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Issue No. 16, Article 5/July 23, 2010

Considerations for Late-Season Herbicide Applications in Soybean

Given this season's frequent and excessive precipitation, many Illinois soybean fields have experienced later-than-normal applications of postemergence herbicides. Coupled with the seemingly ubiquitous growth of waterhemp in Illinois soybean fields, this reality suggests that applications may continue throughout July. But producers should be aware that late-season herbicide applications are not without some risks.

First, larger weeds can be expected to be more difficult to control than smaller ones because the plants are older and spray coverage can be limited. Application rate, volume, and spray additives are important factors to keep in mind, especially if you are trying to achieve good coverage on larger weeds. The possibility of herbicide drift that can injure sensitive vegetation is ever-present.

Almost all postemergence soybean herbicides specify on their labels a preharvest interval or a soybean developmental stage beyond which applications cannot be made (after bloom, for example). Labels of some products may indicate both. Preharvest intervals indicate the amount of time that must elapse between herbicide application and crop harvest. Failure to observe the preharvest interval may result in herbicide residue levels in the crop that exceed established limits. Table 1 presents information on preharvest intervals and grazing restrictions for a number of postemergence soybean herbicides.

Table 1. Preharvest intervals and grazing restrictions for various postemergence herbicides used in soybean.

Herbicide

Preharvest interval

Forage or grazing?

Aim EW

Broadcast: V10 soybean
Harvest aid: 3 days

No

Assure II

80 days

No

Basagran

30 days?

Yes, after 30 days

Cadet

60 days

No

Classic

60 days

No

Cobra or Phoenix

45 days

No

Extreme

85 days

No

FirstRate

65 days

Yes, after 14 days

Flexstar, Flexstar GT

45 days

No

Fusilade DX

60 days

No information on label

Fusion

Prebloom

No

Ignite

70 days

No

Roundup PowerMaxa

Broadcast: through R2
Harvest aid: 14 days

Yes
Yes, after 14 days

Harmony GT XP

60 days

Yes, after 7 days

Poast or Poast Plus

75 days

Hay

Prefix

90 days

No

Pursuit

85 days

No

Raptor

Prebloomb

No information on label

Resource

60 days

No

Scepter

90 days

No

Select or SelectMax

60 days

No

Sequence

90 days

No

Storm

50 days

No

Synchrony XP

60 days

No

Ultra Blazer

50 days

No

Warrant

R2

No

aData, taken from the Roundup PowerMax label, are for broadcast applications in glyphosate-resistant soybean varieties. Intervals change for applications (spot treatment and preharvest) made to nonglyphosate-resistant soybean varieties. Forage and grazing allowances can vary among glyphosate-containing products. Consult the respective glyphosate product label for specific information on forage and grazing restrictions.
bThe Raptor label indicates there is no preharvest interval for any crop, but applications must be made before soybean bloom. 

Nearly all herbicide labels (soil-applied and postemergence) have rotational crop intervals that specify the amount of time that must elapse between herbicide application and planting a rotational crop. This becomes particularly important with late-season applications. Such intervals are established to reduce the likelihood that sufficient herbicide residues will persist in the soil, which could adversely affect the rotational crop. Some herbicide rotational restrictions are based solely on time, while other factors, such as soil pH and the amount of precipitation received after application, can influence the length of the intervals for other products.

Much of the soybean crop across Illinois is in bloom, and a smaller percentage has begun setting pods. Many postemergence soybean herbicide labels caution about making applications after the crop has begun to bloom, as crop injury during this time could reduce yield.--Aaron Hager

Author:
Aaron Hager

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