March nitrogen application critical to wheat yields in Southeast

The most critical time for nitrogen application on wheat in the upper Southeast is January and Febuary and again at Growth Stage 30, which is usually in early March.

Making the right decision in January is essential to making the right decision in March, and both are critical to maximizing wheat yields.

Wheat acreage is up throughout the Southeast, primarily because of price. For growers not familiar with the crop, or who haven't grown small grains in several years, the recommendations have changed significantly over the past few years.

Combine uncertainty over wheat production, especially the timing of nitrogen, with drought-induced late planting of wheat from Florida to Virginia, and the recipe for success is not great. A part of the remedy is knowing when to fertilize wheat to maximize yield potential and minimize weather related damage.

North Carolina State University Small Grains Specialist Randy Weisz says, late January to early February is the time to determine if the crop has enough tillers to optimize yield. If it does, it is best not to apply any nitrogen at that time. Applying nitrogen in January or February when it is not needed results in increased risk of freeze damage, disease, lodging, and can reduce yield.

On the other hand if tillering is low, an early application of nitrogen, followed by a second split at GS-30, usually increases yield. The second critical time, GS-30, is the most important period to apply nitrogen to small grains.

Telling when wheat reaches GS-30 is not always easy. Some varieties reach it in February, others in late March. One clue is that the plants start to “stand up” and get taller. The best way is to pull up several plants and split the stems in half down to the base where the roots grow. Look for the “growing point”. It will be dark green, about one eighth to one quarter inch long, and shaped like a tiny pine cone.

Prior to GS-30 it will be at the bottom of the stem. At GS-30 it will have moved one half inch up the stem. After GS-30 it will be above the soil surface and the first “joint” or “node” will have appeared on the stem. At that point, the ideal time to apply nitrogen fertilizer is over, according to Weisz.

Wheat planted late throughout much of the Southeast is not likely to develop fall tillers. Instead, young wheat plants tend to be just spiking when cold weather starts, and these plants will stay just like that until the weather warms up. The late winter, early spring management for these fields is much different than wheat planted earlier and has a high percentage of fall tillers that are well developed.

If wheat has at least 50 tillers per square foot by late January to early February, there has been fall tillering and the crop is probably in good shape. In these fields, optimum timing for nitrogen applications is early to mid-March. If wheat doesn't have at least 50 tillers per square foot by February, that crop needs nitrogen now.

The first problem is that most growers don't want to get down on their knees in January to count tillers. A good rule of thumb that is easy to calculate is to break tillering into three categories and visually check a field in late January to early February.

  • If you look at a row of wheat and the plant is just beginning to spike, and only has a couple of leaves on each plant, that is about 20-25 tillers per square foot.

  • If the plants are a little thicker, and there is enough leaves that you can see the rows. There are open spaces between rows and thin spots where the rows aren't doing well, but you can see all the individual rows, that is about 50 tillers per square foot.

  • If the rows are closed in and the plants are thick and you can't see a much ground that is about 100-150 tillers per square foot.

Cold temperatures slow small grain growth and tillering, and consequently limit nitrogen uptake. Since little nitrogen can be taken up by the crop during the winter months, nitrogen applied may leach away and is generally of little value to the crop.

Nitrogen management during the winter months consists of making sure the crop does not become nitrogen deficient. Small grains under nitrogen stress in the winter can lose tillers, which may reduce yield.

Indications of a possible nitrogen deficiency are a pale green color, thin, poorly developing stands, and a history of heavy leaching rains after planting.

A small application of 15 to 30 pounds of nitrogen per acre may be made to help green the crop back up. This application can stimulate growth, but the potential for losses to the environment is great.

Weisz says several satisfactory sources of nitrogen are available for top-dressing small grain. Nitrogen solutions (approximately one-half ammonium nitrate and one-half urea dissolved in water) are popular and widely used. These solutions contain from 30 to 32 percent actual nitrogen. Although the solution may cause slight foliar burn, there is little likelihood of reduced yields from this effect if the fertilizer is applied before jointing.

A third source of nitrogen is ammonium sulfate (21 percent nitrogen). Ammonium sulfate is readily available, especially in eastern North Carolina. This source also contains sulfur (24 percent). Sulfur may be important in fields with rather deep sandy surfaces if no other sulfur has been applied within the past 1 or 2 years.

In recent years, urea (45 percent nitrogen) has been readily available. Its popularity stems from its high nitrogen content, usually competitive prices, and low degree of metal corrosion. Under certain conditions, volatility losses can occur when urea is applied to sandy soil.

However, if it is broadcast as early as possible in the spring when the soil surface is moist, wind speed is at a minimum, temperature is relatively low, and humidity is high, no loss should occur. When urea is applied to silty or clayey soil surfaces, volatilization does not seem to be a problem.

The optimism for good yields and high profits remains high across the upper Southeast. To make this optimism come true, growers should be aware of the critical early March time period to insure optimum growth of their wheat crop.

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