Considering the cost of nitrogen, we can ill afford to allow weeds to use this expensive input.

We conducted an experiment for the past 2 years that provided results that truly surprised us in regard to weeds and nitrogen. Basically, we confirmed that weeds are highly effective consumers of nitrogen. As a consequence, the potential exists for weeds to capture some of the nitrogen that the corn needs for optimum yields. We just didn’t expect the effect to be so large.

In term of competition, the biggest risk would seem to be when weeds are allowed to grow too large before being controlled postemergence. Therefore, our experiment was designed to determine how delayed weed control might affect optimum nitrogen rates in corn.

In the experiment, we applied increasing rates of nitrogen over the range of 0 to 200 lb/a. Within these increasing nitrogen rates, we controlled weeds either preemergence, at a recommended 4-inch height, at a 12-inch height, and a weedy check. Weeds were controlled postemergence with glyphosate in the Roundup Ready corn. The 12-inch height was used to mimic a time delay that might occur because of rainy or windy weather or other delays. The time difference between the 4-inch height and the 12-inch height was 8 days in both years.

A few key findings:

SOURCE: Chris Boerboom, Extension Weed Scientist, and Carrie Laboski, Extension Soil Scientist