With the onset of planting season, it is necessary to watch for winter annuals and early emerging summer annuals in corn and soybean fields. Consider applying burndown herbicides to control those weeds when temperatures are above 55 F to improve the effectiveness of herbicide application.
Current cool temperatures increase the risk of failures in terminating cover crops. While cereal rye continues to grow during these conditions, activity of herbicides is reduced. Translocation of glyphosate to growing points is reduced under cool temperatures, slowing activity and increasing the potential for control failures. It is generally recommended to avoid applications when nighttime temperatures fall below 40 F, and we prefer temperatures at application to be at least in the mid-50s with clear skies.
As it has started to warm up, folks are ready to implement early-spring weed management programs, especially in fields with infestations of winter annuals like field pennycress and marestail (horseweed). Spring burndown (preplant) herbicide programs should target winter annual plants at the rosette stage, prior to stem elongation (bolting), for consistent control. Cold nights (<40°F) will reduce activity of herbicides, particularly for glyphosate. With cool weather conditions, it is recommended to wait for a few days until it warms up before applying herbicides.
Due to extensive damage to corn fields last year from the derecho, volunteer corn is a looming threat for many fields this spring. Farmers intending to plant corn in these derecho-affected fields have limited options for managing volunteer corn, thus thoughtful planning is critical. I asked a collective of weed scientists their thoughts on best management practices for controlling volunteer corn prior to planting corn. I received responses from University colleagues in 8 states and Canada. The following is a summary of their thoughts and some resources that you may find helpful.
The plentiful rain in September has provided ideal conditions for establishment of winter annual weeds, thus many no-till fields will have dense stands of these weeds going into winter (Fig. 1). The wetter springs we have encountered recently complicate getting spring burndown applications made in a timely matter.
With the short timeframe for fieldwork this spring prior to planting, early weed management may fall to the bottom of the priority list for many. For those who have persistent issues with winter annuals (field pennycress, horseweed/marestail) in no-till, an early burndown treatment may be worth the extra effort this spring. Winter annuals resume growth soon after the arrival of warm temperatures, so as soon as fields are fit the weeds will be susceptible to spray.
After a decent start to the planting season, cold and wet weather has put the brakes on field work for a while. As we look ahead into next week, or whenever it starts to dry out enough to get back in the field, growers are figuring on being able to spray a day or two before it is dry enough to get back into the planter. This has generated a lot of questions over the last few days as we plan out temperature sensitive applications like no-till burndowns.
I received one report of a cereal rye control failure this week as temperatures warmed and rye growth resumed. Some follow-up questions and photos showed that concerns regarding burndown applications during cool, early-spring conditions have merit.
The focus has been on planting corn, but don't forget about the weeds. In addition to getting preemergence herbicides applied to the corn acres, burndown applications on no-till soybean fields should be a top priority. Timely application of burndown herbicides simplifies controlling many winter annual weeds, particularly horseweed (marestail). Once the stems on horseweed belong to elongate the probability of successful control diminishes rapidly.
Weather conditions this past week (April 13-17) kicked off corn planting for many farmers throughout the state. As soil temperatures rose above 50°F towards the end of the week, farmers were able to dive into fieldwork and planting corn.
See statewide planting progress and field conditions across Iowa here.
