Factors influencing the assessment of the availability of choline in feedstuffs.

Experiments were conducted to test the hypothesis that chicks fed diets designed to be first-limiting in choline will also respond to supplements of sulfur-containing amino acids and that the growth response of chicks fed a choline-limiting diet to a supplement of a feedstuff cannot be accepted as a reliable measure of the choline content of the feedstuff. Chicks fed 25% isolated soybean protein (ISP) diets that were severely limiting in choline showed a growth response to supplements of either choline or methionine. A bioassay was used to determine the availability of choline in soybean meal (SBM). Results indicated that including 10% SBM in the 25% ISP diet (at the expense of corn starch) drastically altered the chicks' response to choline. When 100 ppm choline was added to the 10% SBM diet, growth responses, compared to the original standard curve, indicated recoveries of 400 and 352% in two experiments. Additional studies indicated that chicks fed the 25% ISP + 10% SBM-based diet responded not only to choline additions but also to methionine, cystine, sulfate, and betaine additions. Further, it was shown that chicks fed a practical-type corn-soy-grease-based diet respond equally as well to supplements of 1% concentrated Steffens Filtrate, .2% L-methionine, or 800 ppm choline. The validity of assays of choline availability based on the assumption that choline is the first-limiting nutrient is questioned.