Herbicides 289 



are readily killed by moderate auxin applications, and translocation 

 of the auxin to the roots is not obtained. Consequently the control 

 of thistle is particularly difficult. A clever means of partially cir- 

 cumventing this translocation obstacle is the repeated application of 

 weak auxin concentrations in order gradually to accumulate a lethal 

 level in the roots (Hill and Willard, 1952). 



In some instances with the use of maleic hydrazide as an herbicide 

 it has been found that damaging the plant mechanically after spray- 

 ing may increase the injury obtained. For example, plowing under or 

 mowing perennial weeds after spraying with maleic hydrazide can in- 

 crease the herbicidal effect (Hoffman and Sylwester, 1950, 1953). 



Mixtures of auxins are very generally used. Striking results have 

 been obtained with mixtures of 2,4-D and phenylacetic acid (King, 

 1946) and especially with mixtures of 2,4-D and 2,4,5-T (Offord and 

 Moss, 1948). The latter is commonly used against woody species. 



After extensive evaluations of the effects of herbicides on various 

 plants, researchers in the field are now able to classify a wide variety 

 of herbicides as to their effectiveness in killing specific weeds and as 

 to the tolerance of each crop plant to them. Using this array of in- 

 formation, combinations of herbicides can be tailored to which the 

 crop is tolerant and which will destroy each of the particular weed 

 species expected in a given field (Warren and Larsen, 1952). 



Besides foliar applications, auxin herbicides are highly effective 

 in killing trees when applied to the bark. Because of the corky nature 

 of the bark, esters must be used, and generally in oil. The lethal effect 

 can be increased by stripping off a section of bark at the base of the 

 tree and applying the auxin to the inner bark and cambial layers. . 



Herbicides Applied to Soils 



Slade et al (1945) first demonstrated in England that the appli- 

 cation of auxins to the soil could be effectively used as an herbicidal 

 treatment. This method has achieved widespread use. Its effectiveness 

 is obtained first through exerting a positional effect against the seeds 

 germinating at the surface of the soil and, second, by utilizing the 

 same selectivity factors discussed earlier in this chapter. 



The lethal dosage per plant for soil applications is generally 

 somewhat lower than the amount required for killing by foliar appli- 

 cation. However since the auxin is not concentrated on the plant 

 more auxin in pounds per acre is required. The selectivity between 

 species is somewhat different for soil applications, but in general ap- 

 proximately twice as much auxin is needed for effective soil applica- 

 tion as for foliar treatment. 



