Cell wall digestibility of perennial ryegrass: a strategy to increase forage quality and decrease environmental impact (RAAICELWAND)

Is there a way to improve perennial ryegrass so that cows can convert grass protein more efficiently into milk and meat protein? Perennial ryegrass is by far the most important grass species in Europe because of its high protein and energy content. However, up to 40% of the protein is left unused by the ruminant due to a lack of available energy. The energy content of perennial ryegrass is determined by the content of water soluble carbohydrates ("fast sugars") and structural carbohydrates in the cell wall ("slow sugars"). Further increasing the content of fast sugars is not useful, as this leads to rumen acidosis due to too much energy being released at once. If we succeed in increasing the release of slow sugars from the cell wall, energy will become available more gradually, therefore reducing the risk of acidosis. The current strategy is to reduce the amount of lignin, a glue in the cell wall, thereby increasing the cell wall digestibility and improving the accessibility of slow sugars for enzymatic degradation in the rumen.

We conduct an in-depth descriptive study on the relationship between the developmental stage and the cell wall digestibility of perennial ryegrass. We analyze 15 contrasting genotypes in detail by determining the cell wall composition and fodder quality for different developmental stages (vegetative, heading and flowering), seasons (spring and autumn) and organs (leaf, sheath and stem). Based on this detailed study, a protocol to phenotype a diverse collection of 600 genotypes for cell wall digestibility is created. Further, candidate genes and polymorphisms (genetic markers) are identified in a transcriptome database of 14 genotypes of perennial ryegrass. Selected markers are then characterized in the same set of 600 genotypes. Finally, the phenotypic and genotypic data are used for association mapping, in which genetic markers in several candidate genes are tested for association with increased cell wall digestibility.

The first descriptive study yields better understanding of the complex trait cell wall digestibility in perennial ryegrass. This knowledge on the phenotypic level can be used to improve grassland management and breeding. Phenotypic selection is less efficient compared to marker-assisted selection, which has become feasible thanks to recent developments in biotechnology. The genetic markers that we find in this study can be used in breeding, which offers the advantage that plants can be screened at an early stage of development and selection of ideal combinations of markers is possible. Through the combination of marker-assisted and phenotypic selection, new varieties can be obtained with a well-balanced energy and protein content.