# Pendimethalin

Pendimethalin is an herbicide of the dinitroaniline class used premergently and postemergently to control annual grasses and certain broadleaf weeds. It inhibits cell division and cell elongation. Pendimethalin is a K1-group (in Australia group D, or numerically group 3) according to the Herbicide Resistance Action Committee (HRAC) classification and is approved in Europe, North America, South America, Africa, Asia and Oceania for different crops including cereals (wheat, barley, rye, triticale), corn, soybeans, rice, potato, legumes, fruits, vegetables, and nuts, plus lawns and ornamental plants.

## Use

Pendimethalin protects crops like wheat, corn, soybeans, potatoes, cabbage, peas, carrots, and asparagus. It is used to control annual grasses and certain broadleaf weeds which interfere with growth, development, yield and quality of agricultural and horticultural crops by competing on nutrients, water and light.
Where weed infestation is particularly bad, yield loss can render wheat production uneconomical. Many other crops are grown in Europe that make a fraction of total agriculture. Herbicide options are limited for these minor crops, particularly in the vegetable sector. Long-term field studies performed by the German government and institutions together with farmers call pendimethalin efficient for controlling blackgrass, regarding weed control efficacy, crop yield, treatment costs and environmental impact.
There is some control of Johnsongrass, but other dinitrolaniline herbicides, such as trifluralin and profluralin, showed much stronger effect.
In 2012, 6–12 million pounds (2,700–5,400 t) of pendimethalin was used in the US.

## Mode of action

Pendimethalin acts in both pre-weed-emergence and early post-emergence. Pendimethalin is absorbed into roots and shoots, inhibits cell division and prevents growth, to prevent weeds from emerging, particularly during the development phase of the crop. Its primary mode of action is to prevent plant cell division and elongation in susceptible species. In the HRAC classification of herbicides according their mode of action, pendimethalin is listed in group K1, also called group D (Australia) or group 3 (numeric).

## Risk factor for developing pancreatic cancer

A study in the International Journal of Cancer suggests that Pendimethalin exposure is associated with higher incidence of pancreatic cancer. Mechanistic studies linking Pendimethalin to pancreatic cancer are lacking, warranting additional research. A French study found no association with lung cancer.
Pendimethalin exposure can reduce apoptosis.

## Resistance

Herbicide resistance increases production costs and limits herbicide options, cultivations and rotations. Up to 2009 pendimethalin did not show resistance. It is not cross-resistant with other grass weed herbicides. So pendimethalin supports other supplementary grass weed herbicides using other modes of action. Lolium rigidum has evolved resistance to pendimethalin, at least in part due to increased cytochrome P450 activity. This resistance mechanism in ryegrass (shared with other dinitroanilines like trifluralin, see for longer explanation) is by an opposing mutation to resistance to prosulfocarb, a thiocarbamate herbicide. By evolving resistance to one, the weed devolves its resistance to the other.

## Registrative status

Pendimethalin is registered globally for a wide range of crops, according to human and environmental safety standards by the European Commission, US-EPA, Canada-PMRA, Japan, Brazil-ANVISA and others.

## Toxicology

Pendimethalin is not toxic to mammals, though interestingly the oral LD$_{50}$ for rats and mice is 1050-1620 mg/kg, yet for dogs and rabbits it is much less harmful, at over 5000 mg/kg. For comparison, table salt's LD$_{50}$ is 3000 mg/kg. There may be chronic effects however; repeated or prolonged skin exposure may cause eczema, hives or Quincke's oedema. Prolonged exposure by other routes may affect changes to the liver.

## Soil behavior

Pendimethalin is highly persistent in soil and water. It has high potential for bioaccumulation, and it is moderately mobile in soil, despite it adsorbing strongly into soil.

## Tradenames

Tradenames include Pendimethalin 440, Satellite, Halts, Prowl, PRE-M, Stomp, Stealth and Pendulum, Hilpendi etc.