Impact of Early-Season Weed Competition On Corn Nitrogen Needs

Weeds impact crop yields primarily by competing for limited resources such as sunlight, water and nutrients. Numerous studies have investigated how changing nutrient availability via fertilization influences the competitive relationship. In some situations fertilization favors crop growth over weeds, and therefore reduces yield loss, whereas in others weed growth is benefited more than crop growth. However, most of these studies have allowed weeds to compete for the entire growing season – not a normal practice for agricultural fields.

Chris Boerboom and colleagues at the University of Wisconsin investigated the impact of glyphosate application timing in RR corn on the nitrogen needs of corn. The premise of the study was that early-season weed growth might ‘tie up’ nitrogen, therefore resulting in higher economic optimum nitrogen rates¹ (EONR) with delayed glyphosate applications. Experiments were conducted during 2006 and 2007, evaluating corn yield response to a range of nitrogen rates (0 to 200 lb N/A) and varying lengths of weed competition.

Corn yields were not reduced when weeds were controlled at the 4-inch stage, but delaying application until weeds were 12-inches resulted in a 9 percent yield loss averaged over the two years. Weed densities in the experiments were moderate, with less than 100 weeds/ft2 at the 4-inch application timing.

The presence of early-season competition altered corn response to N fertilization, resulting in higher EONR’s (Table 1). Delaying glyphosate application until weeds reached a height of 12 inches increased the EONR more than 100 lbs per acre in both years of the study compared to the weed-free treatment. In 2007, allowing weeds to reach 4 inch increased the EONR by 40 lbs. These results suggest that the utilization of N by weeds reduces N availability to corn, therefore requiring higher N rates for optimum yields. Although the N used by weeds will eventually be mineralized and become available for plant use, apparently this N did not become available quickly enough following glyphosate application to satisfy the immediate needs of the corn crop. Table 1. Economic optimum nitrogen rates in corn at a 0.15 nitrogen:corn price ratio. 

Table 1. Economic optimum nitrogen rates in corn at a 0.15 nitrogen:corn price ratio

This research shows the complexity of crop-weed interactions, and the risk associated with allowing weeds to grow along with the crop for extended periods early in the growing season. In fields with moderate to heavy weed infestations, the use of preemergence herbicides will reduce or eliminate the effects weeds have on crops prior to post applications. The cost of the preemergence herbicide should be less than the additional N that may be needed to compensate for N usage by weeds.

¹Economic optimum N rate: the point where the last increment of N returns a yield increase large enough to pay for the additional N.

Boerboom, C.M., T.L. Trower, C.A.M. Laboski and T.W. Andraski. 2008. Fertilizing weeds for a profit? Proc. Of the 2008 WI Fert., Aglime and Pest Manage. Conf. 47:223-226.