May 16, 2008

In the coming weeks as your corn gets off to a good start, so will the weeds, competing with the crop for light, water and nutrients. The longer weeds compete with corn, usually the higher the yield losses. The level of yield loss will depend on 

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

When determining whether weed control is economical, it's important to understand whether a given weed infestation is likely to reduce yield if left uncontrolled. This is the basis for the concept - the critical period of weed control (CPWC). This is the period in the crop growth cycle when 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 in achieving an efficient use of herbicides.

Research at the University of Nebraska-Lincoln has shown that each crop has a critical period of weed control during which weeds must be controlled to maintain maximum yields. The length of this period is influenced by cropping practices such as nitrogen level in corn and can vary from field to field.

Nitrogen Effect on Critical Period in Dryland Corn

UNL researchers have studied how nitrogen levels affect the critical period of weed control using plots with velvetleaf, common waterhemp and green foxtail at 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.

Table 1. Critical period of weed control in corn based on 5% yield loss expressed as crop leaf stage (e.g., V1) and days after crop emergence as affected by the level of nitrogen fertilizer.   Nitrogen-Level Time to Control Weeds Time to Control Weeds lbs / acre Corn leaf stage App. Days After Crop Emergence N = 0 V1 - V11 8-45 N = 55 V3 - V10 10-42 N = 110 V4 - V9 15-39 N = 210 V6 - V9 20-39

The results showed that, generally a reduction in nitrogen fertilizer resulted in a longer critical period of weed control, thus corn was the less tolerant crop to weed presence. For example, at zero nitrogen, the critical period was from approximately the 1st to the 11th leaf stage of corn, based on a 5% acceptable yield loss (Table 1). This suggests that when no nitrogen fertilizer is applied, weed control measures should begin early in the season (at the 1st leaf stage of corn) and should be maintained through the 11th leaf stage, approximately the time of crop canopy closure.

This data implies that an increase in nitrogen fertilizer delayed the timing of weed control and increased the corn tolerance to weed presence. From a practical standpoint, insufficient nitrogen can reduce corn's tolerance to weeds and widen the time that weed control is critical. From a nitrogen restricted use and regulatory perspective, anticipated restrictions on the level of nitrogen 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 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 you might decide to select, for example, 2%, 5% or 10% yield loss to signify the beginning of the critical period (time of weed removal). This will allow you to adjust the critical period of weed control, depending on the risk you're willing to take. Our study used an arbitrary level of 5% yield loss to determine the beginning of the critical period of weed control (see the 5% yield loss line in Figure 1).

Figure 1. Corn yield loss and beginning of the Critical Period of Weed Control (CPWC) as influenced by the timing of weed removal and nitrogen rate. (Knezevic and Evans, 2000, University of Nebraska)

To determine the cost of delaying weed control, use the curve above the arbitrarily selected point (the beginning of CPWC). For example, if you select 5%, yield loss will occur if the weeds are removed at the 2nd leaf stage in the 0 nitrogen level (Figure 1). Delaying weed control to the 3rd leaf stage will cause about 7% yield loss, in essence costing the producer a 2% yield loss. Similar trends are observed for later leaf stages at each of the four curves (Figure 1). We concluded that delaying the time of weed removal, after the starting point for the critical period of weed control, will cause an average 2% yield loss per every leaf stage of delay. This is applicable up to canopy closure in corn (about 11 fully developed leaves).

To determine the actual economics of delaying weed control, convert the percentage yield loss of the actual target yield on your farm. For example, if a target yield for corn is 100 bushels per acre, delaying weed control for every leaf stage of crop growth will cost about two bushels per acre (2% of 100 bushels per acre). In terms of actual economic loss, it will be about $10 per acre for every crop leaf stage of delay, assuming a price of $5 per bushel of corn.

Weed Size

Another factor is the weed size at the time of weed control. In the corn study, weeds were about the same size as the crop at the time of removal. If the weeds are taller than the corn, they will shade the crop so control should be initiated four to five days (one to two leaves) prior to the beginning of the critical period of weed control. If the weeds emerge five to eight days after the crop, they will not shade the crop early in the season and 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.

Size of the weed species also will affect the herbicide rate, especially for Roundup or generic glyphosate products in Roundup-Ready soybeans. The 16 to 24 oz rates should provide control of most common annual grassy species (foxtails, barnyardgrass, field sandbur, woolly cupgrass, and 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 Application 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. This would provide adequate weed control for the entire critical period. In order to select appropriate herbicide mixtures for the weed spectrum at your farm, check the herbicide efficacy tables from the Guide for Weed Management in Nebraska (EC-130) and on the UNL Weed Science Web site.

Stevan Knezevic
Extension Weeds Specialist
Haskell Ag Lab, Concord, Northeast REC