Managing nitrogen nutrition makes a big contribution to the yield and quality of winter wheat. Choosing the right source, rate, time and place of nitrogen application improves not only your own profit, but also, food and nutrition security for people around the world.
Since winter wheat often needs a topdress application, choosing the right nitrogen source can help minimize the ammonia loss associated with broadcasting. Urea on the soil surface has high potential to lose ammonia. The proportion of ammonia lost increases with rate of application and, depending on soil pH levels, can be influenced by soil moisture. You can treat sources containing urea with a urease inhibitor — or get a form of fertilizer protected with a slow-release coating — to slow hydrolysis, reduce loss, and supply more ammonium and less nitrate for plant uptake. Ammonium sulfate and ammonium nitrate sources are less prone to loss.
Choosing between fluid urea-ammonium nitrate and granular urea is tricky. Fluids can be applied more uniformly. Less ammonia is lost with fluid, but more leaf burn can occur. You can minimize leaf burn with streamer nozzles and early application. Inhibitors or controlled-release forms do not always pay for themselves in terms of yield increase, but you can also weigh the benefits of reduced loss to the environment when considering these tools.
The correct rate of nitrogen application depends on yield potential and previous crop. You can also use sensors that provide some measure of the canopy cover and its greenness. The optimum rate depends on cultivar as well. Hard red cultivars require more nitrogen for optimum protein levels. Cultivars susceptible to disease require lower rates. Production systems with weather-specific use of fungicide and plant growth regulators can produce higher yields with higher rates of applied nitrogen. For example, research conducted in Ontario, Canada, from 2008 to 2010 demonstrated that, when combined with a fungicide strategy that controlled disease, increasing rates of nitrogen in the spring topdress from 90 to 150 pounds per acre raised average yields from 90 to 112 bushels per acre in soft red winter wheat. These higher yields were accompanied by changes in grain quality considered desirable by millers, including increased protein and decreased levels of fungal toxins.
At wheat seeding in the fall, your soils often contain sufficient nitrogen to start the crop. Exceptions do occur, particularly in sandy soils, or soils that have recently grown a nitrogen-depleting crop. Going into winter, the plant should not be deficient, but excess nitrogen can lead to disease infection and winterkill. Thus, your decision to apply a small amount at seeding should be guided by a soil test. Topdress applications need to be timed according to the growth condition of the crop. Thin stands may benefit from early applications in the tillering stage, but only a low rate should be applied to prevent excessive tillering. You should apply most of the nitrogen by the beginning of stem elongation, or Zadoks growth stage 30, since from this stage, the crop takes it up rapidly. After heading, nitrogen uptake slows down, but foliar applications, or controlled-release forms applied earlier, boost protein in the grain.
The most logical place for nitrogen fertilizer at seeding is with the seed. Wheat seedlings can tolerate the low rates required at planting. For topdress fertilizer, broadcast is often your only choice. Ensuring uniformity of broadcast is important for avoidance of uneven maturity. Uneven applications lose yield to both deficiency and lodging.
Your attention to profitable nitrogen nutrition for your wheat contributes a lot to an essential component of sustainability: the improvement of food and nutrition security.
About the Author
Dr. Tom Bruulsema has been with IPNI since 1994, when it was known as the Potash & Phosphate Institute. Dr. Bruulsema’s research and education programs focus on nutrient stewardship, nutrition security and adapting crop nutrient management to weather.
Dr. Bruulsema resides in Guelph, Ontario, Canada. He earned his BSc and MSc degrees in Crop Science at the University of Guelph in 1983 and 1985, and his PhD in Soil Science from Cornell University in 1994.
Dr. Bruulsema is involved in many professional societies. He is a Fellow of the American Society of Agronomy and of the Soil Science Society of America, and a Fellow of the Canadian Society of Agronomy. He is a Certified Crop Adviser and has served a number of leadership roles in the International Certified Crop Adviser Program.