Nitrogen management continues to be difficult due to transformations of nitrogen fertilizers that are possible when applied to soil and the uncertainties of weather (Cabrera et al., 2008). Nitrogen (N) fertilizer in the form of urea is subject to ammonia volatilization through the activity of the urease enzyme found ubiquitously in soil (Kissel et al., 2008). Nitrate fertilizer is subject to leaching (Randall et al., 2008) or denitrification (Coyne, 2008), depending on the water content of the soil and water movement through the soil. Ammonium forms of N can be fixed (Kissel et al., 2008) or transformed to nitrate through the activities of specific soil bacteria (Norton, 2008). Because of these and other processes, nitrogen use efficiency is low.

Nitrogen often is applied to crops in the north-central region of the U.S. before planting. During the first four to six weeks after planting, corn will require only about 5 percent of the N applied. The following two to four weeks of growth require a large proportion of the total seasonal N requirement.

In winter wheat, very low levels of N are required for overwintering. However, once wheat breaks dormancy, a large proportion of N is required during the next few weeks. In spring wheat, a small of amount of N is required to establish the crop during the first two to four weeks after seeding; however, most of the remaining N is required during the next 30 days.

To address some of the delayed N requirement issues of winter wheat, much of the crop is top-dressed in the spring. In corn, some growers use side-dress applications; however, spring preplant application is most common, with fall application preferred by growers in some Northern states. In spring wheat in the northern Plains, some post-N applications are made.

Because of the lack of rain during the growing season in many years, post-N applications as a source of most of the N requirement are discouraged except under irrigation. To increase nitrogen use efficiency and thereby increase yields or decrease N rates, a number of products have been developed to delay an N transformation process so that the period of time in which the N source is available for uptake is closer to the time the crop needs the available N.

Urease Inhibitors

The compound that most consistently has decreased urea volatilization when mixed with urea or urea-ammonium nitrate solutions is NBPT (N-(n-butyl) thiophos-phoric acid triamide). NBPT is marketed as Agrotain (Agrotain International LLC). The mechanism for NBPT is to lock onto the urease enzyme-binding sites, preventing the enzyme from reacting to the urease (Manunza et al., 1999). Agrotain has at least two possible uses in crop production: One is to protect against seed injury for growers, especially in the northern Plains, who apply urea with small-grain seed at planting. Use of Agrotain has increased the rate of urea that can be applied safely with small-grain seed in some studies.

Agrotain also decreases the rate of ammonia volatilization from urea applied to the surface as dry urea or urea-ammonium nitrate solutions (Brouder, 1996. Ammonia volatilization losses from urea at Brandon, Manitoba, decreased from 40 milligrams (mg) to 2 mg and from 88 mg to 12 mg with Agrotain in two separate studies for a seven-day period after application (Grant, 2004).

In a recent Kansas study (Weber and Mengel, 2009), urea was applied in three site-years to the soil surface after corn emergence using a number of nitrogen-extending additives, including Agrotain. The Agrotain treatment was superior to urea alone by 25 bushels per acre in one of the three site-years. The two locations that received significant rainfall immediately following applications did not receive a yield benefit from the Agrotain treatment. In sorghum, urea + Agrotain and urea + SuperU were 11 and 12 bushels per acre, respectively, greater in yield than with urea broadcast alone (Weber et al., 2009a). At two drier locations, no yield differences occurred between urea + Agrotain and urea alone.

A 14-year study in southern Illinois (Ebelhar et al., 2010) showed a 3-bushel corn yield advantage of urea + Agrotain compared with urea broadcast in conventional till surface and incorporated during 12 years of treatments. In no-till, urea + Agrotain held an 11 bushel/acre advantage over urea surface applied during four years of treatments. Similar results were demonstration by Varsa etal 1995.

In Kentucky, 50 pounds of N/acre was applied preplant to all corn plots (Schwab and Murdock, 2009). Side-dress applications of urea and UAN with several additives or formulations were applied to the soil surface at the six-leaf stage. Higher yields than urea alone were achieved with urea + Agrotain and SuperU. Higher yields than UAN alone were achieved with UAN + Agrotain and UAN + Agrotain Plus (combination of NBPT and DCD formulated for use with UAN).