emergence control of certain weeds in several vegetable crops. It is more 

 effective on weedy grasses than on broadleaved weeds; however, excellent 

 control of henbit and moderate control of chickweed is obtained by pre- 

 emergence treatments. 



The carbamates are of relatively low oral toxicity (LDcq = 3,000 to 5,000 

 mg./kg.) for rats. 



SUBSTITUTED UREA HERBICIDES 



The substituted urea herbicides include 3 -(p-chlorophenyl)- 1 ,1 - dimethyl - 

 urea (monuron); 3-(3,4-dichlorophenyl)- 1 , 1 -dimethylurea (diuron); 3- 

 (phenyl)- 1 ,1 -dimethylurea (fenuron), and 1 -n-butyl-3-(3,4-dichlorophenyl)- 

 1-methylurea (neburon), previously known as CMU, DCMU, PDU, and 

 DMBU, respectively. These compounds are only slightly soluble in water. 

 They are formulated as wettable powders or as liquids and must be applied 

 as suspensions in high volumes of water. They are the first group of organic 

 chemicals to possess sufficient residual properties to be used as soil ster- 

 ilants. At present they are being used for nonselective weed control on 

 noncultivated land. However, diuron and monuron also have shown con- 

 siderable experimental promise and are being used as selective pre- 

 emergence herbicides in cotton and certain other crops. Neburon has the 

 least herbicidal activity and is least toxic of the substituted urea herbicides 

 listed above to many crops, particularly perennial grasses. 



The substituted urea herbicides are relatively low in oral toxicity (LD^q 

 = 3,400 to 7,500 mg./kg.) for rats. 



TRICHLOROACETIC ACID (TCA) 



There are several salts of trichloroacetic acid (TCA) being used as weed 

 killers, including the ammonium and sodium salts. Sodium TCA is used 

 most widely. It has shown varying degrees of effectiveness in controlling 

 quackgrass, Bermudagrass, Johnsongrass, and other annual and perennial 

 grasses. Best results are obtained when it is applied in combination with 

 tillage and cultural practices. Sodium TCA also is being used as a pre- 

 emergence spray for the control of annual grasses and several broadleaved 

 weeds in flax, sugar beets, sugarcane, and certain other crops. The residual 

 toxicity from high rates of TCA for the control of perennial grasses may 

 disappear within a few weeks or may persist for a year or longer depending 

 on the rate of application, soil type, temperature, and soil-moisture rela- 

 tions. Sodium TCA is highly soluble in water, somewhat caustic, and will 

 corrode spray equipment. 



TCA has low oral toxicity (LDgQ = 5,000 mg./kg.) for rats. 



2,2-DICHLOROPROPIONIC ACID (Dalapon) 



This herbicide possesses properties somewhat similar to TCA. In con- 

 trast with TCA, when dalapon is applied to the foliage of grasses in the 

 vegetative stages of growth, it is translocated from the leaves to the roots 

 of most species. Dalapon has proved less erratic and more effective than 

 TCA when applied as a foliage spray for the control of most of the annual 

 grasses. It is much more effective on quackgrass, Bermudagrass, 

 Johnsongrass, and other perennial grasses. The sodium salt of dalapon, 



