PREDICTING OPTIMAL APPLICATION TIME FOR HERBICIDES FROM ESTIMATED GROWTH-RATE OF WEEDS

A growth model originally developed for grassland but also adjusted to Brassica, was modified and used to estimate the growth of annual weed s in spring barley. Growth rates, calculated for weed stands at the ti me of herbicide treatment in 17 experimental fields, were used to divi de weed plant status in terms of growth rate into four classes: (1) no or poor establishment; (2) estimated weed population with constant gr owth rate; (3) with increasing: growth rate, and (4) with decreasing g rowth rate. The statistical analyses showed that a herbicide applicati on performed at increasing growth rate gave the best effect for two te sted herbicides. The conclusion is that environmental conditions that promote growth rate of the weed stand will also increase the herbicide effect. The results were based on data from two field experimental se ries performed at six sites in southern Sweden during 1991-1994. Two h erbicides, dichlorprop-P/MCPA and tribenuron-methyl + wetting agent we re applied at four sub-normal doses on three alternate occasions. Port able weather stations recorded data on a number of weather parameters during the growing season. (C) 1997 Published by Elsevier Science Ltd.