• Large seeded broadleaves like velvetleaf and giant ragweed tend to germinate much earlier than some of our more problematic weeds like waterhemp and Palmer amaranth.
• If left unmanaged for 2-4 weeks, giant ragweed can compete with corn for light, nutrients, and water and reduce yield by roughly 13%.
• Acres that are planted with no preemergence residual will need special attention sooner to prevent yield losses.
• In the event that control isn’t achieved with a systemic herbicide, check the stem for the presence of stem boring insects.

With the excessive rainfalls, this spring season is off to a slow start in most areas, while other areas have been able to slowly get some of the crop planted between rainfall events. With untimely rainfalls, the preemergence residual may have not been applied leaving a crop in the ground with no herbicide on to control the weeds.

In recent travels across the region, I have noticed a lot of giant ragweed and some velvetleaf emerging. The large seeded broadleaf weeds tend to germinate much earlier than some of our more problematic weeds like waterhemp and Palmer amaranth. Giant ragweed seeds germinate with an accumulation of ~150 growing degree units (GDUs) while velvetleaf seeds will germinate with an accumulation of ~150-300 GDUs. Keep in mind that corn will germinate with an average accumulation of between ~90 and 120 GDUs and soybeans with an accumulation of ~130 to 150 GDUs. This will cause early season crop and weed competition, and if left unmanaged, the crop will ultimately be out-competed resulting in significant yield losses. According to Purdue University sources, giant ragweed (Image 1) can compete with corn for light, nutrients, and water and reduce yield by roughly 13% if left unmanaged for 2-4 weeks. While in soybeans giant ragweed control is more time sensitive, if left unmanaged for four weeks giant ragweed can reduce yields by 25%. Similarly, velvetleaf (Image 2) has the potential to reduce soybean yields by 14-27% if the weeds emerge around the same time as the crop and compete for resources (University of Missouri data).

If the preemergence window to control these two large-seeded broadleaf weeds has already passed or a pre-application wasn’t planned, make sure to utilize effective postemergence products. Selecting the correct product as well as applying the postemergence herbicide before the weeds get too large will ensure minimum weed competition. Lastly, if the postemergence herbicide application fails to control the weeds, check the stem for boring insects or “tunneling”. In the last few years we have noticed that stem borers can occasionally be the cause of herbicide failures because they may interfere with herbicide translocation in the plant.

With the spring conditions so far, it is imperative that we check the fields for early season weeds like giant ragweed and velvetleaf and make sure we have a plan to control them and minimize yield loss. If you have questions about control of these two competitive weed species, contact your local FS Crop Specialist.