Herbicide Injury

Mark VanGessel, Extension Weed Specialist; mjv@udel.edu

There has been a lot of herbicide injury observed this spring, more than most years. There are several reasons for this, with Mother Nature at the top of the list. There is a selectivity with herbicides, some plants being susceptible (injured), and others are tolerant. There are a number of ways that crops are able to escape herbicide injury, but this can be a complex interaction between the crop, herbicide, and environment (climate and soil).

One of the most important factors in crop safety is plant metabolism, the biochemical processes at the cellular level that changes the herbicides to an inactive form. Selectivity between crops and weeds is often the rate at which metabolism occurs. Plants are dependent on temperature and sunshine for metabolism to occur and both have been lacking this spring. As a result, crops are not metabolizing herbicides as quickly as they normally do.

Herbicide rate is another critical factor. The application rate of many soil-applied herbicides differs by soil type and organic matter since these factors can “tie up” the herbicide and effect how much reaches the crop seedlings. Heavy rainfall can move more of the herbicide into the soil, making more available for the seedling to take up the herbicide.

The amount of postemergence herbicide(s) that the plant absorbs is impacted by the waxy cuticle on leaf surfaces. Cool, overcast weather results in thinner cuticle layers that can allow more herbicide to enter the plant. Herbicide formulations, spray adjuvants, and fertilizers play a role in herbicide absorption and being cautious with what goes into the spray tank can reduce the risk of injury. If spraying during cool, overcast periods, switch to “softer” additives if the label allows it; for instance, methylated seed oils (MSO) increases the risk of injury over crop oil concentrates (COC); and non-ionic surfactants (NIS or 80-20’s) reduces the risk further. Consider using the lower allowed rate of the surfactant or nitrogen.  Be sure to read the label and see what is allowed by the manufacturer.

We do not want to see herbicide injury, and some is more apparent that others, but crops often outgrow early-season injury with little to no impact on yield. Under these cool, cloudy conditions be cautious about the adjuvants and tank mixes you use to avoid injury. Remember if the crop is more sensitive to the herbicides during these conditions, chances are, so are the weeds.