Herebicide Families Prone to Carryover

This article was originally published on August 20, 2012 and expired on August 27, 2012. It is provided here for archival purposes and may contain dated information.

The 2012 season has generated concern over the carryover potential of herbicides. Around a dozen herbicide families tend to be used in crops, but only a few have the potential to carryover.

ALS inhibitors encompass a wide range of plant herbicides. They include products such as Pursuit, Classic, Beacon, Accent, Broadstrike, etc. Depending upon the product these chemicals can be foliar applied or soil applied. They "do their damage" by causing plant enzymes to discontinue amino acid production. Amino acids are the building blocks of protein, and protein is the building block of "plant parts". Therefore, grasses and broadleaves exhibit a wide range of poor growth symptoms. Since roots of the grass plant are withheld necessary proteins for growth, the roots are stunted, and the plant exhibits yellowing or purpling due to a lack of nutrients. The leaves of the grass plant are often stunted. Broadleaves are also stunted with leaf yellowing or purpling. The distance between nodes often decreases giving the plant a "stacked" appearance. Carryover becomes more likely when soil pH is high, if too much product is applied, or if the soil environment fails to break down the product.

Several chemical herbicides affect cell growth. These "cell growth inhibitors" are soil applied. As the plant absorbs the material from germination to emergence, the material either affects root growth or shoot growth. The root inhibitors include such chemicals as pendimethalin (Prowl) and trifluralin (Preen) causing poor emergence, stunting of plants, and small bulky-looking roots. Shoot inhibitors, such as Dual, cause shoots to be stunted, grasses to leaf out underground, or broadleaf leaves to resemble wadded newspaper. Of these, only trifluralin occasionally carrys over. However, such carryover tends to be rare.

Triazines keep photosynthesis from occurring efficiently. In fact, seedlings will only die after they emerge because photosynthesis is needed for triazines to kill. Leaves may be yellowed around the tips or between veins in broadleaves as the chemicals retard food production. The oldest leaves yellow first. Carryover may be a potential problem when the soil environment slows down the metabolization of the product. However, high pH environments are more prone to triazine carryover. Atrazine and sencor are examples of triazine family members.

The final group of herbicides that "might" carryover is the pigment inhibitors. Pigment inhibitors cause plants to look white or partially transparent as they keep photosynthetic pigments from being produced, thus retarding growth. Those pigments are called carotenoids. Minus the presence of carotenoids, chlorophyll rapidly breaks down damaging the plant. Carryover potential exists with these products where too much product is applied or where the soil environment decreases metabolism.

Growers concerned about carryover may be able to use oats as a good carryover indicator. By seeding oats in pots using soil from suspect areas, growers may be able to see if herbicide is still present enough to damage plants.

Source: Matt Montgomery, Extension Educator, Local Food Systems and Small Farms, mpmontgo@illinois.edu

Pull date: August 27, 2012