Nitrogen: Choosing source and timing for wheat
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.
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