Managing nitrogen fertility and stand density for sustaining Kernza intermediate wheatgrass yields

Kernza intermediate wheatgrass (IWG) [Thinopyrum intermedium (Host) Barkworth & D.R. Dewey] is a promising perennial grain and forage crop, but experiences grain yield decline, potentially due to limited nitrogen (N) and stand overcrowding. We evaluated the effects of N fertilization and stand thinning on grain and forage yield, weed biomass, thousand-kernel weight (TKW), and harvest index (HI). We used a full factorial design with N rates of 0, 75, and 150 kg N ha⁻¹ and thinning intensities of 0%, 25%, 38%, or 50% stand density reduction via banded herbicide at two locations in Wisconsin over 2 years. Fertilization and thinning did not interact. Grain yields increased with N fertilization except at Madison in Year 2. At Lancaster, grain yield increased from 293 with no N to 497 and 701 kg ha⁻¹ with 75 and 150 kg N ha⁻¹, respectively, across years. At Madison, grain yield increased only in Year 1. Forage mass also increased with N at both sites except Madison in Year 2. At Lancaster, forage mass ranged from 4016 to 6500 kg ha⁻¹ across years and N rates. TKW and HI increased with N at both sites, except at Madison in Year 2. Weed biomass was unaffected by treatments. Thinning had no effect on grain yield at Lancaster in Year 1, but in Year 2, grain yield increased from 368 to 505 kg ha⁻¹ with 50% thinning. These results suggest that applying 75 kg N ha⁻¹ is important for maintaining IWG productivity and that thinning can help sustain grain yield in older stands.