June 8, 2007

As the season progresses, not only does your corn grow, but the weeds grow as well, competing with the crop for light, water and nutrients. The longer the weeds compete with the corn, the greater the likelihood of yield losses. The level of crop yield loss will depend on environmental variables and

1. weed species composition within a given field,
2. weed density and
3. time of weed emergence relative to the crop growth stage.

To determine whether a herbicide application will be economically worthwhile, it's important to understand whether a given weed infestation is likely to reduce yield if left uncontrolled. This establishes the rationale for a critical period of weed control (CPWC). This is the time during which weeds must be controlled to prevent yield losses. Weeds that emerge before or after this period may not present a threat to crop yields. This information is essential in deciding the need for and timing of weed control and achieving an efficient use of herbicides.

Research at the University of Nebraska has shown that each crop has a CPWC during which weeds must be controlled to maintain maximum yields. The length of such a critical period is influenced by cropping practices, for example by the nitrogen level in corn.

CPWC in dryland corn as affected by nitrogen

UNL studies were conducted in 1999 and 2000 at Mead and Concord. Predominant weed species at both locations/years were velvetleaf, common waterhemp and green foxtail, with the densities ranging from 80-120 plants per square yard. Nitrogen was applied immediately prior to planting at 46-0-0 and incorporated within one hour after application .

CPWC in corn was affected by the level of nitrogen fertilizer. Generally, a reduction in nitrogen fertilizer resulted in a longer CPWC, thus corn was less tolerant crop to weed presence. For example, at zero N level, the critical period ranged from approximately 1st to 11th leaf stage of corn, based on a 5% acceptable yield loss (Table 1). This suggests that when no N-fertilizer is applied, the timing of weed control should start early in the season (at the 1st leaf stage of corn) and maintained through the 11th leaf stage, approximately the time of crop canopy closure.

This data implies that an increase in N fertilizer delayed the timing of weed control and increased the corn tolerance to weed presence. From a practical standpoint, an insufficient N can reduce corn tolerance to weeds and it can widen the window of a CPWC. From a nitrogen restriction-use and a regulatory perspective, anticipated restrictions on the level of N use in corn may require more intensive weed management programs.

Cost of delaying weed control in corn crop

A common question among producers is “How much is it going to cost me if I delay weed control?” In order to answer this question, we graphed the yield loss data against the crop growth stage at the time of weed removal (Figure 1). In a practical situation a producer might select, for example, 2%, 5% or 10% yield loss to signify the beginning of the critical period (time of weed removal). This range will allow you to adjust CPWC, depending on the risk you're willing to take. In our study, an arbitrary level of 5% yield loss was used to determine the beginning of CPWC (see the 5% yield-loss-line in Figure 1).

In order to determine the cost of delaying weed control, use the curve above the arbitrarily selected point (the beginning of CPWC). For example, if an arbitrarily selected point of CPWC is 5%, the 5% yield loss will occur if weeds are removed at the 2nd leaf stage in 0-N-level (Figure 1). Delaying weed control to the 3rd leaf stage will cause about 7% yield loss, in essence costing a producer a 2% yield loss. A similar trend is observed for the later leaf stages at each of the four curves (Figure 1). We concluded that delaying the time of weed removal, after the starting point of CPWC, will cost a producer an average of 2% in yield loss per every leaf stage of delay.

This recommendation is applicable up to canopy closure in corn (about 11 fully developed leaves). To determine the actual economics of the cost of delayed control, the producer will have to convert the percentage yield loss of the actual target yield on his farm. For example, if a target yield for corn is 100 bushels per acre, delaying weed control for every leaf stage of crop will cost producers about 2 bushels per acre of yield (thus 2% of 100 bushels per acre). In terms of actual economic loss, it will be about $4 per acre for every crop leaf stage of delay, assuming a price of $2 bushel for corn.

Weed size

Weed size at the time of weed control measure is another concern. In the corn study, the weeds were about the same size as the crop at the time of their removal except for the Mead site in 2000. If the weeds are taller than corn they will shade the crop and control should be initiated four to five days (one to two leaves) prior to the beginning of CPWC. If the weeds emerge five to eight days after the crop, they will not shade the crop that early in the season so the control can be initiated 5-10 days (2-3 leaves) after the beginning of critical period, as it is shown with the later start of the CPWC at Mead in 2000. The size of weed species will affect herbicide use rates too, especially the rates of Roundup or various generic glyphosates in Roundup Ready soybeans. Generally Roundup has much better activity on grassy species than broadleaf species. Rates of 16 to 24 ounces should provide control of most common annual grassy species (foxtails, barnyardgrass, field sandbur, woolly cupgrass, panicums) that are 3-8 inches tall. The same rates should control annual broadleaves (velvetleaf, lambsquarters, pigweeds, mustards) that are less than 6 inches tall. For taller grasses and broadleaf species a full rate (32 oz) will be required. Higher rates of Roundup (40-60 oz) will be needed to control species such as ivy-leaf morning-glory, sweet clover, field bindweed, Venice mellow and various smartweeds (lady's thumb, Pennsylvania smartweed, wild buckwheat, etc).

Practical use of the CPWC in herbicide-tolerant crops

A generally sound strategy, for example in Roundup Ready corn, would be to apply Roundup tank-mixed with a residual herbicide at the beginning of the critical period, which will provide adequate weed control the entire critical period. In order to select appropriate herbicide mixtures for the weed spectrum at your farm, we suggest you consult the herbicide efficacy tables from the Guide for Weed Management in Nebraska (EC-130).

Stevan Knezevic
Extension Integrated Weed Management Specialist
Haskell Ag Lab, Northeast REC