Assessing and Adapting in-Season Diagnostic Tests to Guide Winter Grain Nitrogen Topdressing for the Northeastern U.S.

Growers in the Northeastern U.S., both organic and conventional, cite nitrogen (N) management as a key challenge for winter grain production. Failure to provide the correct rate of N at the proper times can reduce grain yield and protein levels. Nitrogen applied at fall planting is subject to over winter losses, and spring N mineralization rates are difficult to predict and often lag behind crop demand. In-season diagnostic tests are used successfully to guide topdress decisions for conventionally grown winter wheat in other humid regions of the U.S. (Alley et al., 2009). Growers minimize fall N application to avoid over-winter losses, and then use tiller density to decide whether and how much N to topdress at spring green-up (Zadok GS25) and tissue N concentration at jointing (GS30) to decide how much N to topdress at this stage. The current study evaluates if these decision tools could be used in the Northeast for conventional and organic production of winter wheat, and whether critical levels need to be adjusted. Extensive N fertilization trials were conducted at three research stations in Maine, New York, and Vermont in 2014 and 2015 where GS25 tiller densities were varied using fall planting rates or preplant manure application, and GS30 tissue N concentrations were varied using GS25 topdress rates. Quadratic, exponential, and quadratic plateau models were evaluated to describe the yield response to five rates of N fertilizer. Optimal N rates were regressed against GS25 tiller density or GS30 tissue N concentration to determine critical levels. A separate series of participatory on-farm trials were conducted at six sites in Maine, Massachusetts, and Vermont in 2014 and 2015 with winter wheat, rye, and triticale to assess current critical level recommendations. Results from both on-station and on-farm trials will be presented.