Nitrogen: Choosing source and timing for wheat

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A couple of warm days in late January got the wheat and grass growing. It’s January 30th and the weather has turned chilly again, but with some warm days in the forecast. Any farmers with wheat or grass who weren’t already thinking about nitrogen are thinking about it now.

Is it time to fertilize wheat and grass yet? Not really. N uptake for wheat and grass during February will be minimal, and there is a risk that fertilizer applied now will be lost before peak uptake time in April and May.

In a 2-year research project near Columbia, Mo., wheat yielded about 10 bushels better with N applied in mid-March than when N was applied mid-February (and 20 bushels better than when N was applied mid-January). The graph below shows the effect of N timing on wheat yield, averaged over 2 years and 5 N sources, all at an N rate of 70 lb N/acre. Applications were made in the middle of each month.

Only these two experiments had N topdressed on wheat in all three months, but we have had 17 experiments over 16 years with N applied in February or March. Over all 17 experiments, at a 70 lb N rate, the March application has given a 6.8 bushel/acre yield advantage over the February application. ‘March’ applications in these studies have been applied just before the crop joints, at a time when a short hollow space (about one-half inch) can be found at the bottom of the main stem where it meets the crown. This is, in my opinion, the ideal time to apply N for wheat in the vast majority of wheat fields. In years with late springs, the pre-joint N applications were sometimes made in early April.

This yield differential between March and February applications can be partially made up by putting out higher N rates in February. We had 15 experiments with a range of rates in February and again in March. For March applications, average optimal N rate was 79 lb N/acre, giving a yield of 67 bushels. For February applications, average optimal N rate was 93 lb N/acre, giving a yield of 64 bushels. Optimal February N rates were 14 lb N/acre higher than in March, while giving yields 3 bushels lower, for a net loss of about $35/acre at current prices. While February applications may be a reasonable choice to get field operations done, it would be appropriate to pencil in a $35/acre penalty when deciding whether to apply N in February rather than March. A better option is to use ESN.

If you must topdress wheat early, use ESN (but not on grass)

If you feel compelled to get some topdressing done in February, ESN from Agrium is a good choice. This is a polymer-coated urea product that slowly releases the urea through the polymer coating. In two years of research, ESN out-yielded urea by 15 bushels when both were applied mid-January, and by 10 bushels when both were applied mid-February. However, when both were applied mid-March, the urea out-yielded the ESN by 4 bushels. Nitrogen release from the polymer capsules is too slow to adequately supply the crop’s needs when ESN is applied in mid-March, and these treatments were visibly N-deficient throughout the spring. It was a surprise that they fell only 4 bushels short of regular urea. But it’s clear that the capsules protected the fertilizer from losses that were experienced when urea was applied in January or February.

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.

Topdressing urea on wheat or grass? Use Agrotain

Urea is a great fertilizer product but with one hitch: surface applications are prone to volatile loss of ammonia gas. Depending on complex interactions between a range of weather parameters, losses can range from 0 to 50% of applied N. A good number to pencil in for average loss is 25%. When urea first hit the world scene, this tendency to volatilize was almost universally managed by tilling it in following broadcast application. As we have moved to less tillage and more targeted goals for tillage, this option has become less attractive and less widely used.

Although ammonia volatilization from urea applied to summer crops like corn, rice, and cotton is widely acknowledged to be a problem requiring focused management, rumors have circulated through agricultural and fertilizer circles that ammonia loss is not an issue in cool-season crops like wheat and fescue. This was never based on any research that I am aware of, and recent research in Montana and Alaska has shown convincingly that ammonia volatilization from urea can be substantial even under cool temperatures, and even from frozen soil. Urea breakdown to ammonium/ammonia is accomplished by an enzyme, but an enzyme that is loose in the soil rather than only in living cells. This means that urea breakdown is a purely chemical reaction and is much less temperature-sensitive than other reactions (like the conversion of ammonium to nitrate) that take place only in living cells. Our research showed about a 5-bushel yield advantage for Agrotain-treated urea over regular urea, and this was true whether these products were applied in mid-January, mid-February, or mid-March. This clearly implies that ammonia was being lost from the urea, and that Agrotain was effective in reducing this loss. Measurements of ammonia volatilization following the March N applications confirmed this idea—12 lb N/acre was lost as ammonia from the urea treatment, while only 1 lb N/acre was lost when the urea was treated with Agrotain.

Agrotain treatment of urea applied to grass in March increased yield by an average of 330 lb N/acre over six experiments. This is only a bit better than break-even in most years, but still supports the use of Agrotain when urea is applied to tall fescue or other grasses, even in cool spring weather. In some of these experiments, rain within a few days of nitrogen application moved the urea into the soil and avoided volatile loss of ammonia. If rain is probable in the forecast within a day or two, it would be reasonable to omit the Agrotain.


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