YIELD RESPONSE OF SPRING WHEAT TO INCREASING DENSITIES OF SPRING OATSAND VARIOUS FORMS OF POSTEMERGENCE WEED-CONTROL

Reductions in the yields of spring wheat (cv Bastion) caused by interf erence from spring oat (cv Selma), considered as a weed, were estimate d in field studies on post-emergence weed control in Dijon (France). T he growth and development of both plants in competition was different for each of the 3 experimental years. Wheat yield losses were signific antly greater on oat-infested plots than on plots treated with diclofo p-methyl. A hyperbolic model provided an excellent fit to data for bot h wheat-grain yield and ear density as a function of oat density, meas ured at the beginning of tillering. On infested plots where the oats w as left to grow until harvest, the number of grains per wheat ear decr eased with increasing oat density, and weight-per-grain was not affect ed. On plots where diclofop-methyl was not applied, chemical control o f broad-leaf weeds did not increase wheat yield. On plots controlled w ith diclofop-methyl, the per cent wheat yield losses differed with the chemical weed control of broad leaf weeds used. When diclofop-methyl and 2,4-D were applied on the same day, the number of wheat ears m-2 w as lower than for other diclofopmethyl + antidicotyledoneous herbicide applications. These results show that competitive interference betwee n weeds and crop plants can be modified by the sequence of herbicides applied, so altering the measured crop yield components.