Adapted from: http://extension.psu.edu/plants/crops/news/2013/07/herbicides-persistence-and-rotation-to-cover-crops-ii-soybean?utm_campaign=Field+Crop+News&utm_medium=email&utm_source=newsletter

Adapted by: Dr. Darren Robinson, University of Guelph, Ridgetown Campus and Kristen Callow, OMAF and MRA, Harrow

The two factors that are important when trying to predict the potential for carryover injury to rotational crops are:

  • how long does the herbicide last or persist in the soil assuming that it has soil activity; and,
  • how sensitive is the rotational crop to potential herbicide residues?

Herbicides with shorter half-lives (the time it takes for 50% of the active ingredient to dissipate) are always less of a concern. Of course several factors influence the rate of dissipation such as rainfall, soil texture and soil pH, etcetera; however, most guidelines generally are for "normal" conditions (e.g. not severe drought). In general, products with a 4 month or less rotation restriction for the species of interest, close relative, or sensitive species should pose little problem. These products typically have half-lives of less than 30 days. Species sensitivity can play a role if only a small amount of residue is necessary to cause injury and the herbicide persists. Quite often, small seeded legumes like the clovers, ryegrass, buckwheat, and mustard species like canola are very sensitive to some herbicides.

The following tables provide some persistence and carryover information for some commonly used herbicides in soybean (Table 1) and corn and soybean herbicides (Table 2). Some of this information is our best guess.

Table 1. Common soybean herbicides, their half lives, and potential to injure subsequent crops.

Herbicide Active Ingredient Normal Rate/Ac Half Life (days)* Cash Crop Restrictions Cover Crops:
OK to Plant		 Cover Crops: Concern For Other
Assure II quizalofop 0.15-0.3 L/ac 60 Most BLs OK OK to plant most broadleaves Concern for all grasses if less than 120 days or at high rates Plant anything after 120 days
Classic chlorimuron 14 g/ac 40 12-30 months for small seeded legumes Cereals and ryegrass Small seeded legumes, mustards, sorghum, buckwheat More persistent in high pH soils (> 7) and with higher soil applied rates
FirstRate cloransulam 8.5 g/ac 8-33 Four months to wheat, 9 mo. to alfalfa, corn, sorghum and oats, 12 mo. for barley, and 18 mo. for tobacco Wheat, triticale, rye Small seeded legumes, mustards, sorghum, buckwheat The restriction for transplanted tobacco is 10 months. Sugarbeet and sunflower have a 30 month restriction.
Pinnacle SG thifensulfuron 3.3-4.8 g/ac 12 Any crop can be planted 45 days after application No restriction for wheat, barley and oats None with 45 day waiting interval  
Pursuit imazethapyr 126-168 mL/ac 60-90 Recrop restrictions range from 4 to 18 mo. Wheat, triticale, rye, alfalfa, clover Oats, sorghum, mustards, buckwheat Any crop can be planted 40 months after Pursuit application
Reflex fomesafen 0.4 L/ac 100 Recrop intervals range from 4 to 18 months Cereal grains Small seeded legumes, mustards, sorghum Since fomesafen is often applied postemergence, soil activity can surprise users
Select clethodim 0.05-0.15 L/ac 3 Most broadleaves OK All broadleaves None assuming at least 30 days Plant anything after 30 days
Sencor 75 DF metribuzin 0.3-0.6 kg/ac 14-60 Recrop restrictions range from 4 to 12 months Cereal grains and ryegrass Slight risk for small seeded legumes and mustards Nonfood/feed winter cover crops allowed
Valtera flumioxazin 56 g/ac 12-20 Recrop restrictions up to 10 mo. for no-till alfalfa, clover and 12 mo. for no-till canola All grasses Small seeded legumes and mustards Based on the half-life, all nonfood/feed winter cover crops should be OK

*Herbicide half-life estimates derived from the WSSA Herbicide Handbook, and from other scientific literature.

Table 2. Herbicides used in both soybean and corn, their half lives, and potential to injure subsequent crops.

Herbicide Active Ingredient Normal Rate/Ac Half Life (days)* Cash Crop Restrictions Fall Cover Crops:
OK to Plant		 Fall Cover Crops: Concern For Other
Glyphosate glyphosate 1-2 L/ac 47 No restrictions preemergence All None Glyphosate does not have soil activity at normal use rates
Liberty glufosinate ammonium 1 L/ac 7 No restrictions for canola, corn and soybean. Small grains have a 70 day restriction All Food or feed residues rather than crop injury may be a concern Glufosinate does not have soil activity at normal use rates
Broadstrike RC flumetsulam 25 g/ac 14-120 Cash crop restrictions from 4 months for alfalfa and cereals to 26 months for canola Cereal grains Small seeded legumes, mustards and annual ryegrass, buckwheat Cover crops and forage grasses are restricted for 9 months
Eragon SG saflufenacil 14.4-28.4 g/ac 7-35 Any crop can be planted 4 months after application All None This product has been reported more persistent in Western Canada
Frontier dimethenamid 305-390 mL/ac 20 4 months for cereal grains and anything the following spring Most crops should be fine Food or feed residues rather than crop injury may be a concern Nonfood/feed winter cover crops should be OK after corn harvest
Prowl H2O pendimethalin 1.68 L/ac 44 Wheat and barley after 4 months. Other rotation crops the following year Cereal grains Small seeded legumes and annual ryegrass Nonfood/feed winter cover crops should be OK

*Herbicide half-life estimates derived from the WSSA Herbicide Handbook, and from other scientific literature.

Original article written by:

  • William Curran, Professor of Weed Science, Penn State University, wcurran@psu.edu
  • Dwight Lingenfelter, Program Development Specialist, Penn State University, dxl18@psu.edu