Effects of defoliation and row spacing on intermediate wheatgrass I: Grain production

Increasing intermediate wheatgrass [Thinopyrum intermedium (Host) Barkworth & D.R. Dewey] grain yield and maintaining yield over the life of a stand will be critical to the economic viability of Kernza (The Land Institute) grain production. Research on perennial grasses has shown that seed yield can be enhanced by (a) mechanically defoliating the stand for hay production and (b) increasing row spacing. We evaluated the interacting effects of row spacing and defoliation across the 4-yr life of an intermediate wheatgrass (IWG) stand in St. Paul, MN. We measured grain yield, harvest index, lodging, and yield components including grain mass and number of tillers, spikes, and grains. Data was analyzed with linear mixed models and partial least squares path analysis. Overall, grain yield declined substantially over time, from a mean of 880 kg ha⁻¹ in 2015 to 276 kg ha⁻¹ in 2018. Wider row spacings tended to increase grain yield. Defoliation increased grain yield in the first 2 yr, but may have decreased stand vigor in later years. Neither management practice fundamentally mitigated yield decline. The main cause of yield decline was the reduction in grain number per high-yielding spike, which dropped by roughly half after the first year. The proportion of spikes that were high yielding also declined over time. Increasing competition among reproductive units likely contributed to yield decline, but there is also evidence that resource allocation to reproduction declined over time. Future research in IWG breeding and management should focus on maintaining high grain number, reducing intra-stand competition, and increasing resource allocation to reproduction.