Nitrogen is an essential nutrient, and often the most yield-limiting, for winter wheat production. It serves to promote tillering, enable photosynthesis, and build protein in the grain. To achieve each of these goals, nitrogen availability needs to be regulated carefully over the growth stages of wheat by managing the timing, rate, form, and placement of nitrogen fertilizer applications. Managers should try to protect against excessive N availability because it can often lead to lodging at harvest or promote diseases, resulting in yield losses.
Determining Nitrogen Rates
The primary role of nitrogen applied at wheat planting is to provide sufficient nutrients for the wheat crop to establish biomass and develop tillers prior to the onset of winter dormancy. It is only necessary to apply around 20 lbs/ac of nitrogen fertilizer at planting to meet these needs, and often no nitrogen is needed at all in fields with a manure history or with a previous legume crop like soybeans. Optimum wheat planting dates should allow for adequate tillering in the fall, but if seeding was delayed or weather conditions were not suitable for good tillering, then adjustments to N fertilizer applications should be made at green-up to promote more tillering.
After the winter dormancy, when plants green-up, a tiller count at Feekes stage 3 can help determine how to approach nitrogen management in the spring. If the tiller count at Feekes 3 is below 70 tillers/ft2, then there is likely a benefit to split applying N in the spring. When tiller counts are below this threshold, an application of 30 to 50 lbs N/ac at Feekes 3 is generally advisable. Use a rate toward the higher end of the range when tiller counts are lower, and a rate towards the lower end of the range as tiller counts approach 70/ft2. This early application of N will help to promote additional tiller development and allow optimal yields to be achieved.
If the tiller count at Feekes stage 3 is greater than 70 tillers/ft2, then there is less likely to be a benefit of split applying spring N, and a single application can be made at the Feekes 4-5 stage (leaf sheaths lengthening, but before jointing). The total N applied in the spring, either across the two split- applications, or in a single application at Feekes 4-5, should be based on the yield goal, with a recommendation of 1 lb N applied per bushel of expected yield. So, if the yield goal was 100 bu/ac, and 30 lbs N/ac was applied at Feekes 3 due to a low tiller count, then an additional 70 lbs N/ac should be applied at Feekes 4-5. Nitrogen applied after Feekes 6 (jointing) provides little benefit to yield. An application of N after Feekes 9 (boot stage) can increase grain protein; however, this is rarely desirable in the soft red winter wheat production typical of Pennsylvania.
Nitrogen Fertilizer Sources
Granular urea and urea ammonium nitrate (UAN) solution are the two most common nitrogen fertilizer sources used for wheat production. Given the importance of sulfur nutrition, and the declining levels of atmospheric sulfur deposition, ammonium sulfate (21-0-0-24S) is also a worthwhile form of fertilizer to consider, and could serve particularly well for the fall applied N when used at a rate of 100 lbs/ac. Because of the sensitivity of wheat to the nitrogen fertilizer rate, particular effort should be made to apply the N fertilizer uniformly. With broadcast spinner spreaders used for granular N, make sure the swath widths and overlaps of the fan pattern have been carefully calibrated to ensure uniform spreading, or consider using a boom drop spreader. For liquid applications of UAN solution, use a stream bar that will deliver closely spaced streams of fertilizer in uniform bands, promoting even availability of N across rows of wheat while minimizing leaf burn.
Controlled release fertilizer (e.g., polymer coated urea) is sometimes used for the spring N application in wheat. It can be applied in mid-January to mid-February when the soil is frozen, preventing compaction. Consult with the manufacturer for specific guidelines on application timing to be sure it will be released by the time it is needed by the wheat. Also consider the risk of fertilizer granules being washed off the soil surface on sloping, frozen soil, and avoid this fertilizer usage in high-risk scenarios.
Manure as a Source of Nitrogen for Wheat
A significant amount of farmland in Pennsylvania receives regular manure applications. While manure is a valuable source of many macro- and micro-nutrients, given the sensitivity of wheat to the rate and timing of N applications, manure poses some real challenges as a source of nitrogen for wheat. The timing and quantity of N available from manure depends on the relative amount of ammonium N and organic N in the manure and the method in which the manure was applied. The gradual mineralization of organic N from manure, which takes place over a period of months and is regulated by soil temperature, is not a good match for a crop that needs specific quantities of N available at precise growth stages.
Nevertheless, when manure does get applied to wheat, consult the manure nitrogen availability factors in the Penn State Agronomy Guide, and use relevant planning protocols in your Manure Management Plan or Nutrient Management Plan to determine appropriate manure application rates and supplemental fertilizer requirements. Any supplemental fertilizer that is required should be applied according to the guidelines discussed above. Nitrogen available from historical manure applications (up to 5 years in the past) should also be credited in the determination of the N requirement from fertilizer or manure, with a 7 lbs N/ac credit when manure was applied in 2 or 3 out of the last 5 years and 11 lbs N/ac credited when manure was applied 4 or 5 years out of the last 5 years.
Adaptive Nitrogen Management in Wheat
There are several technologies and approaches that may be able to help inform nitrogen fertilizer management decisions based on site specific conditions realized during the growing season. In addition to the practice of taking tiller counts at green-up discussed earlier, optical sensors such as the Greenseeker or OptRx can be used to measure the greenness of the wheat canopy prior to spring fertilization, which provides a way to calculate an N recommendation that takes into account the N available to the crop from pre-existing soil stores. In some states, like Maryland, soil sampling to measure nitrate concentrations at planting in the fall is used as a diagnostic to determine whether N needs to be applied in the fall at planting. The methods and interpretation criteria for these adaptive N management tools were developed in other states, so there is some risk in applying a tool developed elsewhere to our environment, but we encourage farmers to explore using these tools in a limited manner to become familiar with them and gauge their potential.
One diagnostic tool that has been tested in Pennsylvania is tissue testing wheat between Feekes growth stage 3 and 5. If the wheat tissue N concentration is above a specific critical level, which depends on the growth stage (Table 1), there is a very low probability of a yield response to additional N fertilizer. Using this diagnostic tool would be most helpful in soils with a heavy manure history or a recent manure application, such as at planting in the fall, to determine sites that have enough N available already and are unlikely to respond to additional N fertilizer, or may even suffer a yield decline from lodging or disease caused by excessive N.
Table 1. Critical tissue N concentrations for whole wheat plants sampled at different growth stages after green-up. Whole plants should be sampled from two feet of row in two representative spots within a field. If tissue N concentration in the plants is above the critical level for the growth stage sampled, no additional N fertilizer is needed. Data from Roth, Fox, and Marshall in Agronomy Journal (1989).
|Growth Stage Sampled||Critical N Concentration of Whole Plant Tissue(%)|
|Feekes 3 (tillers formed)||4.4|
|Feekes 4 (leaf sheaths lengthen)||3.9|
|Feekes 5 (leaf sheaths strongly erected)||3.5|
Managing nitrogen in wheat is one of the most challenging agronomic scenarios because of the sensitivity of the crop to both under- and over- application of N and the importance of appropriate timing for tiller development and yield. The best results will be achieved when spring N applications can be tailored to specific conditions in the field, such as by implementing split applications when tiller counts are low, or by using tissue testing in heavily manured fields. Given the sensitivity of the crop to N rate, it is also important to use the right fertilizer application equipment and make sure the fertilizer spread patterns and application rates are well calibrated. Paying attention to all these details are necessary to achieve optimal yields and make wheat a profitable component of the cropping system.