Weed control tips

Many farmers have witnessed the value of applying herbicides in the fall for the management of winter annual and biennial/perennial weeds (Figure 1). Often, they will see both a reduction in density and a delay in shoot emergence the following spring. This allows a spring planted crop to have a competitive advantage over those undesirable weeds. Unfortunately, weather conditions around the time of application can be quite variable and can influence a herbicide’s effectiveness. Let’s go through five “top tips” to make the most of this application window.

TIP 1

Choose the most effective products, rate, and tank-mixes for the target weeds. Glyphosate is usually the “foundation” of any fall herbicide application. When applied alone, a rate of 1,800 grams of active ingredient per hectare should be used when targeting perennial or biennial weeds. That rate is equivalent to applying Roundup Transorb (glyphosate @ 540 g/L) at 1.34 L/acre.  The addition of a tank-mix partner will depend on the target weed species, and is required for glyphosate resistant weeds that overwinter, such as Canada fleabane (Figure 2). Table 1 provides a summary of weed species that can benefit from a tank-mix partner.

Weed	Tank-mix partner
Alfalfa	2,4-D, dicamba (e.g. Engenia), clopyralid (e.g. Lontrel XC)
Bluegrass	Zidua SC
Burdock	2,4-D, dicamba (e.g. Engenia)
Canada fleabane	Dicamba (e.g. Engenia), Distinct, Eragon LQ, clopyralid (e.g. Lontrel XC)
Canada thistle	Dicamba (e.g. Engenia), clopyralid (e.g. Lontrel XC)
Dandelion	None needed
Field bindweed	2,4-D, dicamba (e.g. Engenia)
Ground cherry	dicamba (e.g. Engenia),
Henbit/Purple Deadnettle	Eragon LQ
Horsenettle	dicamba (e.g. Engenia),
Sow-thistle, Perennial	2,4-D, dicamba, clopyralid (e.g. Lontrel XC), Express SG
Red clover	Dicamba (e.g. Engenia), Distinct, clopyralid (e.g. Lontrel XC)
Vetch	Distinct, clopyralid (e.g. Lontrel XC)
Wild carrot	Express SG

TIP 2

When tank-mixing, be aware of any “plant back” or rotational crop restrictions for next spring. Table 2 outlines the plant back intervals required after application (in warm weather months) so that the desired field crop can be safely planted the following spring.

TIP 3

Apply when air temperatures are above 8˚C for a minimum of two hours after application. This is best accomplished by applying during late morning or mid-day so that the targeted plant is taking up herbicide during the heat of the day

TIP 4

After a frost event, wait two to three days before evaluating weed growth and if the target plants look fine and air temperatures are above 8˚C then resume applications. For example, a bolted Canada fleabane plant is very sensitive to frost (Figure 3), whereas a seedling rosette is quite tolerant to frost (Figure 4). It would not make sense to apply glyphosate to a weed species that has severe frost injury since the leaves are unlikely to absorb any herbicide. Fortunately, weeds like dandelion and wild carrot are tolerant to light frost events (figure 5 and 6), and applications to those species could resume if leaf tissue appears green and healthy.

TIP 5

Wait a minimum of 72 hours after application to perennial weeds before tilling the soil. The longer that you can wait after application before making a tillage pass, the more the herbicide will translocate within the plant and do a more effective job controlling the species.

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