Effects of expanding functional trait diversity on productivity and stability in cultivar mixtures of perennial ryegrass

Abstract

Cultivar mixtures can provide a host of beneficial agroecosystem services in annual grain crops; however, it remains unclear whether these same benefits apply to perennial forage cropping systems, or the degree to which beneficial effects depend on the functional trait diversity of the mixtures. We conducted a field experiment across four locations in the Northeast US in which we grew perennial ryegrass cultivar mixtures varying in cultivar number and range of expression of three functional traits (winter hardiness, heading date, and extended growth) and assessed the effects on dry matter yield and inter-annual yield variability. Trait ratings supplied by the seed company were related to observed variation in perennial ryegrass productivity and/or stability at both the individual cultivar and mixture levels. Winter hardiness was associated with greater perennial ryegrass cumulative biomass, and lower interannual stability. In contrast, extended growth was associated with lower cumulative biomass, and both extended growth and later heading date were associated with greater interannual variability. Overall, cultivar richness was negatively associated with perennial ryegrass biomass and stability; however, the best performing mixtures performed as well as the recommended cultivar for the region. When comparing mixtures with equal cultivar richness, functional trait diversity measured as the additive trait range—the summed range of the three traits associated with the cultivars in that mixture—was positively associated with biomass production and over-yielding, but not interannual variability. Cultivar mixtures of perennial ryegrass can lead to improved forage production when specific functional traits are optimized within mixtures. Our results support the growing understanding that efforts to ecologically intensify agriculture through enhancement of crop diversity are more likely to succeed when they explicitly consider the functional traits of the crops involved rather than solely numbers of cultivars or species.