With harvest rapidly approaching, it is time to make the final management decisions that can maximize wheat revenue. In addition to controlling insects and applying fungicides, growers may want to consider making a late season nitrogen application.
Despite an abundance of wheat worldwide, there is a shortage of high protein wheat which has generated protein premiums at delivery points across Texas the last several years. On average, protein is 1/6th nitrogen and, therefore, soils lacking in adequate nitrogen will result in lower yield and lower grain protein.
Unlike nitrogen applied prior to jointing, which helps to maximize yield, late season applied nitrogen can help to boost protein levels. This is true because wheat is able to uptake nitrogen late in the season, but by heading, the crop has already set its yield potential.
Research has shown that nitrogen applied late in the growing season can raise protein levels up to 0.8 percentage units (Table 1) or more in some situations.
Several factors must be considered when deciding whether or not to apply late season nitrogen, the first of which is the yield potential of the crop. Currently, Texas A&M AgriLife Extension nitrogen fertilization recommendations are to apply 1.5 lb N per bu of expected grain yield (minus any residual nitrate nitrogen that can be credited in the soil). Roughly 1/3 of this should be applied in the fall around planting and the remaining 2/3 at topdress.
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High yielding wheat that exceeds early season expectations often has lower grain protein content. This is usually due to insufficient nitrogen applications earlier in the season to meet the need of the additional grain production.
These situations would benefit most from another application of nitrogen. Studies done in South Dakota on winter and spring wheat increased grain protein 70% of the time when yield goals were exceeded, but only 23% when yield was lower than expected.
Figure 1 shows an example of how grain protein can continue to increase even after yield is maximized following different nitrogen rates applied at the typical spring topdress time in a soft red winter wheat in northeast Texas. Similar reports are documented in hard red wheats.
If yield potential is similar to or lower than expected, late season nitrogen is less likely to provide a bump in protein as adequate nitrogen is available to meet plant needs.
While wheat varieties can differ in grain protein, these differences are often driven by yield in any particular environment. Figure 2 demonstrates the negative relationship between grain yield and grain protein in a hard red winter wheat variety trial.
Lower yielding varieties are nearly always the highest in grain protein and vice versa. Producers are cautioned when making comparisons among varieties for protein levels, and should only make comparisons when varieties are similar yielding.
Looking at protein data from multiple years or locations is also suggested to determine protein consistency across environments.
Other factors to consider are fertilizer costs and expected grain prices. If premiums for protein levels above a certain level (or discounts for those below a certain level) are expected, then a positive return on fertilizer cost is more likely. The projected likelihood of receiving a positive return on fertilizer investment is displayed in Figure 3.
The optimal time for late season nitrogen applications is anywhere from pre-flowering up to two days after flowering. Pre-flowering is favored under dryland scenarios whereas protein is typically maximized at post flowering when rainfall or irrigation can incorporate nitrogen quickly into the soil.
Applying nitrogen about 10 days after heading would be a good rule of thumb to hit this crop stage.
Research has shown that at least 30 lbs/N per acre (10 gallons of 28-0-0) is needed to achieve significant increases in grain protein. Foliar applications using flat fan tips may be the best method for application as leaf pores within leaf cuticles are able to absorb urea, ammonium, and nitrate relatively quickly.
The use of dribble tips or other forms of nitrogen, such as dry urea, can help mitigate leaf burn concerns but will need rain or irrigation to move it into the root zone where it is accessible to plants.
The flag leaf provides the vast majority of nutrients to the developing seed head so it is critical that leaf burn is minimized. Therefore, it is recommended that foliar applied nitrogen is mixed in a 50/50 ratio with water. When able, aim for early evening applications to avoid the hottest parts of the day when leaf burn is most likely to occur.
Fungicide and nitrogen applications should be made independently of each other as tank mixtures can also result in leaf damage.