It’s always fun when rainfall is feast or famine. Dry periods such as the coming week are great for about everything except weed management. From the perspective of making sure residual herbicides work, we like to see a decent rain about once a week. Residual herbicide treatments need to be applied and receive a half to one inch of rain within a week or so after tillage or an effective burndown treatment, to control weeds that will start to emerge at that time. More time than this allows for weeds to emerge before herbicide can be moved down into the soil, reducing the degree of control that residual herbicides are capable of providing. This is especially important for shoot uptake herbicides, such as group 15 – acetochlor, metolachlor, pyroxasulfone, and dimethenamid. Weeds are germinating and emerging more rapidly now compared with a month ago, so timeliness of the rain is more important. Under less than optimal rainfall conditions, it’s possible that herbicides may control the small-seeded weeds that emerge at or just below the soil surface, but not larger-seeded weeds that can emerge from deeper. Herbicide on the soil surface will usually become active once enough rain occurs, even if it doesn't control the weeds that have already emerged. Residual herbicides do vary in the relative amounts of rain needed for “activation”, or movement into the soil to reach germinating seeds. Most growers are applying mixtures or premixes of several products, so we’re not sure these differences are as important as the overriding principle here.
In a tilled situation, a timely rotary hoe can be used to remove some of the weeds that are about to emerge and buy some time for rain. The rotary hoe is most effective when weeds are in the “white stage”. This refers to weeds that have germinated but not emerged yet, and still lack green color. Waiting until weeds have emerged is not the correct strategy here. Two passes with the rotary hoe, spaced a week or so apart, are more effective than one, if rainfall continues to be scarce. It is advisable to avoid the crook stage of soybean development, when the equipment will cause the most damage to emerged soybean.
The good news here is that we have effective POST herbicides to remedy many situations where the residual herbicides are not completely effective. Be aware that lack of residual herbicide activity is likely to result in overall reduced control of the more difficult to control weeds, and a generally higher weed population earlier in the season than is desirable. This may lead to early application of POST herbicides, when weeds are small, and an opportunity for later emerging weeds to go uncontrolled. A two POST application strategy may therefore be most effective for season-long control, where residual herbicide performance is initially reduced.