BROWN-MIDRIB GENES OF MAIZE - A REVIEW

The efficiency of forage for ruminant animals primarily depends on cell-wall constituent content, mostly cellulose and hemicellulose, and on their digestibility, mostly reduced by lignin incrustration. Four brown-midrib genes (bm1, bm2, bm3 and bm4) have been described in maize. Brown-midrib plants were described as having a lower lignin content than normal genotypes; the ratios of hydroxycinnamic acids released after alkaline hydrolysis, and the ratios between monomeric units released after alkaline nitrobenzene oxidation or thioacidolysis also differed in normal and brown-midrib plants. The bm3 gene, allowing an important decrease of lignin content, and a better improvement in plant digestibility, was also the most studied. When cattle were fed bm3 plants, intake and digestibility were higher compared to normal isogenic ones, and the rate of digestion was also higher. However, the agronomical value of the brown-midrib genotypes was distinctively lower than that of their normal isogenic counterparts and this was particularly notable on the bm3 gene. This observation also appeared to be genotype-specific, suggesting the possibility of producing brown-midrib genotypes that are practically as good as the normal ones, but only when using well-adapted breeding methods, with normal lines of a very high agronomical value.