Nitrogen Management for Corn
All the rain we have received makes the management of nitrogen a little more complicated. Most of the corn in SE Arkansas is getting close to tassel, while corn in Central Arkansas may be around V8-V10 , and some as small as V5-V6 in NE Arkansas.
How much of the nitrogen that was sidedressed is still there? While there is a good chance that most of the nitrogen has been lost, the answer to this question will vary among and within individual fields and will depend on many factors such as soil type, field topography, fertilizer source, and time of nitrogen application among others.
Daily nitrogen uptake is still relatively slow between V4-V-8, but delaying the sidedress application past the V10 growth stage significantly increases yield loss potential. This is the time of largest daily nitrogen uptake, so the nitrogen fertilizer needs to be near the root zone by the time the crop reaches this stage. This is even more critical for soils of clayey texture where nutrient movement is more restricted than in loamy soils.
Our observations show that by the time a corn plant tassels, it has used 70-80% of the nitrogen it needs. While a good amount of nitrogen needed to fill the grain comes from translocation from other parts, that remaining 20% comes from soil uptake. Many corn farmers flow 100 lb. urea per acre 1-2 weeks before tassel. This practice has resulted in significant yield gains under the right conditions. Yield gains are normally observed during years where nitrogen uptake is compromised by too much rain and the existing yield potential justifies this third split. This year, the pre-tassel application may be critical to meet the nutritional demands of the corn crop. Our experience shows that the timing of the pre-tassel application is very important. While the nitrogen can be flown a couple of weeks ahead of tasseling, the potential benefits decrease significantly when the nitrogen is applied once the whole field is at the tassel stage. Hybrids may vary on the need for timely application, but once a plant enters into the reproductive stage, it reduces the “amount” of energy available to use for nutrient uptake. Research in Arkansas has shown that the efficiency of the nitrogen applied at pre-tassel is high; almost 100% of the N applied pre-tassel is used by the plant. But even with such high efficiency, if the ground is wet, a urease inhibitor is recommended. Research has shown that the most efficient urease inhibitors are those with NBPT as the active ingredient.
Preliminary results from current research investigating the effect of delaying the N sidedress application past the V10 growth stage, show increasing risk of yield loss potential and lack of response to the pre-tassel application of nitrogen.
Several have asked about the value of doing tissue analysis around this time. Tissue analysis basically shows if the level of a given nutrient is sufficient or not, based on set parameters and collecting the proper plant part. I am not sure we need to do tissue sampling to confirm that the yellow corn we see is a result of deficient nitrogen or sulfur levels. A typical nitrogen deficiency shows yellowing of lower leaves resembling a “V.” A sulfur deficiency appears as more generalized yellowing, which is more pronounced in young (top) leaves.
Nitrogen Management for Cotton
Our current nitrogen recommendations call for about 100 lb. per acre as a base rate, modified by soil texture or situations that may affect efficient nitrogen use. Clayey soils may require an additional 30-40 lb. N per acre. Our research on silt loams shows, that under most conditions, 60 lb. N per acre are needed by pinhead square to develop the appropriate plant structure for a good yield potential, with the remainder 40 lb. N per acre applied no later than first bloom. With the significant amount of rain we have received, farmers are encouraged to be timely on the second split and avoid delaying such application. The use of a urease inhibitor, based on NBPT, is suggested due to weather conditions as well as including ammonium sulfate in the fertility program.