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MISCELLANEOUS PUBLICATION NO. 10 65, U.S. DEPARTMENT OF AGRICULTURE 



pesticides. Concentration of herbicide in true solu- 

 tion in a stream does not vary in relation to rate 

 of flow. A herbicide with low degradability in wa- 

 ter actually becomes a good tracer. 



Consider the following study. Cooperative re- 

 search between the U.S. Department of Agricul- 

 ture and the Washington agricultural experiment 

 9tation revealed that six applications of acrolein 

 could be quite effective for control of aquatic weeds 

 which clog irrigation canals. An average concen- 

 tration of only 0.1 p.p.m. (weight basis) in the 

 water during exposure periods averaging 48.4 

 hours at intervals of 3 to 4 weeks in 1965 controlled 

 pondweeds for 10 to 20 miles from the point of 

 application in a canal with an average flow of 

 2,023 cubic feet per second (4,000 acre-feet per 

 day). Control was still adequate from 20 to 50 

 miles downstream. Pondweeds in many branch 

 laterals were adequately suppressed, particularly 

 those within 20 miles of the point of application. 

 The concentration and total quantity of acrolein 

 required for pondweed control were much less than 

 in previously used methods of application. Acro- 

 lein will kill most species of fish at concentrations 

 required to kill aquatic weeds in irrigation canals. 

 Fish production should not be attempted in canals 

 that are to be treated with acrolein. 



Temperature of the water very much affects loss 

 of acrolein during transport. In July, with water 

 at 70° F., 32 percent of the acroleine was lost dur- 

 ing the first mile of canal flow when the chemical 

 was introduced at 0.1 p.p.m., whereas 42 percent 

 was lost when introduced at 0.6 p.p.m. (weight 

 basis). In cooler water in September — about 61° 

 F. — the losses of acrolein in the first mile of flow 

 were only 20 percent when the chemical was intro- 

 duced at either of the above concentrations. 



Some of the more seriously adverse effects from 

 use of herbicides have occurred as a result of un- 

 toward air transport. To counteract the inadver- 

 tent introduction of these chemicals into the 

 atmosphere during application, research is under- 

 way to (a) develop invert emulsions to reduce 

 spray drift, (b) develop granular and pelleted 

 formulations to minimize volatilization and drift 

 during aerial applications, (c) incorporate the 

 herbicide directly into the soil, (d) combine with 

 invert materials to retard volatilization and photo- 

 decomposition. 



Development of Improved Herbicides. — There 

 is a continuing need for herbicides that are more 



efficient in destroying the target weeds, carry an 

 appropriate degree of degradability, have a mini- 

 mum toxic effect on crop plants, and have no ad- 

 verse effects on humans, farm animals, wildlife, 

 or fish. 



Research has shown that chloroxuron is much 

 more toxic in acetone than in water, and that 

 surfactants enhance the activity of many herbi- 

 cides. In both cases, the enhanced activity means 

 that effective weed control can be obtained with 

 less herbicide. 



Degradability in soil is much more rapid for 

 2,4-D, a chlorosubstituted phenoxyacetic acid, than 

 for fenac, a chlorosubstituted phenylacetic acid. 

 The faster degradability of 2,4-D has been used 

 to advantage in the control of witchweed. 



Development of Alternate Methods of Weed 

 Control. — Edwin Markham in his sad little poem 

 "The Man With the Hoe" sought to portray a 

 human soul engaged in the very essence of drudg- 

 ery. Those of this latter day who have "chopped" 

 cotton or sugar beets know well Markham's mean- 

 ing. 



Mechanization of this art has largely removed 

 the drudgery and evolved effective and economi- 

 cally feasible practices. Mechanization is not new. 

 Jethro Tull invented the horse hoe before 1822. 

 George Esterly patented a straddle-row cultivator 

 pulled by horses in 1856, and a tractor-mounted 

 cultivator became available in 1918. 



An important landmark in mechanical weed 

 control was the report of Cates and Cox in 

 1912 (23) that the principal virtue of cultivation 

 was to kill weeds. They found that there was no 

 merit in plodding down the rows and cutting off 

 roots if there were no weeds to kill. Later work has 

 shown that if the soil crusts over following a rain 

 and impairs soil aeration, breaking the crust has 

 beneficial effects beyond weedkilling. 



Research underway in the U.S. Department of 

 Agriculture and a number of State agricultural 

 experiment stations indicates that nonchemical 

 weed control still has a place. This method of weed 

 control is still predominant in the production of 

 most crops. Use of shallow tillage tools such as the 

 rotary hoe early in the season has been found to be 

 satisfactory for weed control in soybeans, peanuts, 

 and many other crops. 



Flame cultivation has been developed as an eco- 

 nomical and practical means of controlling mid- 

 season and late-season annual broad-leaved weeds 



