Chloracetamides

Introduction

Alachlor was the first herbicide in this chemical group to be commercialized. It was discovered by the Monsanto Corp. in 1966 and introduced to the marketplace in 1969. Alachlor had a big impact on farming and weed control. Alachlor was a herbicide that could be used in both corn and soybeans for broad spectrum (grasses and broadleaved weeds) without soil incorporation (unlike trifluralin in soybeans, one of its competitors in those early days).

Soon thereafter metolachlor was discovered by Ciba-Geigy, a herbicide with very similar herbicidal properties as alachlor. One of those irrepressible Swiss wits in the company came up with the chemical name as some kind of sardonic marketing joke for their alachlor clone: "me-to-lachlor" (me-also-lachlor?).

These herbicides provide selective control of seedling grasses and some dicot weeds: pigweeds; black nightshade; fair control of purslane; suppression or lowered competitiveness of yellow nutsedge. They are used in corn, soybeans, potatoes, white beans (phaseolus spp.), peanuts, cotton, sunflower, cabbage, tobacco, sugarcane.

They are applied typically pre-emergence or pre-plant incorporated. Although some marketing effort was expended on promoting postemergence alachlor, this type of application failed consistently in all my many years of field evaluations.

The "Yield Advantages" and crop safety under stressfull environmental conditions.
In the late 1980's alachlor was implicated as having toxicological problems (causing turbinate carcinomas in mice in feeding trials). Because no one wants cancer producing chemicals in crop production, the EPA went through an extensive Cost-Benefit Analysis of alachlor: was it worth continued farmer exposure to a known cancer causing chemical compared to its benefits to society? The Monsanto lobbyists geared up and won the battle, mainly by proving that alachlor had a "yield advantage" over metolachlor, making it beneficial and worthy. This controversy over relative differences between alachlor and metolachlor was disturbing to me in that small and inconsistent weed control & crop safety differences between these chemicals became the basis of important public policy decisions. Alachlor may have marginally better control of pigweeds, common lambsquarters and wild mustard. Metolachlor may have marginally longer residual in soils.

Chemistry

Other herbicides in this group include propachlor (1965), butachlor (1970), CDAA (1956), CDEA, diphenamide, napropamide, pronamide, propanil.

Acetochlor has been commercialized and has largely replaced alachlor in terms of Monsanto's contributions to this group of chemistry. It has amazed me over the years that acetochlor was introduced to the marketplace. I tested this herbicide for several years and it consistently injured crops. Hopefully the marketed version has greater crop safety.

Physiology and Metabolism of the Chloracetamides in Plants

Mode of Chloracetamide Action

• The physiology of chloracetamides is poorly understood, despite their importance to agriculture and the environment.
• These herbicides have their toxic action by inhibiting protein synthesis (probably many proteins) in shoot meristems and root tips in susceptible species. This protein inhibition results in inhibited cell development, cell division, cell enlargement causing inhibited shoots and roots. It also affects leaf elongation and lipid synthesis, leaf cuticle formation.
• The mode of chloracetamide lethality from this inhibition and growth arrest of shoots and roots results in plant tissue dehydration, and a dead or permanently stunted plant.

Uptake and Movement of Chloracetamides in Plants

• The chloracetamide herbicides are rapidly taken up from the soil, at a maximum of about 4-5 days after emergence (ideally). They are absorbed mostly by seedling shoot organs (coleoptiles, hypocotyls, coteledons). Secondarily, they are absorbed by seedling root organs (roots, radicles, coleorhizas).
• These herbicides translocate in the xylem. As with the triazines, environmental factors that cause rapid translocation (movement of water from the soil, through the roots, out the leaves) move alachlor quickly through the plant system. Translocation is speeded up by adequate soil moisture, higher temperatures, higher winds, and lower air humidity.
• Alachlor and metolachlor accumulate in vegetative plant parts, but have their toxic action in shoot meristems (growing points).

Basis of Selectivity between Plant Species

• Tolerant plant species rapidly metabolize alachlor by joining a small plant protein to the herbicide, rendering it inactive. This degradative metabolism is very similar to the way atrazine is detoxified in tolerant plants.
• Some resistance to alachlor is provided by placement selectivity: separation of the crop seed lower (planted 2-3 inches deep) than the weed seeds (top 0-1 or 2 inches soil depths).

Fate of Chloracetamides in the Environment

Soil.

• Alachlor and metolachlor are readily adsorbed to soil colloids (clay particles, organic matter). They will leach readily down into the soil with rainfall. They require at least 1/4 inch rain within 10 or less days to "activate" them for weed control: rain incorporation down in the soil, adsorption to the soil colloids, and therefore availability for weed control. High rainfall can lead to excessive uptake of the herbicides and crop injury. Lack of rainfall can result in lack of weed control.
• The most important environmental factor affecting alachlor activity, and alachlor rate of active ingredient to be use, is soil organic matter. The greater the absorptive capacity of the soil from greater amounts of soil clay content & organic matter, the slower the degradation of the herbicide.
• Alachlor and metolachlor are volatile if left on the soil surface, and evaporation of these herbicides and entry into the atmosphere is the route and source of much environmental pollution, especially in herbicide residues in rainwater.
• Alachlor is degraded in the soil by both microbes (~90%) and chemical hydrolysis (~10%).
• Alachlor does not persist long in soils: 1-4 kg ai/ha (1-4 lb ai/a) gone from soil in 6-10 weeks, less time in sandy or low organic matter soils.

Water.

• Alachlor and metolachlor leach in soil (solubility in water at 24oC = 240PPM).
• Carcinogenic properties and presence in groundwater a significant source of concern.
• Both alachlor and metolachlor are primary soil groundwater, surfacewater, and rainfall contaminates. It is a continuing source of amazement to me that herbicides that induce cancer are still allowed to be used and find their way into our drinking water.

Air.

• Volatile if soil applied.
• Low or no risk of alachlor or metolachlor drift on non-target plants (remember, postemergence alachlor is poor weed control).

Animal Toxicology.

• Alachlor a carcinogen: turbinate (nasal) cancerous tumors. Its ability to act as a protein inhibitor (alkylating agent) is broader than desired.
• Herbicides in this group may have undesirable estrogenic effects on humans and animals also.

Plant Injury Symptomology of the Chloracetamides

The primary symptoms are inhibition of early seedling growth and emergence after germination.

Leaf Injury.

-Inhibition of grass leaf emergence from coleoptile and stem (below).

-Malformed, inhibited leaf midvein development. Leaf tip inhibition of soybeans ("heart-shaped"; "drawstring" effect) often observed. Increasing injury from left to right on beans:

-Twisted, malformed, dark-green leaves; improper unrolling of monocot leaves from sheath, often stuck together due to cuticle development inhibition; decreased air spaces between leaves in sheath; "onion-leaf" or "buggy-whip" effect.

Meristem inhibition: stunted shoot tips and roots, reduced plant vigor.

Greater plant injury under severe weather (cold, wet) prior to emergence. Greater injury with high rainfall, greater alachlor uptake.

Abnormal release of apical dominance due to chloracetamides can cause growth of corn leaf buds, underground.

©jdekker-1999