June 8, 2007

With the advances of herbicide tolerant soybean (eg. Roundup Ready) there is still a constant dilemma on how to time post-emergence weed control. To decide whether weed control is economically worthwhile, it helps to understand whether a given weed infestation is likely to reduce yield if left uncontrolled. This has led to the concept of 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 determining 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. The length of this critical period is influenced by cropping practices, for example, row spacing in soybean.

Time of Weed Removal as Affected by Soybean Row Spacing

Studies were conducted in 1999 at Mead, and in 2000 and 2001 at Mead and Concord. Predominant weed species at both locations/years were velvetleaf, common waterhemp and green foxtail, with densities ranging from 70-100 plants per square yard.

The critical time of weed removal was significantly influenced by row spacing. Generally, an increase in row spacing resulted in a need for earlier weed removal, thus a less competitive crop. For example, the beginning of the CPWC in wide-row soybean (30-inch) was approximately at the 1st trifoliate stage, based on a 5% acceptable yield loss level (Table 1). This suggests that in wide-row soybeans control measures should start early in the season (at the 1st trifoliate stage). The beginning of the CPWC in 15-inch rows was delayed and corresponded to the 2nd trifoliate stage, and in 7.5-inch rows, the period started at approximately the 3rd trifoliate stage (Table 1).

Table 1: The beginning of CPWC in soybean based on 5% yield loss expressed as crop leaf stage (eg.V1) and days after crop emergence (DAE) as affected by the row spacing, at two locations in 1999, 2000 and 2001. Row spacing (inches) Time to control weeds (soybean leaf stage) Time to control weeds (days after crop emergence) 7.5 V3 19 15 V2 15 30 V1 9

This data implies that reducing row spacing delayed the timing of weed control and increased the tolerance of soybean to weed presence. The mechanism of soybean tolerance needs to be determined yet, although we believe it is related to the crop shading effects. The speculation is that even though the weeds are present in the narrow row soybeans they are not growing as vigorously and they are not as competitive against the crop, due to crop shading effects. Furthermore, from a practical standpoint, these results indicate that a reduction in soybean row spacing increases soybean tolerance to weeds and it may require less intensive weed management programs (eg. weed control measure applied perhaps once or twice).

Cost of delaying weed control: The commonly asked question among producers is “How much is it going to cost me if I delay weed control.” Possible reasons for delayed weed control may include weather constrains (rainfall, wind), and time constrains due to large acreage to spray. In order to answer the above question the yield loss data from the above studies were pooled among years-locations and graphed against the crop growth stage at the time of weed removal in corn and soybean (Figure 1).

The 2% yield loss per every leaf stage of delay passed the critical stage of weed control, was determined as the cost of delaying weed control in soybean. For example, the time to control weeds in 7.5 inch rows soybean is the V3 stage (third trifoliate-Table 1), if weed control is delayed to the V4 (fourth trifoliate), it will cost a producer about 2%in yield losses due to prolonged competition from weeds. The same is true if weed control is delayed past the recommended critical time in other soybean row spacings (Table 1). This recommendation is applicable up to the R3 stage in soybean (beginning pod). If the weed control is delayed further than these indicated stages the yield losses will be much higher than suggested.

In terms of actual economic losses in soybean, it will be about $5 per acre for every soybean leaf stage of delay, assuming a price of $5 bushel and a yield goal of 40 bushels.

Effect of Weed Size

Figure 1. Soybean yield loss and beginning of CPWC as influenced by the timing of weed removal and row spacing.

Weed size at the time of weed control measure is another concern. If the weeds emerge 4-5 days before the crop or they are taller than crop 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 5-10 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.

The size of weed species will effect the herbicide use rates too, especially the rates of Roundup or any generic glyphosate 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 (40-60 oz) will be needed to control species such as ivy-leaf morning-glory, sweet clover, field bindweed, Venice mellow and various smartweed species (lady=s thumb, Pennsylvania smartweed, wild buckwheat, etc).

Practical Use of the CPWC and Timing of Weed Control in Herbicide Tolerant Crops

Roundup-Ready soybeans have received high levels of acceptance in our state. The concept of critical period of weed control is an important part of integrated weed management in answering a fundamental question if and when to apply post-emergence herbicide.

A generally sound strategy in Roundup Ready soybeans 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, consult the herbicide efficacy tables from the Guide for Weed Management in Nebraska (EC130).

Stevan Knezevic
Extension Integrated Weed Management Specialist
Haskell Ag Lab, NEREC