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Corn And Soybean Herbicides And Rotation To Cover Crops

Sep 15, 2016
By Dwight Lingenfelter
Program Development Specialist
 
With silage harvest well underway and corn grain and soybean not far behind, a number of farmers are considering cover crop establishment this fall. Remember that some herbicides can persist and potentially influence successful cover crop establishment.
 
Both the 2015/16 Agronomy Guide (Table 1.10-6) and the 2016 Mid-Atlantic Weed Management Guide (Table 1-5), contain a table titled “Corn and soybean half-lives, cash crop restrictions, and their potential to injure fall cover crops” that includes the major corn and soybean herbicides restrictions and guidelines.  Remember that cash crop rotation restrictions may be due to the concern for herbicide residues accumulating in forage or feed rather than carryover injury.  Cover crops that are not harvested can be planted after any herbicide program, but the grower assumes the risk of crop failure.
 
Two factors become important when trying to predict the potential for carryover injury to rotational crops.  1.) How long does the herbicide last or persist in the soil assuming that it has soil activity, and 2.) 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, etc., 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 (i.e. clovers) 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 herbicide residue is necessary to cause injury and the herbicide persists. Quite often, small seeded legumes and grasses like the clovers and ryegrass and mustard species like canola are very sensitive to some herbicides.  The good news is that for most winter cereals like rye or wheat, most products are fairly safe.  The following table includes some “highlights” for products that can have the potential to be more problematic.  For a more complete listing, refer to the Penn State Agronomy Guide or the Mid-Atlantic Weed Management Guide. 
 
Selected Corn and Soybean herbicide half-lives and their potential to injure fall cover crops.  Herbicides included below are generally more restrictive.
 
Corn Herbicides
HerbicideActive IngredientNormal Rate/acreHalf life                (days)Fall Cover Crops- OK to plantFall Cover Crops- Concern forOther
Atrazine 4LAtrazine11-2 qt15-90Sorghum speciesCereals, ryegrass, legumes, and mustardsMore persistant in high pH soils (>7).
Rates < 1 lb/acre can allow more flexibility
Balance Pro 4L
Balance Flexx 2L
Isoxaflutole3 fl. oz
6 fl. oz
50-120Fall cereal grainsRyegrass, legumes, and mustards15 inches of cumulative precipitation required from application to planting rotation crops except soybean, barley, wheat, sorghum, and sunflower
Callisto (includes Acuron, Instigate, Lumax, Lexar, Halex GT, Realm Q, Revulin Q, Solstice, etc.Mesotrione3-6 fl. oz10-50All grassesSmall seeded legumes, mustardsSequential applications (PRE fb POST) increase the potential for injury.
Corvus 2.63SCIsoxaflutole+
thiencarbazone
5.6 fl. oz50-120Wheat, triticale, ryeSmall seeded legumes, mustards, sorghum15 to 30 inches of cumulative precipitation from application to planting for sensitive crops
Python 80WDG (Hornet and Surestart)Flumetsulam1 oz.14-120Cereal grainsSmall seeded legumes, mustards, and annual ryegrassCover crops and forage grasses are restricted for 9 mo.
Simazine 4L (Princep)Simazine1-2 qt.60Sorghum speciesCereals, ryegrass, legumes, and mustardsSoil pH >7
Stinger 3S (Hornet and SurestartClopyralid5 oz.40All grassesSmall seeded legumes 
 
1A  recent study conducted by Mueller et al. at the University of TN shows that the half-life for atrazine in soils where it has been frequently used over a number of years is considerable less than expected with degradation rates averaging 6 times faster in soils with previous atrazine use.  The paper is currently under review in Pest Management Science.
 
Soybean Herbicides
HerbicideActive IngredientNormal
Rate/acre
Half life 
(days)
Fall 
Cover 
Crops
-OK to 
plant
Fall 
Cover 
Crops 
-Concern 
for
Other
Classic 25DF (Canopy, Envive, Authority XL, Fierce XLT, Valor XLT, Synchrony, Trivence, etc.)Chlorimuron0.5-2 oz.40Cereals and ryegrassSmall seeded legumes, mustards, sorghumMore persitent in high pH soils (>7) and with higher soil applied rates
Pursuit 2S (Optill) imazethapyr4 fl. oz. 60-90 Wheat, triticale, rye, alfalfa, clover Oats, sorghum, mustards Any crop can be planted 40 months after Pursuit application
 Python 80WDG flumetsulam 1 oz. 14-120 Cereal grains Small seeded legumes, mustards, and annual ryegrass Cover crops and forage grasses are restricted for 9 mo.
 Reflex 2E/Flexstar 1.88E/(Prefix) fomesafen 1.5 pt. 100 Cereal grains Small seeded legumes, mustards, sorghum Since fomesafen is often applied postemergence, soil activity can surprise users