#2. Does your answer change if question #1 is changed to “What is the best timing to manage weeds in corn?”

Answers: 

 

Question 1. The key to question #1 is that “control” means “to kill”. If the herbicide (such as glyphosate) lacks residual control and is able to control large weeds, the best answer is “C”. Spraying later when weeds are larger can result in better control because fewer weeds would emerge after the herbicide application. In contrast, weeds sprayed with glyphosate early (answer A) should have good initial control, but weeds emerging later in the season would reduce the level of control by the end of the season. Of course, some herbicides may not be able to control larger weeds and they should be applied before weeds exceed the size stated on the label for control.

 

Question 2. The answer “A” is wrong. There is definitely a difference between controlling weeds and managing weeds when it comes to profit. Managing weeds is “killing weeds with purpose” and that purpose is to protect crop yield. The correct answer to this question is “B - Yes”. The best timing to manage weeds postemergence in corn is before they exceed 4 inches. For example, when weed control was delayed for 8 days past the 4 inch stage in 2006 and 2007, corn yields were reduced 6 to 8%, costing $60 to 90/a at a corn price of $5/bu (Figure 1). 

Since every field can’t be sprayed on the exact day when weeds reach 4 inches, is it better to err and spray early or spray late? 

Of the two risks, the early-season weed competition will likely cause more yield loss than weeds emerging late in the season. 

Figure 1. Corn yields when weeds were controlled at a 4 inch weed height compared to when weeds were controlled at a 12 inch height. Application dates differed by 8 days at Arlington.

 

Action: Start postemergence herbicide applications early rather than increasing the risk of yield loss with delayed applications. Consider adding residual herbicides with early applications if the primary postemergence herbicide lacks residual activity.