Alix Carpenter –
University of Missouri Extension
Following a drought, herbicide carryover to the next season’s crop is always a concern. After a herbicide is applied, it is broken down by one of many processes, with microbial degradation being the most common. Microbial breakdown is slowed under drought conditions. Soil microbes which degrade herbicides are not active in very dry soils, making carryover possible. And, as little breakdown occurs at temperatures below 50 degrees F, recent rains and snow melt will not have an effect on carryover until soil temperatures rise. Breakdown, regardless of method, usually results in the inactivation of the herbicide.
If a sufficient amount of herbicide residue carries over into the next crop year, significant damage to the subsequent rotational crop may result. Because of this, it is important to if any possible herbicide remains in drought-affected fields. The persistence (and likelihood of carryover) varies greatly between herbicides. In general, herbicide carryover is more likely when the application occurred later in the season, or at a high rate than typical.
The herbicide labels of compounds used in the 2012 cropping year will provide information on the suggested interval between herbicide application and planting of specific crops. Additional information on the label will indicate if the likelihood of carryover is greater under specific environmental conditions. Rotational restriction data is also available in the 2011 Missouri Pest Management Guide, which is available for purchase through local extension offices.
Producers can test for carryover. If testing is done once soil temperatures reach a temperature of 50 degrees F or lower, test results should indicate levels approximate to those which will be present at next year’s planting time. Testing can be done in two ways: a laboratory analysis, or a bioassay. Laboratory analyses can be costly.
The following steps are involved in performing a bioassay:
1. Sample soil from a three inch profile in no-till soil and a six inch profile in a conventional soil (or soil tilled since last year’s crop).
2. Take samples from a representative area of the field (samples should be taken randomly, much like sampling for nutrient and pH analysis).
3. Place the soil sample in a milk jug (top removed and small holes in the bottom for drainage) and place in a well lit window. Be sure to collect a similar soil sample from another area that is thought to not have a carryover issue. This will serve as a control.
4. Plant six to eight seed of the variety to be planted in 2013 year at a one inch depth (do not plant too many seeds or the effects of the carryover herbicide may be diluted, making symptomology difficult to visualize). As an alternative, one could plant several seeds of a more sensitive crop such as oats.
5. Wait eight to 20 days; water the soil as needed to allow for crop growth. If triazine injury occurs, older leaves turn yellow or chlorotic at the edges, and symptoms move progressively in to the center of the leaf. Sometimes this also can appear as speckling.
6. Compare the symptoms of the crop in the suspected and control containers to be sure that injury is not due to some disease or other factor.