Rumen digestibility of starch and nitrogen in near-isogenic lines of wheat

Comparisons between cereal species have shown that wheat starch is fermented more rapidly in the rumen than starch from maize, barley, potato or sorghum, probably due to the protein matrix surrounding the starch granules, but there are few comparisons of starch digestion from different cultivars of wheat. Although wheat varieties are classified as being hard or soft, genetic factors affecting nutritional value cannot be evaluated using name alone because any two named varieties may be distantly or closely related. Near-isogenic lines allow the nutritional implications of specific characteristics to be investigated against a comparatively uniform genetic background. The purpose of this study was to investigate genetic effects on starch and nitrogen degradation kinetics in sacco for four pairs of near-isogenic wheat lines. Pair A comprised one hard and one soft line; Pair B comprised two hard lines, one of which contained the 1B/1R rye translocation; Pair C comprised a hard line with the 1B/1R rye translocation and a soft wheat without the 1B/1R translocation; Pair D comprised two lines differing in the visco-elastic properties of dough. Minor differences in the soluble and degradable fractions of starch and nitrogen were found within pairs. Only in Pair C, where the 1B/1R translocation was accompanied by a change in hardness, was there a significant effect on effective rumen starch and nitrogen degradability (soft non-1B/1R versus hard 1B/1R, starch digestibility=0.86 versus 0.88, standard error of the difference (S.E.D.) 0.001; nitrogen degradability=0.80 versus 0.76, S.E.D. 0.008). Rumen by-pass starch was influenced mainly by the starch content of the wheat rather than differences in rumen digestion. It is concluded that in ruminants, unlike poultry, genetic differences in wheats have very little impact on their overall nutritive value, which is determined mainly by total starch content.