The emergence pattern of giant ragweed varies every spring. This spring giant ragweed started emerging in mid-March and continues to this day.   The book says that giant ragweed mostly emerges in the early spring.  A few years ago we collected giant ragweed seed from plants in a fence row here at the station. We also collected some giant ragweed seed from a soybean field in Lake County.  We planted them in a field at the station in November.  That next spring the giant ragweed seed sourced from the fence row all emerged within 7 days from February 28 to March 4.  The giant ragweed we collected from the Lake County soybean field started emerging February 28^th and continued to June 3^rd.  Moreover, the Lake County giant ragweed was highly resistant to glyphosate while the giant ragweed from the station could still be controlled with glyphosate.  Take home point is that we have developed a new biotype of giant ragweed in our row crop fields that is not only glyphosate resistant but has expanded its’ emergence pattern making it a much harder target.

                Since giant ragweed starts emerging so early, it is essential that the crop emerges clean.  Burndowns that include dicamba plus glyphosate typically work well.  For larger giant ragweed, rates higher than 8 oz/A are needed to give good control.  Tankmixing an ALS inhibitor in with that burndown can help curtail any regrowth that may occur.  Hands down the most effective herbicide on giant ragweed is Ignite.  However, during the burndown timing if is often cold which greatly reduces the effectiveness of Ignite.  Gramoxone Inteon is a more consistent cold weather burndown option, but if giant ragweed is over 6” tall you will see regrowth.  

                Giant ragweed now is a major threat to emerge in crop as well.  I have yet to find a pre applied herbicide that provides good control of this weed.    FirstRate applied at 0.6 oz/a plus PowerMax at 44 oz/a applied post emergence is the only application that has provided good control in RR soybeans. Ignite in LL soybeans or cotton would also be a very good way to manage this weed.