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Timing Post-Emergent Weed Control in Corn

As the season progresses, not only does your corn grow, 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 yield loss. The level of crop yield loss will depend on (a) environmental variables, (b) weed species composition within a given field, (c) weed density and (d) time of weed emergence relative to the crop growth stage.

In addition, to decide whether or not weed control is economically worthwhile, there is a need for understanding if a given weed infestation is likely to reduce yield if left uncontrolled. This establishes the rationale for introduction of the concept of critical period of weed control (CPWC) . The CPWC is a period in the crop growth cycle 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 making decisions on the need for and timing of weed control and in 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. However, we also concluded that the length of such critical period is influenced by the cropping practices, for example by the nitrogen level in corn.

CPWC in Dry-land Corn as Affected by Nitrogen: 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 as 46-0-0 and incorporated within one hour after application .

The 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 the less tolerant crop to weed presence. For example, at zero N level, CPWC ranged from approximately 1 st to 11 th 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 measure should start early in the season (at the 1 st leaf stage of corn) and needs to be maintained through the 11th leaf stage, approximately the time of crop canopy closure.

Table 1: Critical period of weed control in corn based on 5% yield loss expressed as crop leaf stage (eg.V1) and days after crop emergence as affected by the level of nitrogen fertilizer.

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. Furthermore, 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: A common question among producers is “how much is it going to cost me if I delay weed control”. In order to answer such question we graphed the yield loss data against the crop growth stage at the time of weed removal (Figure 1). In a practical situation one may decide to 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 adjustment of the CPWC depending on the risk one is 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, the curve above the arbitrarily selected point (the beginning of CPWC) should be used. For example, if an arbitrarily selected point of CPWC is 5%, the 5% yield loss will occur if the weeds are removed at the 2 nd leaf stage in 0-N-level. Delaying weed control to the 3 rd leaf stage will cause about 7% yield loss, in essence costing producer a 2% yield loss. Similar trends are observed for the later leaf stages at each of the four curves.

Therefore, we conclude 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 so the control should be initiated 4-5 days (1-2 leaves) prior to the beginning of CPWC. If the weeds emerge 5-8 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 effect the herbicide use rates too, especially the rates of Roundup or various generic glyphosates in Roundup-Ready soybeans. It is well known that Roundup has much better activity on grassy than broad leaf species. Therefore the rates of 16 to 24 oz 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 oz - 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).

Figure 1: Corn yield loss and beginning of CPWC as influenced by the timing of weed removal and N-rate.

Practical use of the CPWC and timing of weed control in herbicide tolerant crops: A generally sound strategy, for example in Roundup-Ready corn will 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 to consult the herbicide efficacy tables from the Guide for Weed Management in Nebraska (Extension Publication, EC-130) (SK)

David P. Shelton
Extension Agricultural Engineer

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