Nitrogen: Choosing source and timing for wheat
click image to zoom Missouri research with ESN applied in spring to tall fescue has been carried out only in March. As with wheat, March applications of ESN performed poorly, reducing yield by about 800 lb/acre compared to urea. Research on the effects of spring N timing on fescue yields is thin to non-existent for both ESN and other N sources. If fescue parallels wheat, earlier timing of ESN may give acceptable results and out-perform other N sources applied in January or February. However, March is probably still the ideal time for N application to grass, especially hay. Pastures may benefit from delaying application into April or even May in order to push the grazing period farther into the summer.
Topdressing early to stimulate tillers
The one case where we would recommend an early (greenup) N application to wheat is when tiller number is low. Ideally wheat should form several tillers per plant in the fall. If you don’t see at least two good tillers per plant (in addition to the main stem) at greenup, an N application at that time will stimulate formation of additional tillers. An quickly available N source (NOT ESN) is required to accomplish this. Your tillers are your yielding population, and if there are not enough, yield will suffer. Fields in this situation generally have an appearance of thin stand at greenup, and individual plants are small. For anyone with the stomach to count tillers (not me any more), fields with less than 80 tillers per square foot should probably receive their N at greenup rather than just before jointing. Stimulating spring tiller formation can’t completely make up for lack of fall tillering, but is a lot better than doing nothing about a poorly tillered stand.
Should I split my topdress N?
In some environments, research has shown that splitting spring applications can produce better wheat yields than a single spring topdress. Missouri is not one of those environments. In 15 Missouri experiments comparing split to single spring N applications, split applications came out $6 to $14/acre behind putting all N on in a single application just before jointing. Responses to split spring N applications have come mainly in coastal plain environments with dominantly sandy soils—these fields often need a greenup N application because the sandy soils have not held N through the fall and winter to support adequate tiller formation. But a single N application at greenup will sometimes be leached below rooting depth in these sandy soils, leading to the need to split N. If it makes sense to split spring N on wheat anywhere in Missouri, it would be on sandy river- or creek-bottom soils.