Weed species shifts in response to broad spectrum herbicides in sub-tropical and tropical crops

Weed species shifts in response to broad spectrum herbicide use in cotton, maize, rice, citrus and rubber and oil palm plantations are reviewed using data from long term studies available from the literature. Broad spectrum herbicide use has changed the composition of the weed flora in all of these crops. In maize and cotton, zero tillage regimes have resulted in successional changes towards ephemeral broadleaf weed species. Shorter term shifts and cyclical changes in abundance of weeds have been documented in citrus and in oil palm and rubber plantations; in the latter the response of perennial grasses to non-selective herbicides is strongly influenced by mulching. In irrigated rice, weed species shifts (species composition and abundance) are rapid and responsive to both method of crop establishment and herbicide use, competitive release being one mechanism by which relative abundance is altered. The absence of sufficient knowledge of life histories of tropical and sub-tropical weeds means that it is difficult to speculate on the impact of long-term use of non-selective herbicides in resistant crops on the weed flora and emphasises the need for experimental assessment.