Will winter annual weeds be an issue this year?

Summer annual and herbicide-resistant weeds often overshadow winter annual weeds when producers formulate weed management strategies. With the unusually early spring and mild winter, winter annuals weed growth is proceeding at a rapid pace and they will be more troublesome this year as compared to past years especially in no-till soybean acres. As I have driven around the state over the last couple of weeks I have noticed an increase in the amount of winter annuals showing up in corn stubble and can only assume these acres will be planted to no-till soybeans. The amount of time between now and herbicide burndown applications and subsequent soybean planting in those fields is dependent on the weather, but no doubt at least a couple of weeks away. If the weather stays warm and dry, then burndown applications and planting will start relatively early on the calendar and should not be adversely affected by these winter annual weeds. Although if the weather turns cold and wet, keeping sprayers and planters out of the field, then the already heavy winter weed pressures will increase and become much more difficult to control. Increased winter annual infestations will slow soil drying, further delay planting, cause mechanical planting interference, and are harder to control with herbicide burndown applications. As well as fields with prolonged infestations of henbit and purple deadnettle can be at risk of high soybean cyst nematode populations.

Winter Annual Weed Burndown Applications
Some of the common winter annual weeds encountered in Indiana are henbit, chickweed, purple deadnettle, shepherd’s purse, and field pennycress. These common weeds, while fully capable being troublesome for the reasons mentioned above, are typically not considered to be hard to control weeds. A burndown containing a non-selective herbicide such as glyphosate (Roundup) plus either 2,4-D, saflufenacil (Sharpen, Verdict, Optill, and Optill PRO), or chlorimuron (Classic, Synchrony XP), will effectively control these weeds when applied at an appropriate time. Other burndown programs to consider are those that would replace glyphosate with Liberty or paraquat. The use of these programs will reduce selection pressure for glyphosate-resistant weeds. However it is important to use Liberty and gramoxone properly, that is, add the appropriate tankmix partners to ensure complete control of winter annual weeds.

Burndown applications are most efficient when made to young actively growing weeds. The likelihood of making soybean burndown applications to young winter annuals this spring is unrealistic as many are already flowering, but a number of practices can be used to assure that a burndown application is effective. Producers may consider making applications to relatively less mature weeds a few weeks prior to soybean planting. This will also allow time for dense mats of winter annual weeds to breakdown resulting in higher soil temps at planting as well as less mechanical planter interference. If choosing to make early burndown applications, producers should also include a residual herbicide to keep the field clean of weeds up to and through soybean planting. Those choosing to wait until just prior to soybean planting to make burndown applications should consider using increased herbicide rates to assure an effective kill of mature winter annuals.

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