Dietary Histidine: Broiler Responsiveness With and Without Endogenous Histidine Reserve Consideration

In the past, histidine warranted little concern from poultry nutritionists because it was seldom deficient in practical broiler diets. With most essential amino acids now commercially available as feed-grade forms to partially replace intact protein ingredients, histidine and its minimum in least-cost formulation are receiving fresh attention. Published histidine requirement data for broilers are sparce, and many of these studies have been conducted on males within the first three weeks post-hatch. Moreover, rudimentary dose-response experimental designs may not be applicable to estimate histidine requirements in later growth periods because histidine is uniquely stored within the body. In times of dietary histidine deficiency, endogenous histidine reserves can be catabolized to furnish free histidine and possibly confound requirement estimates. Therefore, two experiments were conducted to evaluate broiler responsiveness to dietary histidine concentrations with and without endogenous histidine reserve consideration. Chapter II utilized a 2 x 4 factorial design to determine whether female broilers responded differently to increasing digestible histidine to lysine ratios from 22 to 42 days when previously fed different digestible histidine to lysine ratios from 0 to 21 days. Live performance data were collected for both feeding phases and broilers were processed on day 43 to evaluate carcass characteristics. No interactions were found between the starter and grower diet digestible histidine to lysine ratios for any parameters assessed. Increasing the grower diet digestible histidine to lysine ratio elicited linear and quadratic responses for 42 day final body weight, 22 to 42 day body weight gain, 22 to 42 day feed conversion ratio, and 43 day total breast meat yield. The optimal digestible histidine to lysine ratio was calculated to be 34% for 43 day total breast meat yield and 35% for 22 to 42 body weight gain and feed conversion ratio. Chapter III utilized a dose-response design to assess male broiler responsiveness to digestible histidine to lysine ratios of 28 to 43% from 15 to 35 days. A positive control diet differing in the crude protein concentration was formulated to determine the efficacy of the test diet. Live performance and carcass characteristics were measured as response criterion. Increasing the digestible histidine to lysine ratio elicited linear responses for final body weight, body weight gain, feed conversion ratio, and carcass yield. Linear and quadratic responses were observed for both Pectoralis major and minor yield. The optimal digestible histidine to lysine ratios were calculated to be 37 and 39% for Pectoralis minor and major, respectively. Despite the fact that further reductions in dietary crude protein often lead to impaired broiler performance, several benefits could be achieved in doing so, especially during times when prices for intact protein ingredients are high. Broiler responsiveness to increasing dietary histidine concentrations suggests that a deficiency could occur during practical formulation. Thus, understanding requirements for histidine and other less limiting amino acids are some of the steppingstones in the quest towards the more sustainable production of chicken meat