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 starch lost through the bag pores without being degraded, with two drying methods and four grinding sieves. Starch losses were the lowest for oven-dried at 40°C and 3-mm ground grains. In a second trial, two types of maize differing in the texture of their endosperm, dent or flint, were harvested, respectively, at four and five stages of maturity, between 22 and 78 days after silking. Each maize was characterized 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 differential association between starch granules and protein matrix in each fraction of endosperm alters the accessibility of starch granules to ruminal bacteria. Starch degradation decreased with maturity and this trend was greater for the earlier harvest dates and for flint than dent maize. 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 related to the increase in the vitreousness of the endosperm with maturity (r = 0.93). At silage stage (30% dry matter in whole plant), the difference in starch degradation between the two genotypes was high and similar to that determined at maturity of the grain. These results imply that at silage stage ruminal starch escape can be modified by genetic selection.