Uncommon Plant Growth Regulators for the Control of Nutgrass and Oxalis

Abstract

THE recognition of the herbicidal activity in the phenylacetic T acids by Pybus et al. (2) is a matter of particular interest to those whose work is concerned with weed control in crops unusually sensitive to 2,4-D (2,4-dichlorophenoxyacetic acid) and its derivatives. Many crops will be injured by 2,4-D applications at the rates used in weed control, but are not particularly damaged by the use of 2,4-D in a spray directed at weeds, and from which only incidental amounts of 2,4-D come into contact with the foliage of the crop. However, there are some crops, e.g., cotton, which are severely damaged by 2,4-D at rates far below those in common herbicidal use. The pineapple plant is of about the same order of sensitivity to 2,4-D, but in a different way. Although pineapple may not be greatly injured by a chance application of 2,4-D in drift amounts (say 2 to 5 grams per acre) the crop may be forced to flower out of season, and the fruits produced on the immature plants are then too small to meet the standard can size. Although the formulation of the hormonal herbicide quite regularly spells the difference between success and failure in a commercial application, it has been our experience that an exceptionally wide range of 2,4-D and 2,4,5-T derivatives are ruled out of commercial use for weed control on pineapple plantations. Some 19 esters and other derivatives (alcohols, acetonitriles, acetamides, ethylsulfates, and the like) all have had more or less toxicty to nutgrass, but also have induced flowering in pineapples. Work by Wain (3), and his colleagues, has shown that there are a number of crops which are not adversely affected by chlorinated phenoxy-n-alkyl acids with an even number (more than two) of carbons in the side chain, although weeds common in the fields may be killed by these compounds. For some plants, the explanation for this apparently lies in the ability of the sensitive plants to convert 4-(2,4-dichlorophenoxy)butyric acid, for example, to 2,4-D, which is then toxic. Unfortunately, pineapple is forced to flower by 4-(2,4-dichlorophenoxy)butyric acid, although whether the compound is active in inducing flowering in its own right or only after beta-oxidation to 2,4-D may be debated. In any event, if growth regulators are to be used for weed control in the crop, some compound other than one containing the 2,4-D moiety must be used. Two weeds of increasing importance in pineapple fields under Hawaiian conditions are nutgrass, Cyperus rotundus, and a bulb-