With the adoption of glyphosate-tolerant crop technology, there is less tendency to use soil-applied herbicides for pre-emergent control of weeds. This means that early-season weed infestations will be common.

One of the advantages of using glyphosate with glyphosate-tolerant crops is that post-emergence weed control can be delayed until a large population of weeds has emerged. Glyphosate can control relatively large plants of numerous weed species. Thus, there may be a tendency to delay the first application of glyphosate to ensure the maximum benefit.

Corn is totally dependent on soil and applied sources for nitrogen nutrition. Nitrogen fertilizer is the single most expensive input in corn production.

Generally, a large percentage of the nitrogen fertilizer requirement for corn is applied prior to or at planting. As a result, this expensive input is available to both weed and corn plants that are growing in the same fertilized area.

Without a pre-emergent herbicide application, weed plants may emerge before, at, or soon after corn emergence. Thus, they will compete with emerged corn for nitrogen that was applied to supply the needs of the corn crop.

Current prices for nitrogen fertilizer have resulted in reduced amounts of nitrogen being applied by many corn producers in order to lower production costs. In these cases, there will only be the minimum nitrogen available to produce the corn yield goal. Any nitrogen used by weeds will reduce the amount of nitrogen available to the corn crop and this may reduce yield potential.

Research conducted by Hellwig, Johnson, Scharf, and Hans in Missouri determined how much nitrogen is accumulated by grass weeds that are not killed soon after their emergence. When grasses were allowed to grow to 12 inches in height, they accumulated 16 to 50 pounds of nitrogen per acre. When additional nitrogen was applied to corn as a side-dress before weeds were killed, nitrogen accumulation in weeds was an additional 10 to 16 pounds per acre.

They concluded that postemergence control of grass weeds in corn should be done before the weeds reach 6 inches in height and before an in-season nitrogen application to ensure full availability of applied fertilizer to corn and not weeds.

The costs of nitrogen lost to weeds that are not controlled early in corn are significant. For instance, at a nitrogen cost of 33 cents per pound, 15 to 25 pounds of nitrogen used by grass and not corn results in a loss of $5 to $8 per acre of nitrogen fertilizer. At a nitrogen cost of 40 cents per pound, the loss is $6 to $10 per acre.

The accumulation of nitrogen by weeds will result in a lowered corn yield because of less nitrogen available to corn. If the nitrogen accumulated in weeds and lost to corn results in 10 to 15 bushels of corn less per acre, then $25 to $40 of net income is lost using a $2.50 per bushel price for corn grain.

The total losses represented by the above figures amount to $30 to $50 per acre. With higher prices for corn grain than the $2.50 per bushel used here, monetary losses will be even greater. Thus, control of small rather than large early-season weeds in glyphosate-tolerant corn should be considered an economic necessity.

The extra cost of a pre-emergent herbicide or an additional glyphosate application that may be associated with this weed management decision will be small in relation to the risks associated with nitrogen lost to weeds.