Bill Johnson and
Glenn Nice

Purdue University Weed Science Extension
Weather Related Weed Management Items
Created: 5/20/2010     www.btny.purdue.edu/weedscience
 

Recent wet, rainy weather has created some weed management challenges for Indiana growers.  In this article we will hit on a few key points to consider based on current challenges.

Delayed weed control in corn.  Indiana corn growers rely heavily on atrazine premixes in corn.  Rain will not have completely washed all of the herbicide away, but may have compromised overall activity.  Scout fields as soon as possible to determine if weeds are escaping.  Obviously giant ragweed is a big concern, but wet conditions and dilution of atrazine can result in failures to control cocklebur, sunflower, velvetleaf, burcucumber, morningglories, and waterhemp.  If corn is less than 12 inches tall and you haven’t used all of the atrazine allowed by the label, it would be wise to add atrazine to the other postemergence herbicides being applied to corn to control the above mention weeds and provide some soil residual activity.  If corn is more than 12 inches tall, you cannot use any additional atrazine. 

Weed escapes in large corn in all of Indiana.  When the warm weather hits us late this week and through the weekend, corn will grow rapidly.  Postemergence corn herbicide options become limited when corn is 12 inches tall, and really limited on corn at the V6 or later growth stage.  Also keep in mind that large weeds are much more difficult to control.  To avoid crop injury on corn under other stresses, try to keep spray out of whorls, especially with ALS inhibitors and contact products.  See table 6 in the weed control guide for the height restrictions of postemergence corn herbicides.

 
 
Figure 1. Maximum Crop Size for Postemergence Corn Herbicides [Taken from the 2010 Weed Control Guide for Ohio and Indiana WS-16].

Herbicide Maximum Crop Size

2,4-D Amine Broadcast up to 8-inch corn; directed spray before tassel stage.
2,4-D Ester Broadcast up to 8-inch corn; directed spray before tassel stage.
Accent Q, NIC-IT Broadcast up to 6 collars or 20-inch corn; directed spray up to 10 collars or 36-inch field corn.
Aim Apply up to 8-collar corn.
Atrazine Apply before corn is 12 inches tall.
Basagran  
Beacon Broadcast 4 to 20-inch corn; directed spray before tassel emergence.
Bromoxynil Apply before tassel emergence.
Bromoxynil + arazine Apply before corn is 12 inches tall.
Cadet Broadcast or directed from 2 collar stage up to 48 inches tall, and before tassel emergence.
Callisto Apply up to 30-inch or 8-leaf corn.
Capreno Apply broadcast from V1 to V6 corn; directed spray up to V7 corn.
Celebrity Plus Broadcast up to 6 collars or 24-inch corn.
dicamba Do not apply with COC when corn height exceeds 5 inches. Broadcast up to 5th-leaf stage or 8-inch corn; directed spray up to 36-inch corn.
dicamba + atrazine Do not apply with COC when corn height exceeds 5 inches. Apply broadcast up to 5-leaf stage or 8-inch corn.
Halex GT Broadcast up to 30-inch or 8-leaf corn.
Hornet Broadcast up to 20-inch corn or 6 collars; directed spray up to 36-inch corn.
Ignite Broadcast up to 5-collar corn.
Impact Apply broadcast or directed up to 45 days before harvest.
Laddok Apply before corn is 12 inches tall.
Laudis Broadcast up to V8 corn.
Laudis + atrazine Broadcast up to 12-inch corn
Lightning Broadcast up to V6 or 20-inch corn; directed spray up to 45
days before harvest.
Northstar SURF, COC or MSO up to 12-inch corn. Only SURF between 12 and 36-inch corn. Broadcast 4 to 20-inch corn; directed spray up to 36-inch corn.
Option Apply broadcast in the V1 to V6 stage; directed spray up to 36-inch corn.
Permit/Sandea Halomax Apply through layby stage of corn.
Resolve Broadcast up to 12-inch or 6 collar corn.
Resolve Q Broadcast up to 20-inch or 6 collar corn.
Shotgun Apply before 12-inch corn.
Spirit Broadcast 4 to 20-inch corn; directed spray up to 24-inch corn or after 6 collar corn.
Starane Broadcast up to the V5 stage; directed spray after the V5 stage.
Status Broadcast from 4 to 36-inch corn (rates up to 5 oz/A)
Steadfast Q Broadcast up to and including 6 collars or 20-inch corn.
Steadfast ATZ Broadcast up to and including 6 collars or 12-inch corn
Stinger Up to 24-inch corn.
WideMatch Broadcast up to the V5 stage; directed spray after the V5 stage.
Yukon Apply broadcast or directed up to 36-inch corn.

 
 

Big Bad Broadleaves.  Be on the lookout for giant ragweed and burcucumber.  Every year is a good one for giant ragweed, and frequent rains are good for burcucumber.  Both weeds have relative long emergence patterns and herbicides with both foliar and residual activity are needed.  If corn is not 12 inches tall, consider adding some atrazine to the postemergence product to extend the residual window of activity on these weeds.

How do I pick a postemergence herbicide for corn? Going on the assumption that most of the corn grown in Indiana is Roundup Ready or Liberty Link, we assume growers will use glyphosate or Ignite as the base herbicide. Many different herbicides can be tankmixed with glyphosate or Ignite to broaden the spectrum of weeds controlled and provide residual activity in the soil. The best advice we can give you is to consult table 4 on page 28 of the weed guide to help in selecting tankmix partners for weeds you are trying to control.

 
 
Table 2. Weed Response to Postemergence Herbicides in corn. [PDF]
 
 

Large weeds in fields intended for soybean.  We have observed some fields (both planted and unplanted) with dandelions flowering for the second or third time, cressleaf groundsel in excess of 3 feet tall, common pokeweed, hemp dogbane, and milkweed in excess of 2 feet tall, and flowering thistles.  In no-till fields that have not yet been planted, increase glyphosate rates for burndown to at least 1.5 lb ae/A to improve control of large weeds.  Wait at least 1 day before pulling a drill or planter through the field to allow the weeds to absorb glyphosate.  Since it is late in the spring, if marestail is present, add FirstRate or Classic or Sharpen to glyphosate. If you know the marestail is ALS resistant, add Sharpen to glyphosate. 

In fields that will conventionally tilled, consider using no-till instead.  It is difficult to completely control large weeds with spring tillage tools other than plows. Disks, field, cultivators, and soil finishers will knock down large weeds which then recover and become difficult to control with herbicides. If you do conventional tillage and have weeds in excess of 1 foot tall, apply glyphosate prior to tillage to ensure effective control. Wait at least one day between glyphosate application and tillage for annual weeds, and 2 to 3 days for perennial weeds.  The good news is that glyphosate products work much more rapidly when air temperatures are high and knockdown activity will be more rapid than we are used to with early spring applications.

 

 

 

 
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