III. WEED CONTROL IN FIELD CROPS 



CORN 



(1) Pre -emergence . --For control of annual grasses and broadleaved weeds, such as 

 crabgrass, foxtail, ragweed, pigweed, lambsquarters, and others: An ester or amine 

 salt of 2,4-D at 1 to 2 pounds in 5 to 20 gallons of water per acre applied any time 

 after planting but prior to emergence will often give good control. Pre -emergence 

 treatment will not control perennial grasses, such as Johnson grass and quackgrass, 

 or other perennial weeds such as Canada thistle or field bindweed, but treatment may 

 temporarily inhibit their growth and make them easier to control by cultivation. 

 NOTE: In must States the ester formulations are preferred for pre-emergence treat- 

 ment, and some States specifically suggest that the amine salt formulations not be 

 used for pre-emergence weed control in cotn. ^^ ^. . _., , ■, -; , 



Comments and precautions. --Treatment is not advised on sandy soils. Use lower 

 rate of application on loam soils and higher rate on clay soils. On muck soils and 

 heavy clay soils high in organic matter 2 to 4 pounds of 2,4-D per acre may be 

 required to control weeds. Although the amine salts may be used on heavy soils, the 

 esters of 2,4-D are not as likely to leach through the soil as amine salts, and are 

 less likely to cause injury if heavy rains follow application. Lack of soil moisture 

 may reduce effectiveness of treatment, but under such conditions weed populations 

 are usually not serious. Pre-emergence treatments are especially valuable when 

 excessive rainfall prevents cultivation for extended periods after corn emerges. 

 Pre-emergence treatments may fail to control weeds if extended droughts follow the 

 application. The low volatile ester or amine salts of 2,4-D should be used for pre- 

 emergence weed control in fields adjacent to susceptible crops such as cotton, 

 tobacco, grapes, and certain vegetables. 



(2) Post-emergence . --For control of pigweed, ragweed, lambsquarters, field bind- 

 weed, annual morning-glory, cocklebur, smartweed, and other broadleaved annual 

 weeds: Ester or amine salt of 2,4-D at 1/4 to 1/2 pound in 5 to 20 gallons of water per 

 acre applied when weeds are small and corn is 4 to 24 inches tall. Treatment does not 

 control annual or perennial grasses, but may temporarily inhibit the growth of such 

 perennial broadleaved weeds as Canada thistle, milkweed, and horsenettle. 



Comments and precautions. --In the western Great Plains and Intermountain 

 region, 2,4-D at 1/2 to 1 pound per acre is usually suggested for control of most 

 weeds in corn. 



Some injury to corn from 2,4-D applications made at any time from emergence to 

 tasseling may occur if the treatment is applied during conditions favoring rapid 

 growth. Avoid applications when temperatures are high and corn is growing at a 

 maximum rate. Use the esters of 2,4-D at the lower rate and the amine salts at the 

 higher rate in the range. Severe reductions in seed set may occur if applications 

 are made during the 2 -week period just prior to silking and until the silks are dry. 

 Hybrids vary in their susceptibility to 2,4-D but differences are of little importance 

 at rates below 1/2 pound per acre. Avoid cultivation for several days after treatment 

 in order to reduce stalk breakage if brittleness develops. 



(3) Directed post-emergence treatment at layby .- -For control of annual grasses and 

 broadleaved weeds from after last cultivation until harvest: Ester or amine salt of 

 2,4-D applied at rate of 1/2 pound per acre to base of cornstalks and in such a manner 

 as to spray weeds in the row (post-emergence), and at rate of 1-1/2 pounds per acre 

 to soil between rows to prevent new weed growth (pre-emergence). Spray is applied 

 with drop nozzles having different volume capacities. Example: Prepare a solution in 

 proportion of 1/2 pound of 2,4-D to 5 gallons of water. Direct a nozzle delivering 



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