Nitrogen is the primary fertilizer nutrient required by wheat and barley. A wheat crop contains 33 to 37 pounds of nitrogen per 1000 pounds of grain in a range of grain protein from 12 to 14%. A barley crop will contain about 20% less nitrogen than wheat at similar yield levels due to lower grain nitrogen concentration. The actual total amount of nitrogen contained in the crop will vary greatly depending on yield level and protein content of the grain. If high yields are expected or high protein content is desired, the grower needs to increase nitrogen fertilizer rate accordingly. The expected crop nitrogen content and optimum nitrogen fertilizer rate are not identical due to nitrogen that may be available in the soil and water or losses of nitrogen fertilizer that may occur throughout the season.
Small grains obtain nitrogen not only from fertilizer, but also from the soil and irrigation water. In a study conducted at the Maricopa Agricultural Center with durum, about half of the nitrogen taken up by the crop was from fertilizer and the remainder was from soil and water.
The actual amount of fertilizer nitrogen required for optimum yields varies from 150 to 300 pounds per acre for wheat and 120 to 240 pounds per acre for barley. Additional nitrogen fertilizer is usually needed for wheat after heading to increase grain protein content, but nitrogen fertilizer applied after heading usually does not affect grain yield. Nitrogen fertilizer requirement can be affected not only by yield potential and grain protein content but also by crop residues, soil type, irrigation practices, the efficiency of fertilizer application, and the nitrogen contained in the soil and water. Insufficient nitrogen application reduces grain yield. Excessive nitrogen application increases fertilizer cost, reduces yield, and increases lodging.
Most crop nitrogen uptake occurs between the 5-leaf stage until heading, when crop nitrogen uptake is most rapid. Therefore, nitrogen fertilizer applications should precede or coincide with crop N needs. Nitrogen applied up to the heading stage has the potential to influence yield, while applications from heading to 2 weeks after flowering will have a minimal effect on yield but can increase protein content of the grain. Nitrogen fertilizer applications can be scheduled ahead of time if approximate irrigation dates are known. Excessive nitrogen applications before planting should be avoided since nitrogen fertilizer efficiency is about 20% less when applied pre-plant compared to during the growing season. However, enough nitrogen fertilizer should be applied pre-plant to supply the crop with nitrogen until the next planned nitrogen application date.
Durum grain protein content needs to be 13% or greater to meet end-use requirements. Grain protein content of wheat can be increased by nitrogen fertilizer applications between heading and 2 weeks after flowering. Nitrogen fertilizer applications after heading will have minimal effect on grain yield. Thirty pounds of nitrogen per acre applied near flowering will increase grain protein by about 1 percentage point. Nitrogen fertilizer application later than 2 weeks after flowering will not increase protein content as effectively as earlier applications. High yielding varieties that tend to have protein levels below 13% are most likely to benefit from late season nitrogen application.