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Weed Emergence Sequences

April 20, 2001
Last year we enclosed an 8-1/2 inch by 11 inch glossy bulletin titled "Weed Emergence Sequences" in the Bulletin. This year for those who have access to this newsletter on the Web, a link to "Weed Emergence Sequences" bulletin can be accessed here.

This bulletin helps focus our attention on emergence sequences of different weed species throughout the growing season and the duration of their emergence. This bulletin can also be used as an identification guide of 16 common weed species in the Midwest. Each weed species is characterized by an emergence date relative to growing degree-days (GDD) as well as the length of time (weeks) each species emerges.

You may wonder why it is important to know when these different weed species emerge. Knowing when a species emerges and the length of time that it emerges helps you make good decisions on how to manage that particular weed species. A good example is waterhemp. Looking at the "Weed Emergence Bulletin," we see that waterhemp is one of the later-emerging species. It is characterized to emerge after corn emergence and when there are more than 350 GDD. It can also be characterized as emerging over a prolonged period (8 to 10 weeks). Knowing these characteristics about waterhemp can help refine management decisions. With its later emergence date, applying a soil-applied herbicide closer to planting can often extend the control of waterhemp later into the growing season compared with applying the same herbicide several weeks prior to planting. Also, understanding that waterhemp emerges over a prolonged period during the season provides an opportunity to select a sequential program to improve overall control. Frequently, a timely postemergence herbicide application and/or cultivation will help control later-emerging waterhemp that may have escaped the residual activity of a soil-applied herbicide. Another good example is eastern black nightshade. Nightshade is a weed that many producers often don't realize they have until the soybeans start to mature and drop leaves. Knowing when to look for this weed species may help producers plan a timely postemergence herbicide application to control this troublesome species.

So where are we currently in regards to weed emergence in Illinois? Over the past week we have seen the emergence of many of the Group 1 weed species, including giant ragweed, common lambsquarters, and Pennsylvania smartweed. As corn planting is beginning in earnest we need to take into consideration that many of these species will be present at planting and should be considered when planning burndown strategies for no-till fields. For burndown efficacy ratings, refer to Table 6, issue no. 2 of the Bulletin. In the next couple of weeks we will start to see the emergence of a number of the Group 2 weed species, including common ragweed and giant foxtail in certain parts of the state. So during planting over the next couple of weeks keep your eyes open to what weeds have emerged and follow along with the bulletin. Knowing what weeds are present and which weeds will soon emerge will help you make good decisions on how to manage a number of these species.--Christy Sprague and Aaron Hager

Author: Aaron Hager Christy Sprague

The Pest Management and Crop Development Bulletin
Executive Editor: Kevin Steffey, Extension Entomologist

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