Genetic variation and breeding strategies for improved cell wall digestibility in annual forage crops. A review

Abstract

Forage plants are basis of ruminant nutrition, and cell wall digestibility is limiting factor of their feeding value. Cell wall digestibility is therefore the target for improving feeding value of forage crops. Among annual forages, maize cropped for silage making is most widely used, and much research in genetics, physiology and molecular biology of annual forages is devoted to maize. Sorghum, immature small grain cereals and straws of small grain cereals are also given to cattle. Some dicotyledons are or were also used, such as forage beets, kales, canola in temperate areas and amaranths in tropical and subtropical areas. Large genetic variation for cell wall digestibility was proved from both in vivo and in vitro experiments in numerous species. Among regular maize hy- brids (excluding brown-midrib ones), NDF in vivo digestibility nearly doubled from 32.9 to 60.1%. Correlations between in vivo and in vitro estimates of cell wall digestibility were often close to 0.75, but in vitro estimates of cell wall digestibility significantly reduced range of variation be- tween genotypes. Despite lignin content is well known as major factor making cell wall undigestible, breeding for a higher digestibility of plant only from a lignin content trait appeared im- possible. Correlations between lignin content and cell wall digestibility were indeed greatly variable according to genetic background. Moreover, enzymatic solubilities were excessively dependent on lignin, and correlation between in vivo estimates of cell wall digestibility and lignin content were always lower than correlation between in vitro estimates of cell wall digestibility and lignin content. Among brown-midrib genes, bm3 mutant in maize, and bmr12 (and possibly bmr18) mutant in sorghum, which are both altered in COMT activity, appeared as most efficient in cell wall di- gestibility improvement. Moreover, a great genetic variation in efficiency of maize bm3 gene for cell wall digestibility improvement was observed according to genetic background, with a lower efficiency when normal germplasm was of better cell wall digestibility. Efficient breeding maize and others annual forage plants demands a renewing of genetic resources. Because resources of interest in cell wall digestibility improvement could be of poor agronomic value, best is likely to