Breeding for Silage Quality Traits

Abstract

Forage plants are the basis of ruminant nutrition. Among cereal forages, maize cropped for silage making is the most widely used. Much research in genetics, physiology, and molecular biology of cereal forages is thus devoted to maize, even if silage of sorghum or immature small-grain cereals and straws of small-grain cereals are also given to cattle. Cell wall digestibility is the limiting factor of forage feeding value and is, therefore, the first target for improving their feeding value. Large genetic variation for cell wall digestibility was proven from both in vivo and in vitro experiments in numerous species. Among the regular maize hybrids [excluding brown-midrib (bm) ones], the cell wall digestibility nearly doubled from 32.9% to 60.1%. Genetic variation has also been proven in cell wall digestibility of sorghum and wheat, barley or rice forage, or straw, with lower average values than in maize. Despite lignin content is well known as an important factor making cell wall indigestible, breeding for a higher digestibility of plant needs the use of specific traits estimating the plant cell wall digestibility. Quantitative trait loci (QTL) analysis, studies of single-nucleotide polymorphism (SNP) × feeding value traits relationships, studies of mutants and deregulated plants, and expression studies will contribute to the comprehensive knowledge of the lignin pathway and cell wall biogenesis. Plant breeders will then be able to choose the best genetic and genomic targets for the improvement of plant digestibility. Favorable alleles or favorable QTL for cereal cell wall digestibility will thus be introgressed in elite lines through marker-assisted introgression. Efficient breeding of maize and others annual forage plants demands a renewing of genetic resources because only a limited number of lines are actually known with a high cell wall digestibility. Among bm genes, the bm3 mutant in maize and the bmr12 (and possibly bmr18) mutant in sorghum, which are both altered in the caffeic acid O-methyltransferase (COMT) activity, appeared as the most efficient in cell wall digestibility improvement. Genetic engineering is both an inescapable tool in mechanism understanding and an efficient way in cereal breeding for improved feeding value. Moreover, gene mining and genetic engineering in model plant and systems (Arabidopsis, Zinnia, Brachypodium, …) are also essential complementary approaches for improvement of cell wall digestibility in grass and cereal forage crops.