June 26, 2009

Each year soybean growers face the question of when to time post-emergence weed control.

 

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	Days after crop emergence
7.5	V3	19
15	V2	15
30	V1	9

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. The critical period of weed control (CPWC) is the period in its growth cycle when weeds must be controlled to prevent yield losses. Weeds emerging before or after this period don't threaten yield. This period can be influenced by several factors, including row spacing.

UNL research has shown that an increase in row spacing resulted in a less competitive crop and the need for earlier weed removal. The studies found that in wide-row (30-inch) soybeans, the CPWC began at the 1st trifoliate stage, based on a 5% acceptable yield loss level (Table 1). In 15-inch rows the CPWC began at the 2nd trifoliate stage and in 7.5-inch rows, it began at the 3rd trifoliate stage.

This data implies that reducing row spacing delayed the timing of weed control and increased the tolerance of soybean to weed presence. From a practical standpoint, a reduction in soybean row spacing increases soybean tolerance to weeds and may require less intensive weed management programs.

 Cost of Delaying Weed Control

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

Producers often ask "How much is it going to cost me if I delay weed control?" when they're faced with weather constraints (rain, wind) or time constraints due to number of acres needing treatment. To answer this question yield loss data from these studies were graphed against the crop growth stage at the time of weed removal in corn and soybean (Figure 1).

The 2% yield loss per leaf stage of delay after 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 is the V3 stage (third trifoliate, see Table 1). If weed control is delayed to the V4 stage (fourth trifoliate), it will cost 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 weed control is delayed further than these indicated stages, 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.

Weed Size

Weed size at the time of application is another concern. If weeds emerge four to five days before the crop or they are taller than the crop, they will shade the crop. In these instances, control should be initiated four to five days (one to two 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. In this case, control can be initiated 5-10 days (two to three leaves) after the beginning of critical period.

Weed size also affects the herbicide rate, especially the rate for Roundup or any generic glyphosate in Roundup Ready soybeans. Roundup has much better activity on grassy than broadleaf species. Rates of 16 to 24 oz should control most common annual grasses (foxtails, barnyardgrass, field sandbur, woolly cupgrass, and panicums) that are 3-8 inches tall. The same rates should control annual broadleaves (velvetleaf, lambsquarter, pigweeds, and 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, and wild buckwheat).

Practical Use Of The CPWC to Time Weed Control

Roundup Ready Soybeans have been readily adopted in Nebraska. The CPWC can be an important tool in deciding if and when to apply postemergence 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 throughout the period.

To select appropriate herbicide mixtures for the weed spectrum on your farm, consult the herbicide efficacy tables

• in the Guide for Weed Management in Nebraska (UNL EC-130) or
• on the UNL Weed Science Web site.

Stevan Knezevic
Extension Weeds Specialist
Haskell Agricultural Laboratory, Concord