Herbicide performance and the control of Emex australis in an annual pasture.
In field trials conducted on loamy sand at Wongan Hills Research Station, Western Australia, during 1988-1991, the effects were evaluated of a mixture of diuron + 2,4-DB (100 + 300 g/ha), applied post-em., and imazethapyr alone (50 g), applied pre- and post-em., for the control of weeds (mainly E. australis, Trifolium subterraneum and Hordeum glaucum) in Medicago polymorpha cv. Santiago. Herbicide performance was highly variable, with significant effects on weed survival only in single seasons. In most cases, seed bank densities declined monotonically from an initial level of 1900/m2. Seed banks decreased after seasons when E. australis contributed <25% to pasture DM. The annual decay rate of the seed bank was estimated to be 70%. It was concluded that chemical control of E. australis was limited, as low concn of herbicide had to be used to protect pasture legumes and E. australis can tolerate periods of population decline. It was suggested that E. australis could be managed more effectively by maintaining high densities of annual grasses and legumes, against which it is a poor competitor.