to have similar nutrient:TMEn ratios. Each basal diet was supplemented with four levels of lysine to provide four LB:TMEn ratios calculated, and subsequently found, to be the same in each basal diet series. Each of the eight diets was diluted with five levels of cellulose to ensure a range of intakes under ad libitum feeding. Three individually housed chicks were assigned to each of the 40 diets. Carcasses were assayed for water, nitrogen, lipids, ash, and gross energy; changes in these variables during the experiment were the response criteria. The fat content of the diet had no effect (P greater than .05) on the chick responses to LB:TMEn ratios. At a fixed energy intake the body weight gain, retained water, retained energy as protein and retained ash increased with the LB:TMEn ratio, with no conclusive evidence of a maximum response having been reached for any variable at an LB:TMEn ratio of .83 g/MJ. Retained energy was independent (P greater than .05) of the LB:TMEn ratio but the energy retained as neutral lipids tended to decrease as the ratio increased. The data support the view that the lysine requirement of the broiler chick varies according to the response criterion used in its assessment.