Herbicide Timing in Roundup Ready Corn

Chris Boerboom

Herbicide timing is critical in corn when using one-pass postemergence programs. If the herbicide application is delayed, early season weed competition can reduce yields. This is true whether the corn is conventional or Roundup Ready. An excellent article was published last fall that documented the importance of herbicide timing in corn. As we approach this year’s season, it is worth reviewing these results and considering what it might mean for programs that are being planned for this year.

The study was coordinated by weed scientists from the Ohio State University. It had 35 experiments that were conducted over 2 years in nine Midwestern states. Roundup Ready corn was planted in each experiment and 0.75 lb ae/acre glyphosate was applied when the weeds were 2, 4, 6, 9, or 12 inches tall. The weed height was based on the tallest weed, which was usually giant foxtail. Additional treatments were Harness + atrazine to create weed free plots for yield comparisons. A second set of treatments were included where the 2, 4, and 6 inch timings were resprayed with glyphosate to control late emerging weeds. The 9 and 12 inch timings did not have a significant number of weeds emerge after these timings.

The most common weeds in the experiments were giant foxtail, velvetleaf, common lambsquarters, and pigweed, which are all common weeds for Wisconsin. The total weed density ranged from 2 to 325 weeds per ft2 with an average of 73 weeds per ft2. Since these experiments were conducted on research stations, some of the sites had weed densities that were higher than typical of farm fields. This makes the results somewhat conservative.

The grass weed control from a single application of glyphosate was greater with the later applications because additional grasses emerged after the early applications (Table 1). The latest glyphosate application was very effective in controlling the grasses despite their large size because grasses are fairly sensitive to glyphosate. A similar trend was measured for broadleaf control. There were fewer broadleaves emerging after the early applications so those ratings look better than for grasses. The broadleaf control at the 12 inch timing was more erratic (data not shown) because larger broadleaf weeds are not controlled as easily as grasses with glyphosate.

Table 1.  Grass and broadleaf weed control following a single application of glyphosate and corn yield

The corn yields were not fully protected by a single application of glyphosate regardless of timing. The weeds that emerged after the early timings competed with the corn to cause some yield loss. Despite the good weed control with the later timings, corn yield was lost from the earlier weed competition. Note that the poor overall weed control with the 2 inch timing did not reduce the corn yield severely. This shows that there is much less weed loss from late emerging weeds than the early emerging weeds. We need to focus our control efforts on the early weeds and cannot wait until all the weeds have emerged.

A second component of the study was to evaluate the effect of early season weed competition without any competition of late emerging weeds (Table 2). In this case, significant amounts of corn yield were lost when the timings were delayed after the 4 inch weed height. I estimated a yield loss and economic cost for these applications based on 150 bu/a corn at $2/bu. An application delay from the 4 inch height to the 6 inch height would have caused a loss of $12/a. It may take only 4 days for weeds to grow this amount. An application delay from the 4 inch height to the 12 inch height would have caused a loss of $54/a and it may only take 10 days for this amount of weed growth.

Table 2.  Corn yield when weeds were controlled at the specified weed heights and later emerging weeds were also controlled. The number of days between timings are estimates based on experience in Wisconsin. Corn yield and dollars lost are based on 150 bu/a corn at $2/bu.

These results clearly show the importance of early weed control in corn when using total postemergence programs. Although weed control may still look great with later applications, there is a real risk of losing money with later applications. Fortunately, we can minimize these risks with some planning. We need to consider the number of acres to be sprayed and which fields are at greater risk because of moderate or heavy weed pressure. We also need to be realistic and factor in the number of days when wind or rain will keep us out of the field and delay planned applications. Based on this information, some or many fields should probably have a preemergence herbicide applied, which will greatly extend the time when the postemergence herbicide needs to be applied. It is possible that the preemergence herbicide can be applied at a reduced rate to manage costs. Although this preemergence herbicide will add an additional cost, it can be viewed as insurance to protect against losing profits, such as the $54/a estimated loss based on the average results from this study.

Sources

Gower, S. A. and others. 2003. Effect of postemergence glyphosate application timing on weed control and grain yield in glyphosate resistant corn: Results of a 2-yr multistate study. Weed Technology 17:821-828.