Nebraska Research: How Nitrogen Affected the Critical Period of Weed Control in Dryland Corn

Weed control studies were conducted at Mead and Concord in 1999 and 2000 to examine the critical period of weed control in dryland corn. Predominant weed species were velvetleaf, common waterhemp, and green foxtail, with densities ranging from 80 to 120 plants per square yard. Nitrogen was applied immediately prior to planting as 46-0-0 and incorporated within one hour after application.

CPWC in corn was affected by the level of nitrogen fertilizer. Generally, an increase in nitrogen fertilizer delayed the timing of weed control and increased the corn tolerance to weed presence.

For example, at zero nitrogen, CPWC ranged from approximately 1st to 11th leaf stage of corn, based on a 5% acceptable yield loss (Table 1). This suggests that when no nitrogen is applied, weed control should start early in the season (at the first leaf stage of corn) and 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 and days after crop emergence as affected by the level of nitrogen fertilizer.
Nitrogen-Level	Time to Control Weeds	Time to Control Weeds
lb/acre	Corn Leaf Stage	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

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Figure 1. Corn yield loss and beginning of the critical period of weed control as influenced by the timing of weed removal and nitrogen rate.

As your corn grows, so do the weeds, competing for light, water and nutrients. The longer weeds compete with your corn, the more yield loss you can expect, however, total weed control throughout the season isn't practical or economical. Fine-tune your weed management program by timing weed control to get the most economic return. The level of crop yield loss will depend on

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

To decide when weed control is economically worthwhile, it helps to understand at what point a given weed infestation is likely to reduce yield if left uncontrolled. This period in the crop growth cycle when weeds must be controlled to prevent yield losses is the critical period of weed control (CPWC). Weeds that emerge before or after this period may not present a threat to crop yields. Understanding when this period occurs is essential in deciding on 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 and that this period can be influenced by cropping practices, for example nitrogen level in corn. (See box, at right)

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 we graphed the yield loss data against the crop growth stage at the time of weed removal (Figure 1).

In a practical situation you can decide to select, for example, 2%, 5% or 10% yield loss to signify the beginning of the critical period of weed removal. This range allows you to adjust the 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 in both crops.

In order to determine the cost of delaying weed control, the curve above the arbitrarily selected point (the beginning of CPWC) should be used. Identify the potential yield loss that could occur, for example 5%, and find the point on the curve directly above this point. For example, a 5% yield loss will occur if the weeds are removed at the 2nd leaf stage in 0-N-level (Figure 1). Delaying weed control to the third leaf stage will cause about a 7% yield loss, in essence costing the producer a 2% yield loss. A similar trend is observed for the later leaf stages on each of the four curves (Figure 1).

This shows that delaying weed removal after the starting point of CPWC will cost a producer an average 2% in yield 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 delayed 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 will cost producers about two bushels per acre of yield (2% of 100 bushels per acre). In terms of actual economic loss, it will be about $9 per acre for every crop leaf stage of delay, assuming a price of $4.50 bushel for corn.

Effect of Weed Size on the CWPC

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 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 five to ten days (two to three 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 the herbicide use rates too, especially the rates of Roundup or various generic glyphosates in Roundup Ready® crops. It is well known that Roundup has much better activity on grassy than broad leaf species. Therefore the rates of 22 to 32 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 (22 to 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).

Applying the CPWC with Herbicide-Tolerant Crops on Your Farm

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. This will provide adequate weed control the entire critical period. In order to select appropriate herbicide mixtures for the weed spectrum on your farm, consult UNL herbicide efficacy tables, available in the Guide for Weed Management in Nebraska (Extension Publication, EC-130) and on the UNL Weed Science site.

Stevan Knezevic
Extension Weeds Specialist
Haskell Ag Lab, Concord