No. 15 Article 4/July 13, 2012

Precautions with Late-Season Herbicide Applications

Performance of many foliar-applied soybean herbicides has been challenged by the adverse environmental conditions common across much of Illinois. Weeds that survive an initial herbicide application are often resprayed, with the same herbicide or a different one. The likelihood of controlling larger, moisture-stressed weeds at this point in the growing season continues to decline, for myriad reasons. Apart from that, be reminded that late-season herbicide applications increase the probability that herbicide residues will persist long enough to injure sensitive rotational crops.

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

Soil moisture is often the most critical factor governing the efficacy and persistence of soil-residual herbicides. Many herbicides are degraded in soil by the activity of soil microorganisms, and populations of these microorganisms can be greatly depressed when soil moisture is limited. Also, dry soils can enhance herbicide adsorption to soil colloids, rendering the herbicide unavailable for plant uptake and degradation by soil microbial populations. Some herbicide rotational intervals are increased if a specified amount of precipitation is not received by a certain date.

Please keep in mind that the labels of almost all postemergence soybean herbicides indicate a preharvest interval or a soybean developmental stage beyond which applications cannot be made. Labels of some products may indicate both a developmental stage (before soybean bloom, for example) and a preharvest interval. Preharvest intervals indicate the time that must elapse between the herbicide application and crop harvest. Failure to observe the preharvest interval may result in herbicide residue levels in the harvested portion of the crop exceeding established limits. Also, livestock grazing or foraging treated soybean is prohibited by the labels of many postemergence soybean herbicides. Table 1 details preharvest intervals and grazing restrictions for a number of postemergence soybean herbicides.--Aaron Hager

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

Herbicide

Preharvest interval

Forage or grazing ok?

Aim EW

Broadcast: V10 soybean
Harvest aid: 3 days

No

Assure II/Targa

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/Tackle

85 days

No

FirstRate

65 days

Yes, after 14 days

Flexstar/Rhythm

45 days

No

Flexstar GT

45 days

No

Fusilade DX

60 days

No information on label

Fusion

Prebloom

No

Liberty

70 days

No

Roundup PowerMax1

Broadcast: through R2
Harvest aid: 14 days

Yes
Yes, after 14 days

Harmony SG/Unity2

60 days

Yes, after 7 days

Poast or Poast Plus

75 days

Hay

Prefix

90 days

No

Pursuit

85 days

No

Raptor

Prebloom3

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

1Data, 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 non-glyphosate-resistant soybean varieties. Forage and grazing allowances can vary among glyphosate-containing products. Consult the glyphosate product label for specific information on forage and grazing restrictions.
2Unity does not allow treated areas to be grazed or fed as forage.
3The Raptor label indicates there is no preharvest interval for any crop, but applications must be made before soybean bloom.

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