Summary.The effectiveness of pre‐emergence applications of 2,4‐D for weed control in maize was studied In five experiments, in comparison with other herbicide treatments. 2,4‐D‐aimne applied before crop emergence gave satisfactory weed control if the subsequent conditions were wet, but post‐emerge nee applications only killed some species (Chenopodium albumandAtriplex patula.) A combination of 2,4‐D‐amine, applied pre‐emergence to a moist soil and subsequently as a foliar spray, controlled completely C.album, A. patula, Stellaria mediaandVeronicaspp. Pre‐emergence applications of simazine gave better weed control with less need for rain soon after application. OnlyA. patulawas usually resistant. Pre‐emergence treatments of atrazine killed all weeds whilst linuron controlledS. media, A. patula., C. albumandSonchus oleraceusbut notVeronicaspp.Irrigation experiments showed that weed competition for water was largely responsible for the depressions in the early growth of maize when the conditions were dry. Final yield largely reflected the degrees of weed control attained by the herbicidal treatments. Only 2,4‐D applied pre‐emergence, and followed by heavy rain caused a direct depression in yield.Effects des herbicides sur la productivityé du mals fourrage avec référence particuliére aux applications