Nitrogen Fertilizer Rates for Optimized Grain Production of Intermediate Wheatgrass.

Perennial crops could reduce soil loss and nitrogen pollution to waterways compared to annual crops, however there are few perennial crop options that are functionally equivalent to the common annual grains that occupy a majority of agricultural acres in the US. Here we report grain and biomass yields from an improved breeding population of intermediate wheatgrass; a perennial grass that is being domesticated to serve as the first perennial grain crop in the US. Our objective was to measure grain and biomass yields of this improved grain-type intermediate wheatgrass (TLI-C2), an unimproved forage variety of intermediate wheatgrass (Rush), and switchgrass in response to variable nitrogen (N) fertilization rates. We measured seed and biomass yields when managed with five N rates ranging from 0 to 200 kg N ha^-1 to determine agronomically optimum N rates (AONR) for maximizing yields. AONR for TLI-C2 was lowest during year two, where 61 kg N ha^-1 produced in an estimated grain yield of 915 kg ha^-1. When fertilized with AONRs, forage biomass yields ranged from 9.2 to 12.3 Mg ha^-1, which demonstrates the potential to manage TLI-C2 as a dual-use cropping system for both grain and forage. TLI-C2 grain yields declined at high N rates likely related to increased lodging. Plant height did not consistently explain variation in lodging as a response to increased N rates; therefore other physiological changes are influencing lodging. Grain yields showed a consistent decline with stand age at all N rates and locations. Research is needed to understand the mechanisms driving lodging and declining grain yields with stand age. Until these agronomic challenges are met, forage biomass from TLI-C2 can be considered an economic safety net during the development of this perennial grain crop.