WEED control: applied botany 409 



affliction, thinking of them in terms of the dandelions in their lawns, the 

 purslane in their petunia bed, or the ragweed that makes them sniffle and 

 sneeze in the fall. The cases cited above reveal the fact that weeds are 

 responsible for losses — huge, real losses that affect your income and mine. If 

 I were to add the case for Johnson grass in cotton and vineyards, for bindweed 

 in corn, for wild oats in peas and flax, for "lilies" in rice, for nutgrass in 

 vegetable crops, for commelina and goosegrass in sugar cane, medusa head 

 in ranges, etc., the estimates that place weeds first in the list of agricultural 

 pests would surely be accepted. In the 1920's the Agricultural Committee of 

 the United States Chamber of Commerce estimated losses due to weeds in 

 this country at 3 billion dollars annually. Since that time all attempts to prove 

 this to be an overestimate have failed. 



There are several texts devoted to weeds and weed control (Ahlgren, Kling- 

 man, and Wolf, 1951; Muenscher, 1949; Robbins et al., 1952) that ade- 

 quately describe the older work in this field. 



Weed-control methods. Recent developments in chemical weed control 

 have far-reaching significance and application. Not only are these of interest 

 to agriculturists, they are also fascinating to plant physiologists and bio- 

 chemists, for they present new evidence on many aspects of plant metabolism. 



Chemical methods can be classified as follows: 



Selective Nonselective 



1. Foliage applications 1. Foliage applications 



a. Contact a. Contact 



b. Translocated b. Translocated 



2. Soil applications 2. Soil applications 



Three methods falling outside this scheme are the jar method for treating 

 perennials by soaking their tops in a solution of a herbicide, the cut-surface 

 method wherein the concentrated formulation of 2,4-D or 2,4, 5-T is applied 

 to cuts through the bark of trees, and the use of substituted benzenes in irri- 

 gation water to kill submerged aquatic weeds in ditches. 



Functionally speaking, all effective herbicides are absorbed into the plant 

 body, and at some site they bring about a biochemical reaction resulting in 

 the death of the plant. Some are translocated; others act close to the site of 

 absorption. Some are selective; others kill all plants they touch. And finally, 

 with some the final killing effect may be complex, involving several mecha- 

 nisms. 



Absorption. The older materials — sodium arsenite, sulfuric acid, and 

 sodium chlorate — were applied in massive doses, and they brought about 

 rapid destruction of foliage by their drastic and corrosive action. With the 

 introduction of the dinitro herbicides, when dosage was scaled down to a 

 few ounces per acre (Crafts, 1945), absorption was a different problem. With 



