Residual effect of some herbicides on Medicago species in the Victorian Mallee

Abstract

The use of soil persistent herbicides for controlling skeleton weed (Chondrilla juncea) on fallows and in crops might lead to problems of residual toxicity towards establishing medic pastures. Data from 1967 to 1969, taken from both chemical fallow and crop spray experiments on light soils in the Mallee, Victoria, are presented which show the extreme persistence of picloram applied at very low rates and its high toxicity towards both annual medics and lucerne. Other herbicides studied (an atrazine-amitrole mixture, fenac and 2, 4-D) had little or no effect at moderate application rates. It is considered that the very low rainfall in 1967 was largely responsible for the low rate of herbicide decomposition.