Richard Taylor, Extension Agronomy Specialist; rtaylor@udel.edu and Amy Shober, Extension Nutrient Management and Environmental Quality Specialist; ashober@udel.edu
The recent cold, cloudy, wet weather has impacted many wheat fields turning them off-color and creating some confusion as to whether the crop needs an additional nitrogen (N) application. Let’s review the uptake and impact of N on wheat after head emergence is complete (Zadoks’ growth stage (GS) 58 or Feekes’ GS 10.5.
Virginia Polytechnic Institute and State University conducted a lot of research on intensively managed wheat and related N applications to the physiological development of the wheat plant. Around Zadoks’ GS 30 or Feekes’ GS 5, wheat begins stem elongation and enters the most rapid growth phase of the crop. Two crucial factors are involved in establishing yield at this time since tillering by the crop is essentially complete. The first factor is the establishment of the potential number of kernels per head as embryonic head formation occurs. The second factor is the very rapid uptake of N. Inadequate N availability at this stage will cause an increase in tiller abortion, resulting in a reduction in the number of heads per acre and a smaller overall head size. This phase of rapid growth and N uptake is a critical stage when N applications should be considered.
From Zadoks’ GS 30 through GS 58 (initiation of flowering), N plays an important role in the development of adequate leaf area and ovules (unfertilized kernels), which are important for achieving maximum yields. Application of N fertilizer at this stage provides the N that is critically needed during this period of plant production. In addition, leaching loss of N from the soil is less of a concern during this phase since wheat has an extensive root system, relatively high rates of evapotranspiration, and a huge demand for N allowing rapid uptake at the beginning of this period. Therefore, there is little chance that applied N moves through the soil and below the root zone.
At Zadoks’ GS 58 or Feekes’ GS 10.5, stem elongation is complete, the flag leaf has fully emerged, and there is no chance of increasing the leaf area of the plant needed for photosynthesis (Ps). Our goal at this point is to conserve as much of the leaf area as possible from insect feeding and destruction from diseases. Nitrogen uptake from the completion of head emergence through maturity is very low compared with the amount taken up by the crop during stem elongation. Virginia Tech researchers evaluated the impact of small N applications at head emergence and later but found that even a foliar application of 10 to 20 lb N/acre at this growth stage did not increase yield; however, N application did increase the grain protein content in some cases. A real concern with foliar application at this stage is foliar burn, which can occur if dilution of the N fertilizer is not adequate. Foliar burn can lower yield potential.
In summary, N applications to wheat after head emergence is not recommended because there will be essentially no impact on yield potential from additional applied N. While small foliar N application at this stage could raise wheat grain protein levels, the risk of foliar burn and lower yield outweighs the benefits.