Maize genotype and ruminant nutrition

Maize breeding objectives have, for a long time, been limited to agronomic criteria, but a high variability in nutritive value for ruminants also is o bserved among hybrids. Nowadays breeding takes into account the effect of c hemical composition in the modification of maize nutritional value. The amo unt and composition of whole plant cell walls, especially their lignificati on, influences both digestibility and voluntary intake when maize is given as silage. Consequently, hybrids differ in their capacity to be utilized by dairy and beef cattle. Starch, the main constituent of grain, is degraded rather slowly in the rumen, and differences among genotypes exist. Among no rmal maize hybrids, the differences are mainly related to grain vitreousnes s, i.e. to the ratio of vitreous to floury endosperm which characterises fl int and dent grains, respectively. Dent grains contain proteins that are mo re degraded in the rumen. As a consequence, starch granules are more access ible to microbial enzymes. Floury varieties obtained by mutation give rise to extreme cases of dent varieties. Waxy and amylose-extender maizes, origi nating from a mutation that modifies the ratio at amylose to amylopectin, a lso show differences in their starch degradation rate. High-oil maize is no w being developing. No specific problem occurs when high-oil maize is used in ruminant feeding. Compared to other lipidic sources of the same composit ion, maize lipids do not have particular consequences on either the efficie ncy of production or the quality of products.