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CAB Reviews

A reviews journal covering agriculture, global health, nutrition, natural resources and veterinary science

CAB Review

High-sugar grasses.

Abstract

Perennial ryegrass diploids with elevated concentrations of water-soluble carbohydrate (WSC), commonly termed 'high-sugar grasses' (HSGs), have been promoted as a tool for increasing the efficiency of the use of protein (nitrogen (N)) in the rumen and thus offering scope for increasing milk production and animal growth rates, while reducing N losses (in the form of urine) to the environment. Much controversy has arisen about efficacy, largely because benefits have not been seen in all trials, and partly because of variation in the degree to which the sugar trait has been present, or expressed, in several cases reflecting gene × environment or gene × management interactions. Combining the data from multiple trials shows that there is a continuum of response in N-use efficiency (NUE) to WSC:crude protein (CP) ratio, and so 'proof of concept' has been shown. A considerable amount of research is still required to demonstrate a consistently high expression of the trait, and to demonstrate reliably the potential for yield gains and reduced N loss. Associated reductions in nitrous oxide emissions (a potent greenhouse gas (GHG)) are yet to be confirmed. Even more uncertain is the prospect for reducing methane emissions, whether per hectare or per unit energy intake or animal product. Nonetheless, there has been a determined effort to pursue this trait, and to confirm the long-standing hypothesis for NUE. There is no basis as yet for dismissing the prospects of success. The trait remains one of very few low-cost tools being investigated for its potential to mitigate the environmental footprint of livestock production.