INFLUENCE OF GENOTYPE AND STAGE OF MATURITY OF MAIZE ON RATE OF RUMINAL STARCH DEGRADATION

This trial was carried out to determine the influence of genotype and stage of maturity of maize on ruminal starch degradation by an in situ technique. In a first methodological trial, we measured the part of s tarch lost through the bag pores without being degraded with two dryin g methods and four grinding sieves. Starch losses were the lowest for oven-dried at 40 degrees C and 3-mm ground grains. In a second trial, two types of maize differing in the texture of their endosperm, dent o r flint, were harvested, respectively, at four and five stages of matu rity, between 22 and 78 days after silking. Each maize was characteriz ed by its vitreousness. At maturity, ruminal starch degradability was greater for dent than flint maize (61.3 vs 40.1%) due essentially to a greater rapidly degradable fraction (25.2 vs 15.7%). Dent maize had a lower vitreousness than the flint type, 48.1 vs 72.3%. The differenti al association between starch granules and protein matrix in each frac tion of endosperm alters the accessibility of starch granules to rumin al bacteria. Starch degradation decreased with maturity and this trend was greater for the earlier harvest dates and for flint than dent mai ze. This decrease came from a lower rapidly degradable starch fraction (51.0 vs 20.4%) and a lower degradation constant rate (14.2 vs 3.8% h (-1)) for the two genotypes. This decrease in starch degradation was r elated to the increase in the vitreousness of the endosperm with matur ity (r = 0.93). At silage stage (30% dry matter in whole plant), the d ifference in starch degradation between the two genotypes was high and similar to that determined at maturity of the grain. These results im ply that at silage stage ruminal starch escape can be modified by gene tic selection. (C) 1997 Elsevier Science B.V.