Herbicide Residues and Re-cropping to Potatoes

Herbicide residues from previous crops grown in rotation with potatoes can affect yield and tuber quality factors such as size, shape and specific gravity. The vigor of seed produced from fields with an herbicide residue can also be negatively impacted. The herbicide residue is concentrated in the tuber resulting in poor seed performance.

All farmers should keep detailed records of herbicide use for each field. When leasing or purchasing new land, review the field history for potential herbicide residues.

Cultural and environmental factors, as well as the properties of the herbicide, influence the persistence of herbicide residues in the soil. These factors (discussed below) are highly variable between fields and growing seasons. Soil texture and moisture are the two most important factors affecting the re-cropping interval.

Herbicides that could potentially harm potatoes.

Trade Name-Manufacturer	Group	Active Ingredient	Potato Re-cropping Restriction
Amitrol 240 - Nufarm	11	amitrole	Residues may affect crops for 8 months following application.
Atrazine - Syngenta Laddok - BASF Primextra – Syngenta	5	atrazine	Sensitive crops such as potatoes may be affected 22 OR MORE months after application.
Curtail M - Dow AgroSciences Prestige - Dow AgroSciences Prevail - Dow AgroSciences Eclipse – Dow AgroSciences FlaxMax Ultra – BASF	4	clopyralid + MCPA	Do not grow potatoes until 22 months after application.
Lontrel – Dow AgroScience	4	clopyralid	Do not sow potatoes into fields treated with Lontrel until the third growing season after application.
Banvel II - BASF Dyvel II – BASF Dyvel DS – BASF	4	dicamba	Do not grow potatoes until 22 months following an application if greater than 240 g/acre active or if applications are made after September 1 or if dry weather persists after application.
Muster – Dupont Muster Gold – Dupont	2	etha metsulfuron methyl	Do not grow potatoes until a bioassay is conducted at 22 months after application.
Accent – Dupont	2	nicosulfuron	Do not grow potatoes until a field bioassay is performed.
Amber – Syngenta Unity – Aventis	2, 6	triasulfuron	Note: Not for use in Manitoba. In other provinces do not grow potatoes until a field bioassay is performed.
Velpar – Dupont	5	hexazinone	Insufficient information available. Velpar residues will persist for 2 or more years after application. Do not grow potatoes until a field bioassay is performed.
Attain – Dow AgroSciences	4	fluroxypyr	Do not grow potatoes until 22 months after application.
Assert – BASF	2	imazamethabenz	Do not grow potatoes until a field bioassay is performed.
Odyssey – BASF	2	imazamox & imazethapyr	Do not grow potatoes until a field bioassay is performed.
Pursuit – BASF	2	imazethapyr	Do not grow potatoes until a field bioassay is performed.
Ally Toss-N-Go - Dupont Escort – Dupont	2	metsulfuron methyl	DO NOT APPLY on farms where potatoes are included in the rotation.
Tordon 202C- Dow AgroSciences	4	picloram	DO NOT APPLY on farms where potatoes are included in the rotation.
Accord – BASF	4	quinclorac	DO NOT APPLY on farms where potatoes are included in the rotation.
Princep - Syngenta Simazine - United Agra Products	5	simazine	Do not sow treated field to any crop until 22 months after application.
Sundance – Monsanto Anthem – Monsanto	2	sulfosulfuron	Sundance – For use in Manitoba only. Only apply on soils with greater than 4% organic matter. Allow at least 22 months following application and conduct a field bioassay prior to planting. Anthem – For use on Brown and Dark Brown Soils of Saskatchewan only. Allow at least 22 months following application and conduct a field bioassay prior to planting.
Frontline – Dow AgroSciences Spectrum – Dow AgroSciences	2, 4	florasulam	Do not grow potatoes until a field bioassay is performed.
K2 - Dupont	2	thifensulfuron methyl + tribenuron methyl	Do not grow potatoes until a field bioassay is performed.

The above list may be incomplete because of the registration of new herbicides since this document was published.
Consult with the chemical manufacturers label before making re-cropping decisions.

Re-cropping Interval refers to the length of time between the application of the herbicide and re-cropping
to potatoes. One year after application refers to one cropping year. For example if an herbicide was applied in 2000 and the recommendation is to not re-crop to potatoes for one year after application then do not re-crop until
2002.

A field bioassay is a test strip of potatoes grown to maturity in a field to determine whether herbicide residues will affect a crop. For example, 10 months after a particular herbicide was applied, a test strip of potatoes could be grown to determine whether potato yield and/or grade will be affected 22 months after initial herbicide application.

Select an area of the field that mostly closely approximates the soil conditions of the entire field. Test strips should be planted perpendicular to the direction in which the site was sprayed using standard planting equipment and following normal cultivation practices. Test strips should be of sufficient length to transect several spray swaths and should be at least one planter width wide. Insects and foliar diseases, such as late blight, must be controlled in order to accurately assess the affect of residues on yield and grade. Reliability of the bioassay results will increase with the number and length of test strips planted. The test strips should be examined for possible stand reductions and other symptoms of injury such as reductions in plant vigor, yield and grade. If any injury, stand reduction or yield reduction occurs, do not plant potatoes until another bioassay is conducted the following growing season. It should be noted that yield losses within a test strip might not be measurable unless the yield can be compared to an untreated area adjacent to the treated test strip.

A laboratory bioassay is the growing of plants in a greenhouse or growth chamber to determine if herbicide injury will occur. Soil samples are collected from a field suspected of having an herbicide residue and submitted to a laboratory for a bioassay test. The lab will grow a species of plant known to be sensitive to a specific herbicide or class of herbicides in the sample soil, along with the crop to be planted. If injury occurs to the test bioassay plants, then the potential exists for significant field crop injury. Contact the Alberta Research Council for details regarding soil sampling procedure and prices:
Alberta Research Council
Crop & Plant Management
Hwy 16 A & 75 Street
Vegreville, AB T9C 1T4
Contacts: 780-632-8238 or 780-632-8217

Organic Matter, Soil Texture and Moisture

The effect of soil organic matter, texture, moisture and microbial activity on herbicide residue is interrelated. Sandy soils have a lower percentage of organic matter and soil moisture, whereas clay soils have a higher percentage of organic matter and soil moisture. The interval between the application of the herbicides in Table 3.4-3 and re-cropping to potatoes is shorter in loams than sands because 1) finer textured soils have more moisture, which increases microbial activity (see below) and 2) herbicide residues bind to organic matter and clay particles in loams, causing them to be less biologically active.

Microbial Activity

Soils with a higher organic matter and clay content tend to have a higher moisture holding capacity. Generally, the population of soil microbial flora and fauna is larger and more active in soils with a higher soil moisture level, organic content and temperature. The breakdown of herbicide residues are accelerated in situations where microbial populations flourish.

Precipitation and Irrigation

The amount and distribution of moisture (precipitation plus irrigation) received during the growing season(s) between the herbicide application and re-cropping to potatoes is the most important factor determining the rate of residue degradation. Degradation is accelerated in soils with high moisture content. Higher precipitation and/or irrigation amounts increase available soil moisture, which in turn increases soil microbial activity and the rate of herbicide breakdown.

Farmers should be very cautious if the growing season(s) following the application of an herbicide known to cause injury received:

• below average precipitation, or
• average to above average precipitation, but poor seasonal distribution causing short periods of drought.

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