Herbicide Carryover and the Drought - Rotational Restrictions for Winter Wheat  

Dean Volenberg and Chris Boerboom, Door County Extension Agent and Extension Weed Scientist

Although some areas of the state have received normal or above normal rainfall throughout the growing season, many areas have experienced brief or prolonged drought conditions.  During a drought, herbicides do not degrade as fast as under normal soil moisture conditions thereby increasing the chances of injuring succeeding susceptible crops.  Several factors determine the persistence of a herbicide.

How does dry weather increase the persistence of many herbicides?

The length of time that a herbicide persists is dependent on: 1) the chemical properties of the herbicide, 2) the soil type and organic matter content, and 3) the environmental conditions (temperature and moisture) that occur after herbicide application.  During a drought, the herbicide is not available for plant uptake and metabolism.  Lack of soil moisture also limits soil microbe populations which play a major role in the degradation of most herbicides.  Herbicides are also strongly adsorbed to soil particles as soil moisture declines.  Therefore, as soil moisture declines, herbicides become unavailable for plant and microbe degradation.   It is worth noting that some herbicides are degraded by chemical decomposition in a process called hydrolysis.  Hydrolysis occurs when herbicides react with soil water.  When soil water becomes limited, chemical degradation of herbicides is reduced. 

What to do if you are planting winter wheat this fall?

If you are considering planting winter wheat this fall in fields that were drought stricken, first check your herbicide records and determine which herbicide(s) were applied this growing season.  Next check herbicide labels and A3646 Pest Management in Wisconsin Field Crops for rotational crop information.  Many of the most common herbicides used in short season crops such as sweet corn, silage corn, soybeans, and processing crops have a rotation interval (time from application to planting the rotational crop) of 3 to 4.5 months (Table 1).  If these herbicides persist under drought conditions, the risk of winter wheat injury increases.

Are there ways to reduce the risk of herbicide carryover?

One way to reduce potential herbicide carryover is using tillage.   Tillage helps to dilute a residual herbicide by mixing it within the soil profile.  If planting winter wheat this fall on fields with potential herbicide carryover, consider thorough tillage prior to planting.  Also consider altering your crop rotation to a more tolerant crop as specified by the herbicide label. Lastly, watch the weather.  If the drought continues into the fall, the potential for herbicide carryover in susceptible spring seeded crops may exist.

Table 1. Selected silage corn, sweet corn, snap bean, and pea herbicides and the time interval from herbicide application until winter wheat may be planted.  Under extreme drought conditions, the risk of potential carryover injury may still exist after these intervals (based on our estimation).

Note: Some rotational intervals are established to prevent illegal herbicide residues in the rotational crop even though the crop is not at risk of being injured.