More than a century since Charles Darwin stated that diverse grasslands produce more herbage than monocultures, scientists still debate the relationship between species diversity and ecosystem function. Postulated benefits of diversity in experimental grasslands include greater and more stable primary production along with more efficient nutrient use. These benefits have been extrapolated to forage and grazing land systems with little supporting objective data. Most information on the potential benefits of increased plant diversity comes from studies of synthesized grasslands that have not included domestic grazing animals. We explore this debate relative to the management of temperate forage and grazing lands. Plant species diversity refers to the number of species (richness) and their relative abundance (evenness) within a defined area. Plant relations influence biodiversity responses through positive (e.g., facilitation, N2 fixation, hydraulic lift) and negative interactions (e.g., competitive exclusion, allelopathy). Early 20th century research on complex mixtures of forage species (limited to grasses and legumes) for pasture indicated equivocal results regarding benefits of species‐rich mixtures and typically recommended using the best adapted species in simple grass–legume mixtures. Recent research indicates potential herbage yield benefits from species‐rich mixtures for pastures. Limited animal productivity research on species‐rich mixtures indicates variable responses and much more research is needed. Grazing land productivity is a primary focus for biodiversity benefits because of the direct economic relevance to producers. However, taking a broader view of the multifunctionality of grazing lands to include environmental and aesthetic benefits to humans reveals a great scope for using biodiversity in grazing land management.

There has been mounting interest over the proposed production and environmental benefits from using perennial ryegrass cultivars bred to have higher water soluble carbohydrate content (high sugar grasses). Here, we objectively review published evidence, from the EU and New Zealand, of the effects of these on milk yield per cow, liveweight gain in sheep, N utilisation and wider trophic interactions. The literature reveals substantial variation in animal responses, though some of the uncertainty in interpretation can be resolved by combining the data from multiple trials, and showing this forms a continuum of response to diet quality. It also reveals variation in the degree to which the sugar trait has been expressed, possibly reflecting a gene x environment interaction. Achieving a more consistent, and probably greater than current, expression of the high sugar trait would be a valuable goal. We suggest 'proof of concept' has been shown, notably for the potential for improving N utilisation in the rumen, and so reducing the proportion of N intake lost in urine. The evidence suggests that this may be a greater challenge, albeit a more valuable goal, because of the relatively high N (crude protein) content forages that predominate in the New Zealand pasture industry. Keywords: animal performance, high sugar grass, Lolium perenne, perennial ryegrass, nitrogen utilisation, trait expression, water soluble carbohydrates

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