Effects of intercropping perennial legumes on intermediate wheatgrass productivity

Abstract

Context

Intermediate wheatgrass (Thinopyrum intermedium; IWG) is a perennial, cool-season grass undergoing domestication as a grain and forage crop. Intercropping perennial legumes with IWG could reduce synthetic N fertilizer inputs, improve forage quality, and provide pollinator resources. However, legumes may also compete with IWG for resources, the extent to which may depend on traits associated with legume species identity and IWG planting density.

Objective

Our objectives were to determine the effects of IWG planting density and legume species identity on IWG grain and vegetative biomass yield in an intercropping system.

Methods

A field trial with IWG planted at two densities, each as monocultures and bicultures with four intercropped legume species, was conducted two sites in Minnesota, USA for two years. Grain and vegetative biomass were measured from the IWG, and vegetative biomass was measured from the legumes. The legume species tested were alfalfa (Medicago sativa), Canada milkvetch (Astralagus canadensis), Illinois bundleflower (Desmanthus illinoensis), and white clover (Trifolium repens).

Results

High planting density increased IWG grain and biomass at most site-year combinations. Legume biomass in the intercrop was highly variable among species, sites, and years, yet rarely did it reduce IWG grain and biomass yields below that of the unfertilized IWG monoculture. There was no consistent negative effect of intercropping legumes on total biomass productivity, and when we used the relative yield metric to quantify the land-use efficiency of growing IWG in an intercrop with legumes, we found that for most site-years there was a neutral or positive effect of intercropping legumes. Alfalfa was the most productive legume, with a maximum yield of 5993 kg ha⁻¹ at one site-year, and across all treatments higher legume biomass was associated with lower grain yields.

Significance

This study demonstrates that legumes can be intercropped with IWG to generate both grain and forage while possibly limiting synthetic nitrogen inputs. Although vigorously growing legumes may impose competitive effects causing a decline in IWG grain production, competition may not reduce IWG vegetative biomass or overall biomass productivity.