a1 Faculty of Veterinary Science, University of Sydney, Camden, New South Wales 2570, Australia
a2 Department of Poultry Science, College of Agriculture and Life Sciences, North Carolina State University, Raleigh, North Carolina 27695-7608, USA
The nutritional antagonism of arginine (Arg) and lysine (Lys) was first identified and investigated in the 1950's and 1960's. The results of this early research suggest the optimum Arg:Lys ratio to fall between 0.8 and 1.7, depending upon dietary levels of electrolytes such as sodium, potassium, and chloride. Calculations from the more recent optimum amino acid balances included in widely referenced authoritative sources suggest the optimum Arg:Lys ratio to be in the range from 0.90 to 1.18. Changes from the ‘optimum’ value of the ratio can have adverse effects on the performance of growing poultry. The effect is more evident with an excess of lysine (low Arg:Lys ratio) than with an excess of arginine (high Arg:Lys ratio). Studies with heat-stressed broilers have shown that the optimum Arg:Lys ratio varies with ambient temperature. The Arg:Lys ratio for optimum broiler body weight gain and feed efficiency increases at high temperatures, probably because of a reduced uptake of arginine from the digestive tract. The improved response of broilers to increasing dietary Arg:Lys ratio is most clearly seen during heat stress with diets containing minimum concentrations of NaCI. This response diminishes with high dietary NaCl and with NaHCO3 supplementation, when the optimum dietary Arg:Lys ratio declines from the high ratio (~1.30) observed with low dietary NaCl. Furthermore, the nature of the methionine activity source influences the optimum dietary Arg:Lys ratio for heat-stressed broilers. The performance of broilers fed 2-hydroxy-4-(methylthio) butanoic acid (HMB) is optimised at high Arg:Lys ratios (1.35) whereas broilers fed equimolar supplements of DL-methionine (DLM) tend to optimise performance at lower Arg:Lys ratios (1.05). The selection of the correct methionine activity source as a dietary supplement is likely to become more important if the current trend to exclude animal protein feed ingredients with low Arg:Lys ratios from poultry diets continues.