EFFECTS OF DOSE AND APPLICATION TIMING ON THE SEED PRODUCTION OF 3 WEED SPECIES TREATED WITH MCPA OR TRIBENURON-METHYL

The seed production of Fallopia convolvulus (L.) A. Love, Galium spurium L. and Thlaspi arvense L., treated with MCPA or tribenuron-methyl, was studied in a pot experiment. The herbicides were applied in doses from 1/16 to 1/1 of a full dose at five different growth stages. The largest reduction of seed production was observed on plants treated when one true leaf had developed (F. convolvulus, both herbicides) or at the beginning of the generative phase (G. spurium treated with tribenuron-methyl and T. arvense treated with either herbicide). Tribenuron-methyl (0 . 375 g a.i. ha(-1)) or MCPA (0 . 1875 kg a.i. ha(-1)) applied at bolting stage was enough to reduce the seed production of T. arvense by 83% and 100%, respectively. The seed production of F. convolvulus, treated with 0 . 75 kg MCPA at the cotyledon stage, was reduced by 70%. Plants of G. spurium with 5-9 axillary shoots, treated with 3 g tribe-nuron-methyl, produced 45% fewer seeds than the control. Both herbicides (at least at the highest doses) reduced seed production of all three species more than shoot dry matter.