LEVEL OF THRESHOLD WEED DENSITY DOES NOT AFFECT THE LONG-TERM FREQUENCY OF WEED-CONTROL

Weed control thresholds are often presented as a means to reduce unnec essary control measures, thereby increasing the effectiveness of weed management. While the threshold is a useful tool for cost-effective ap plication of control on a single-year base, its role over the longer t erm is more complicated. It is shown that long-term application of thr esholds results in a control frequency that is independent of threshol d level, and in aperiodic dynamics of the weed population which may ca use uncertainty about what control frequency and hence what costs of c ontrol are expected over a given period. We conclude that the economic underpinning of the threshold concept is deceptive and does not provi de a base for rational use of weed control in the long term. (C) 1997 Elsevier Science Ltd.