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Soybean Crop Staging and
Herbicide Application Timing

Author: Mike Cowbrough - Weed Specialist/OMAFRA Guelph
Creation Date: 06 June 2007
Last Reviewed: 06 June 2007

What happens when there is an abundance of weeds at or near the maximum leaf-stage for effective control with an herbicide, yet your soybean crop is not at the growth stage specified on the product label? As a producer you are now faced with the ultimate risk/benefit analysis. Do I sacrifice the level of weed control, or risk possible crop injury?

Is there any risk in applying an herbicide before its specified label stage?
  1. In general, yes. The application stage specified on the label is there because:
    There is an increased chance of crop injury when the product is applied outside the application window, or
  2. Research on the product was conducted at the stages specified on the label. Therefore crop tolerance to applications outside those stages is unknown.

In any case, there is a level of risk to applying an herbicide before the recommended crop stages Refer to Table 1 for a breakdown of ideal crop application stages. The decision to "pull the trigger" will ultimately come down to what growth stage the weeds are at. For example, lets assume you scout your field and the soybean crop is at the cotyledon stage, yet common lamb's-quarters is at the 4-6 leaf stage and at extremely high densities. In this situation, it is probably more advantageous to spray a suitable herbicide as soon as possible versus waiting until the specified crop stage. Refer to Table 2 for a list of maximum weed leaf stages where a specific herbicide will still be effective.

Table 1. Soybean application stages for a number of post-emergent herbicides.

  • Critical Weed-Free Period in Soybean: First Trifoliate to Third Trifoliate Stages
  • Basagran Forte, CleanSweet: Unifoliate to Second Trifoliate Stages
  • Blazer, Classic, Firstrate Pinnacle, Reflex: First Trifoliate to Third Trifoliate Stages
  • Assure 11, Excel Super, Poast Ultra, Pursuit, Select, Venure L: Cotyledon to Third Trifoliate Stages

 


Critical Weed-Free Period in Soybean stages


Bottom Line: Don't make the decision on your own

If you are in a situation where there are advanced weeds in a soybean crop not yet at the labeled application stage, have your agronomist and territory sales representative scout the field with you so that you can make the best decision together.

Table 2. Maximum weed leaf stage whereby the herbicide is still effective according to the product label

Broadleaf Herbicides
Product
Maximum Weed Leaf Stage Controlled
Basagran Forté
(0.9 L/ac)
4 leaf giant ragweed, velvetleaf
6 leaf common ragweed, hairy galinsoga, hairy nightshade, purslane, shephard's-purse, stinkweed
8 leaf lamb's-quarters
10 leaf cocklebur, flower-of-an-hour, lady's-thumb, wild mustard
Blazer (1 L/ac)
 2 leaf lamb's-quarters
4 leaf cocklebur
6 leaf eastern black nightshade, redroot pigweed
8 leaf
common ragweed, lady's-thumb
10 leaf jimsonweed, wild mustard

Classic (14 g/ac)
 4 leaf velvetleaf
6 leaf common ragweed
8 leaf dandelion, redroot pigweed, yellow nutsedge
bud stage annual and perennial sowthistle
Firstrate (8.5 g/ac)
4 leaf jimsonweed, velvetleaf
6 leaf giant ragweed
8 leaf cocklebur, common ragweed
Pinnacle (2.2 g/ac)
8 leaf redroot pigweed, lady's thumb, wild mustard
Pinnacle (3.2 g/ac)
4 leaf velvetleaf
6 leaf lamb's-quarters
Grass and Broadleaf Herbicides
Pursuit (125 mL/ac)
 2 leaf cocklebur*, eastern black nightshade, ragweed*, old witchgrass, wild buckwheat, wild mustard
4 leaf green foxtail, yellow foxtail
6 leaf barnyardgrass*
8 leaf velvetleaf
12 leaf redroot pigweed


Grass Herbicides
Product Maximum Weed Leaf Stage Weeds Controlled
Assure II
(150 ml/ac)		 6 leaf volunteer corn
to early tillering green foxtail, volunteer cereals
Assure II
(200 ml/ac)		 to early tillering barnyard grass, fall panicum, old witchgrass, proso millet, yellow foxtail
Assure II
(300 ml/ac)		 6 leaf quackgrass
Excel Super
(268 ml/ac) 		6 leaf
barnyard grass, crabgrass, fall panicum, green foxtail, yellow foxtail, witchgrass, proso millet, volunteer corn
Poast Ultra
(130 ml/ac)		 6 leaf barnyard grass, crabgrass, fall panicum, green foxtail, yellow foxtail, witchgrass, proso millet
Poast Ultra
(190 ml/ac) 		6 leaf volunteer cereals
Poast Ultra
(450 ml/ac)		 3 leaf quackgrass (supression)
Select (50 ml/ac) 4 Leaf green foxtail, yellow foxtail, volunteer cereals
Select (50 ml/ac) 6 Leaf barnyard grass, fall panicum, witchgrass, proso millet, volunteer corn
Select (75 ml/ac) 6 Leaf
volunteer cereals, quackgrass (supression)
Select (150 ml/ac) 6 leaf
quackgrass
Venture L
(320 ml/ac) 		4 leaf barnyard grass, volunteer corn (only needs 240 ml/ac rate for control of volunteer corn)
Venture L
(400 ml/ac)		 4 leaf

crabgrass, green foxtail, yellow foxtail, fall panicum1, old witch grass1, proso millet1 quackgrass (supression)
Venture L
(800 ml/ac)		 5 leaf quackgrass, wirestem muhly


*Use the 168 ml/ac rate when targeting this species.
1Can be controlled up to the 5 leaf stage.

For more information:
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Local: (519) 826-4047
E-mail: ag.info.omafra@ontario.ca