Although it appeared like we took a step backward last week, warmer days are ahead and our winter wheat fields will be greening up and starting growth. Reports from around the state indicate that wheat planted shortly after the fly-free date looks better than the wheat that was planted late. Laura Lindsey, OSU Extension Soybean and Wheat specialist recommends evaluating stands for yield potential prior to spring nitrogen application. Fields should not be evaluated until completely green from warmer temperatures for at least 10 to 14 days. Stand evaluations will be more accurate when made during weather periods that promote growth. Yield potential is reduced if tiller numbers fall below 25 per square foot after green up. Pick about 10 to 15 spots in the field and count the number of plants per foot of row. A stand with an average of about 12 plants per foot of row may still result in a good population of head-bearing tillers per acre. For those fields with tillers, 15 tillers per square foot is considered minimum for an economic crop. The number of tillers per square foot is equal to the number of tillers in 19.2 inches of 7.5-inch wide rows. OSU studies have shown that under adequate weather conditions, tillering may compensate for relatively poor initial stand establishment.
OSU recommends applying nitrogen between green-up and Feekes Growth Stage 6 (early stem elongation), which is generally the latter part of April. The potential for nitrogen loss will decrease by waiting to apply nitrogen closer to Feekes 6 because plant nitrogen need is greater and nitrogen uptake is more rapid. Nitrogen recommendations come from the Tri-State Fertility Recommendations bulletin and are based upon the yield potential of the crop, thus the importance of a field evaluation to help set a realistic yield potential before applying nitrogen. If the wheat stand looks good one method to select a realistic yield potential is to look at your wheat yield from the past five years. Throw out the highest and lowest wheat yield, and average the remaining three wheat yields. This three-year average should reflect the realistic yield potential.
The Tri State guide recommends 110 lbs. nitrogen/acre for yield goals of 90+; 70 lb. for 75 bu; and 40 lbs. of nitrogen/acre for 50 bushel yield goal (these recommendations are for total N and include any fall N). If you prefer to be more specific the following equation may be used for mineral soils, which have both 1 to 5% organic matter and adequate drainage:
N rate = 40 + [1.75 x (yield potential – 50)].
The Tri-State bulletin recommends that you subtract from the total (spring N) any fall applied N up to 20 lb. /A. Nitrogen rate studies at the Northwest Agricultural Research Station have shown the optimum rate varies depending on the year. However, averaged over years, yield data from these studies correspond well with the recommendation equation given above. These studies have also shown that regardless of the year, yields did not increase above a spring rate of 120 lbs. nitrogen per acre.