Reducing crude protein and increasing limiting essential amino acid levels with summer-reared, slow- and fast-feathering broilers

Abstract

Values for the limiting essential amino acids (EAA), according to AgriStats averages for 1999 [1], that are used in broiler feed formulation generally exceed those advocated by NRC [2]. In turn, satisfying these general industry levels for TSAA, lysine, and threonine with commercially available purified forms enables CP to be decreased [1] below minimums recommended by NRC [2]. Imposing further increases in dietary levels of the three most limiting EAA while forcing a decrease in CP was evaluated as a means to improve performance of summer reared broilers. Average values from AgriStats, Inc. [1] for ME, CP, lysine, TSAA, and threonine served as the basis in developing an industry-representative, four-feed regimen while NRC [2] minimum recommendations were used for all other EAA. Four factorial feed treatments involved a forced reduction of 1% CP from the representative level combined with 10% increases in lysine, TSAA, and threonine from industry levels. Slow- and fast-feathering males received each of the four feeds in floor pens to 47 d of age. Responses attributed to alteration in CP, EAA, and feathering were largely independent of one another. Alteration of CP elicited the most dominant effects; body weight gain, amount of abdominal fat, chilled carcass yield, grade A, and skinless boneless breast meat were favored using the industry standard, whereas the converse was true for feed conversion. Accumulation of abdominal fat when the CP was reduced was less extensive with slow- than fast-feathering males. Presumably, additional productive energy was derived from an improved overall amino acid balance with decreased CP; however, this added energy and additional feather cover appeared to complicate an already close relationship with effective protein. Providing additional amounts of lysine, TSAA, and threonine only enhanced early live performance and likely occurred because all other EAA were more than satisfactory to realize additional benefits.