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

Abstract

-Forage plants are the basis of ruminant nutrition, and cell wall digestibility is the limiting factor of their feeding value. Cell wall digestibility is therefore "the" target for improving the feeding value of forage crops. Among annual forages, maize cropped for silage making is the 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 the regular maize hybrids (excluding brown-midrib ones), the 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 the range of variation between genotypes. Despite lignin content is well known as the major factor making cell wall undigestible, breeding for a higher digestibility of plant only from a lignin content trait appeared impossible. Correlations between lignin content and cell wall digestibility were indeed greatly variable according to the 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, the bm3 mutant in maize, and the bmr12 (and possibly bmr18) mutant in sorghum, which are both altered in the COMT activity, appeared as the most efficient in cell wall digestibility improvement. Moreover, a great genetic variation in the efficiency of the maize bm3 gene for cell wall digestibility improvement was observed according to the genetic background, with a lower efficiency when the 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, the best is likely to cell wall digestibility / ingestibility / annual forage / maize / genetic variation / breeding / lignin Résumé -Amélioration de la digestibilité des parois des fourrages annuels. Les plantes fourragères sont à la base de l'alimentation des ruminants, mais leur valeur nutritive est limitée par la digestibilité variable des parois végétales, dont l'amélioration est alors la cible essentielle des programmes de sé-lection. Parmi les fourrages annuels, le maïs est la plante la plus cultivée, et c'est sur cette espèce que porte l'essentiel de la recherche en génétique, physiologie et biologie moléculaire. Toutefois, le sorgho, les céréales à paill...

Genetic variation of maize silage ingestibility in dairy cattle

“…But, conversely, it allowed to obtain higher accurate maize values in sheep, due to a higher number of replicates. It was then possible to study the relationships between values observed in cows and these value in sheep considered as average acute values, all the more because genotype × year interactions were proven low for cell wall digestibility traits [3,6,9]. Moreover, this methodology allowed to obtain robust in vivo feeding values for a large collection of maize hybrids, and then to study, from a breeding point of view, the genetic variation for feeding value in maize [6,9].…”

“…Genetic variation for maize silage digestibility and energy value was proved in numerous works from in vivo measurements, either between normal hybrids or in comparisons with bm3 hybrids (review in [9]). Recent unpublished data of Inra Lusignan showed that organic matter digestibility measured on wethers in digestibility crates ranged from 65.3 to 74.4% in a set of 232 registered early maize hybrids (mean value was 69.9%), and reached 76.8% in bm3 hybrids.…”

Dilute acid pretreatment of sorghum biomass to maximize the hemicellulose hydrolysis with minimized levels of fermentative inhibitors for bioethanol production

“…Therefore, development of brown midrib (bmr) sorghum varieties has become a significant achievement for the bioenergy applications (Chen and Dixon 2007;Vermerris et al 2007;Dien et al 2009) and forage digestibility (Barriere et al 2003;Guo et al 2001;Jung and Allen 1995;Vogel and Jung 2001).…”

Genetic and physical fine mapping of the novel brown midrib gene bm6 in maize (Zea mays L.) to a 180 kb region on chromosome 2

“…Moreover, the labor and time required for cultivation and harvesting are lower than for many forage crops, which greatly reduces the cost per unit of dry matter (Lauer 1995). Cell wall digestibility is the key factor determining forage quality (Andrieu et al 1993;Barrière et al 2003Barrière et al , 2004b. In Europe, maize is widely grown as forage crop with *4.6 Mha surface area coverage (Barrière et al 2004a).…”