BREEDING SILAGE MAIZE WITH BROWN-MIDRIB GENES - FEEDING VALUE AND BIOCHEMICAL CHARACTERISTICS

The effects of the maize brown-midrib genes bm3 and bm1 on feeding val ue and agronomic traits were investigated by animal studies and bioche mical approaches. When sheep were fed bm3 silage, an increase in diges tibility was observed, equal to 2.4 points for digestibility of organi c matter (DOM), and 8.1 points for digestibility of crude fiber (DCF). Phenotypic variation was reduced for all observed traits of bm3 hybri ds. This allowed the assumption that the bm3 gene acted by levelling t o a similar value of almost 74% DOM, quite independently of the origin al value of the normal counterpart. The bm1 hybrids were intermediate between normal and bm3 ones for DOM, DCF and intake, but the dry matte r yield was not reduced for bm1 hybrids. From experiments with young b ulls, feed efficiency of INRA260 bm1 was not significantly improved, c ompared with normal INRA260, but INRA260 bm3 was a lot more efficient. Despite its lower adverse effect on agronomic characteristics of hybr ids, the bm1 allele did not appear to be of any interest in improving the silage maize feed efficiency. Variations for lignin content due to the bm3 gene was similar for the 3 hybrids studied, despite their var iation for susceptibility to root and stalk lodging. Ratios and conten t of alkali-labile hydroxycinnamic acids behaved similarly for the 3 h ybrids, with a low yield of para-coumaric acid. Structural studies of lignin, after thioacidolysis, strengthened the results previously obta ined, with a clear decrease of syringyl units, and the occurrence of t he 5-hydroxyguaiacyl residue. The lignin of bm3 genotypes also appeare d as a more condensed polymer than that of the normal types. The decre ase of lignin content was observed in only 1 bm1 hybrid and appeared t he same between normal and bm1 hybrids as between normal and bm3 hybri ds. The lower yield of para-coumaric acid, and the equal yield of feru lic acid after alkaline hydrolysis of bm1 material were in good agreem ent with previous work. After thioacidolysis, the release of monomeric syringyl units was similar between bm1 and bm3, about 30% of the norm al counterpart. However, it was very different for guaiacyl units, wit h a large decrease of the release of these units from the bm1 hybrid. The ratio between syringyl and guaiacyl units was about 0.3 for bm3 hy brids and about 1.3 for the bm1 hybrid. The lignin of this bm1 hybrid appeared as a very condensed polymer compared with the normal type. A large variation for digestibility was also found among normal hybrids. However, inbred lines inducing high digestibility often included unfa vorable agronomic traits, such as poor yield, susceptibility to lodgin g or stalk rot. Second or third cycle lines following pedigree breedin g might combine all traits, even if stalk standability and stalk diges tibility are partly antagonistic. This method may be considered as eas ier than the bm3 method, but at present, the 2 ways are probably compl ementary, and worth investigating for improving silage digestibility, and simultaneously increasing knowledge on silage maize digestibility.