Control of Leafy Spurge in Pastures Using Dicamba and 2,4-D

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Abstract
In the greenhouse, dicamba amine (dimethylamine of 3,6-dichloro-o-anisic acid) at 1.1 and 2.2 kg/ha (a.i. [active ingredient]) controlled young seedlings of leafy spurge (E. escula L.) and prevented shoot formation 66 days after herbicide application. The 2,4-D amine (dimethylamine of 2,4-dichlorophenoxyacetic acid) did not effectively control the growth of leafy spurge at 0.6 and 1.1 kg/ha, but gave good control at 2.2 kg/ha. In a separate test, dicamba at 2.2 kg/ha controlled growth of shoots and roots of 100-day-old stands of leafy spurge more effectively than 2,4-D at the same rate. Dicamba killed mother stands and prevented the production of new shoots; 2,4-D suppressed root growth of mother stands but induced more new shoot growth than found in untreated check plants. In a 5-yr (1977-1981) field study (Brandon, Manitoba [Canada]) of naturally established leafy spurge in a Carlton smooth brome (Bromus inermis Leyss.) pasture, dicamba at 2.2 kg/ha, applied each year from 1977 to 1979, controlled leafy spurge satisfactorily in 1977 and 1979, but not in 1978 and 1981. The mixture of 2,4-D (2.2 kg/ha) and dicamba (1.1 kg/ha) improved weed control and increased smooth brome yield. The smooth brome yield was inversely proportional to leafy spurge control. Under field conditions from 1978 to 1979, 14C-dicamba in the plant translocated to the lower part of stems and accumulated in roots of established leafy spurge more readily than did 14C-2,4-D, measured at 7, 47 and 350 days after herbicide application. Dicamba applied each year gave better spurge control than 2,4-D and resulted in a large yield increase of smooth brome due to killing young seedlings and mother stands. This prevented the spread of root system and seed multiplication of leafy spurge on pasture.