Can a late N topdressing help wheat yields this year?
With the wheat being unusually late and small this year, and still pale or yellowish in some cases, producers may be wondering whether they would still get any benefit from topdressing with nitrogen (N) at this point in the season.
Reasons for pale, yellowish wheat at this point in the year
If a topdressing application of N had been made earlier and the wheat is still short and pale, the cause is most likely because: (1) root development is poor due to dry conditions, limiting total nutrient uptake, (2) the applied N hasn’t yet been moved into the soil by precipitation, (3) only a small portion of the N in the soil has been moved to the surface of the plant roots (which have been stunted by the dry conditions) with water via mass flow, the primary N uptake process, (4) the N has been tied up by surface residue or lost due to volatilization, or (5) another nutrient, such as sulfur, may be limiting.
With a rain, most of these issues may be resolved. In many of these dry soils, the breakdown of crop residues and organic matter will kick into high gear once it rains, releasing N and S. Root growth will also kick into high gear and expand the exploited zone of soil. As the water begins to flow towards the roots, mobile nutrients like nitrate and sulfate dissolved in that soil water and will flow to the plant roots for uptake. But if the crop wasn’t topdressed, and soil N supplies are deficient, or if a rain does not look likely, producers may wonder if an application of N or S now would help the yield potential of pale or yellowish N-deficient wheat.
When yield components of wheat are determined
Keep in mind some of the basics of wheat physiology and yield when considering late fertilization. There are three primary components of yield: the number of heads per foot of row; the size of the head or number of kernels per head; and the size of the individual kernels. Number of heads is a function of the initial plant stand, tillering, and tiller survival.
In many Kansas fields initial stands were spotty, but tillering was adequate. As the dry spring progressed, tillers began to abort due to drought stress. Head size is determined around Feekes 5, prior to first hollow stem. The plant responds to environmental conditions and produces a head of the size that it can successfully fill under existing conditions. If conditions are bad, which was the case in many fields, small heads are formed, and the plant sheds tillers through tiller abortion, further reducing total potential kernel numbers. So by jointing and stem elongation, the maximum number of kernels which can be produced is already set.