292 Auxins in Agriculture 



Hermansen, 1950). The high sensitivity of crop yields to auxins ap- 

 plied at the time of flower initiation is borne out by many workers. 



In some instances moderately injurious effects to the crop can be 

 tolerated in order to obtain larger beneficial effects in terms of re- 

 moval of weeds. For example, Buchholtz (1952) recommends the 

 application of approximately 0.17 Ib./acre of MCPA to remove thistles 

 from peas, even though this application will damage the peas, for 

 the presence of thistles would be much more undesirable than the 

 reduction of yield obtained. 



The time at which auxin application is made will have a strong 

 bearing on the success. It has been pointed out in several different 

 contexts that the stage of growth at the time of auxin application has 

 a strong effect on the susceptibility of a plant. Advantage can be taken 

 of this point to remove weeds when the crop is least susceptible or 

 when the weed is most susceptible. For example, orchard bindweed 

 can be removed from a stand of apple trees in the fall when the apple 

 trees have ceased growth and are not at a susceptible stage (Bryant 

 and Rasmussen, 1951). Again, mustard weeds may be removed from 

 clover at a time when the mustard is in bud or in flower and the 

 clover is not (Fertig, 1952). The removal of weeds at an especially 

 susceptible stage is advantageous not only because of the attendant 

 greater selectivity of the auxin, but also because less herbicide need 

 be applied to obtain the desired effect. 



The effects of various environmental and physiological factors on 

 the effectiveness of auxin applications have been described in chapter 

 VI. It may be worth special note here that the moisture level in the 

 soil is critical to the effectiveness of auxin herbicide applications 

 (Buchholtz, 1948). Reports concerning irrigated crops show con- 

 sistently more effective herbicidal action where irrigation has kept the 

 moisture level high (e.g. Tafuro and Marshall, 1952). In general, low 

 temperatures lead to reduced effectiveness (Marth and Davis, 1945), 

 although some exceptions have been noted (Brown and Weintraub, 

 1952). As would be expected from the known effects of light on trans- 

 location, most effective herbicidal action has been obtained under 

 high light intensities (Mitchell and Brown, 1945; Weaver and DeRose, 

 1946). An occasional exception to the light intensity factor has been 

 noted (Robertson-Cuninghame and Blackman, 1952), perhaps because 

 the auxin was not sprayed onto the foliage. 



Simultaneously with the development of auxin herbicides, there 

 has been rapid development of a variety of other herbicidal materials. 

 Some of these are as highly selective between species as the auxins, for 

 example the fractionated oils, the phenols, carbamates and trichloro- 



