Despite high grain prices, nitrogen (N) management has become more challenging in small grain production because of continued increases in fertilizer prices. Both grain and forage yield potential are reduced without adequate fertilizer N. N use efficiency is an additional consideration. For grain production, the efficiency is around 33 percent. This compares to around 60 percent efficiency in a wheat forage system. The difference is mainly because plants are harvested or grazed in a forage system before flowering.
The traditional recommendation from Oklahoma State University is 2 pounds per acre of actual N for each bushel of realistic yield goal of wheat grain. Long-term research on wheat at Lahoma, Okla., has shown that one-third of the time this recommendation is right on the money, one-third of the time N was under-applied and the other one-third it was over-applied.
Besides the rate of nitrogen, the time of application will also play an important role in the N use efficiency. If the nitrogen is all applied in the fall, then you have to apply N fertilizer based on soil test and previous crop history. But, if it is split into two applications as preplant or postemergence and topdress, then topdress rates could be adjusted as needed depending on the crop growing conditions. For example, for a 50-bushel yield goal, the nitrogen recommendation is 100 pounds per acre of actual N. But what if you split the application into 40 pounds per acre actual N at planting and the rest as topdress? At the topdress time, we can adjust the topdress application according to weather and market conditions as well as increase the nitrogen use efficiency.
The nitrogen source plays an important role apart from the rate of application. Most sources of nitrogen can be used for small grain production, but the most limiting factors are availability and price. Solid fertilizers like urea and ammonium nitrate can be used at any time in the growing season and can be split-applied. Urea is subject to volatilization losses (losses of the chemical from the soil through vapor). When temperatures are high, liquid N in the form of urea ammonium nitrate (UAN) should be used with caution over a growing crop, as it will burn the leaves. Anhydrous ammonia is usually used during preplanting in the fall since it can damage the root system if used as a topdress application. Care should be taken to apply each product efficiently to minimize N loss.
Legumes will provide very little N credit if the field is cut for hay and only a little N credit if it is harvested for grain. A legume, such as soybeans or cowpeas, would eventually provide nitrogen to the soil, but it may be too late for the wheat crop.
Nitrogen use efficiency can be further improved by using a topdress application method called Ramp Calibrated Strips (RCS). RCS applies varying levels of N in strips across the field in fixed distance increments and is applied either in preplanting or in the early season of the crop growth. The levels will range from 15 to 300 pounds actual N per acre. During the topdress time, a N rate recommendation can be determined by visual inspection or can be done using a GreenSeeker® handheld sensor. The maximum recommended rate is where you don't see any difference between a lower rate and its next higher rate or where there is no change in the GreenSeeker® reading on the sensor between two rates.
