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Timing herbicides to maximize yields in Roundup Ready corn

Earl Creech Published on 07 June 2012

Weeds compete with crops for nutrients, space, sunlight and water. Compared to other crops, corn is particularly sensitive to early season weed competition.

The traditional weed management program in corn was centered on the use of soil-applied herbicides such as atrazine.



The primary benefit of this strategy is the elimination of weeds early in the growing season to protect corn yield. Post-emergence herbicides were used as a followup to soil-applied products to provide season-long weed control.

Roundup Ready (RR) corn came onto the market in the late 1990s and has grown in popularity to encompass nearly 90 percent of U.S. corn acres.

A glyphosate-based system has many attractive features including low cost, excellent crop safety, broad spectrum of activity and application flexibility.

The effectiveness of glyphosate has caused many growers to discontinue use of soil-applied herbicides and to rely solely on total postemergence (POST) weed control with glyphosate.

Although reliance on a total POST program is not necessarily bad, many growers delay these applications until the majority of the weeds have emerged. This reduces the number of applications needed for season-long weed control. The problem with such a strategy is that a great deal of yield loss can occur by allowing corn and weeds to compete early in the season.


Proper weed control timing in RR corn
To determine the proper timing of glyphosate in RR corn, field studies were conducted at 35 sites across nine states in the north-central U.S. over two years.

In these studies, a single application of glyphosate was applied when the weeds reached two, four, six, nine or 12 inches in height. After this initial application, no additional herbicide applications were made.

Weed control and corn grain yield after a single application of glyphosate
At the end of the season, a single glyphosate application at two-inch weeds resulted in 74 percent grass control and 84 percent broadleaf control (See Table 1).

With such levels of late-season weed infestation, most growers would be unsatisfied. However, compared to a weed-free check, 100 percent of the corn yield was protected by controlling weeds early in the season. Late-emerging weeds had little to no effect on corn yields.

Contrast this with delaying application until weed height reached 12 inches. Grass and broadleaf control were 95 and 93 percent, respectively, which is excellent weed control by anyone’s definition. However, corn yield was reduced by 21 percent compared to weed-free check.

Despite the near-perfect weed control from delaying application, the corn was unable to overcome the yield damage that resulted from early season weed competition.


Although this experiment did not specifically measure silage production, grain and silage yields usually respond similarly to early season weed competition. This is primarily because the ear makes up approximately 40 to 50 percent of silage dry matter and contributes a great deal to silage quality.

Costs and causes of delay
Table 2
Table 2
shows the effect of delayed application in terms of yield and dollars lost. The glyphosate application timing at two-inch weeds had maximum yields and resulted in no reductions in yield or return.

At the four-inch weed timing, an average of six bu per acre or $39 per acre had been lost. Similar trends continue as weed height increases, culminating in the 12-inch weed height causing a 42-bu-per-acre yield loss or $273 per acre in lost revenue.

Of particular interest is how delaying application for a few days can result in significant yield losses. It took only 15 days to progress from two- to 12-inch weeds, and 21 percent of the overall yield was lost. That is equivalent to a 1.5 percent loss in yield per day.

Sometimes the delay in making a POST application of glyphosate is intentional, where a grower attempts to wait as long as possible until most of the weeds have emerged. Other times the delay is unintentional.

How often are we kept out of the field in the spring due to wet or windy conditions? What about if we have a lot of acres to cover, an equipment breakdown or other pressing activities on the farm that need attention? Sometimes the delay can easily extend for days or even weeks on end. Keep in mind that each day the weeds are allowed to compete with corn, yields are dropping.

Include a soil-residual herbicide
Corn yields are maximized when POST herbicides are applied in conjunction with herbicides with soil-residual. For some growers the answer has been to employ a two-pass system where a soil-applied herbicide is applied pre-emergence (PRE), then use the POST glyphosate application as a backup treatment.

The PRE herbicide reduces weed density and delaying emergence, creating a wide window for POST application. Even a low rate of a PRE herbicide is adequate to protect corn yield in the event that POST application is delayed.

A second strategy is to tank mix a soil-residual herbicide with POST glyphosate. Such an approach does not alleviate the narrow application window (two-inch to four-inch weeds) of a glyphosate-only program. It does, however, allow for season-long weed control to be achieved in a single pass.

POST herbicide strategies to protect yield in RR corn:

  • Scout fields – Regardless of strategy (PRE, POST, or combination), all fields should be scouted around two to three weeks after planting.

  • Know your enemy – A good understanding of the dominant weed species and their emergence and growth patterns is necessary to plan the best strategy for a specific field.

  • Control weeds early – If fields are clean for eight to 10 weeks after planting, yields should be unaffected by later-emerging weeds.

  • Include a residual herbicide – Corn yield is maximized when POST herbicides are applied following a reduced rate of a PRE herbicide or early POST with residual herbicides.

  • Target small weeds – To avoid yield loss from early season weed competition, apply POST herbicides when weeds are no more than four inches tall.  FG

Bill Johnson is an Extension weed specialist at Purdue University.

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Earl Creech
  • Earl Creech
  • Extension Agronomist
  • Utah State University