Weeds and yields of spring cereals as influenced by stubble-cultivation and reduced doses of herbicides in five long-term trials

The influence of herbicides at reduced rates and repeated stubble-cultivation on weeds and crop yields was estimated in five field trials with spring-sown cereals situated in the south of Sweden during the autumn of 1989 until the spring of 1997. Stubble-cultivation was accomplished during 1989-1996, while herbicides were applied at 0, 1/8, 1/4 or 1/2 of full dose during 1990-1996. In the spring of 1997, i.e. after 7 years without herbicide application, seedling densities 3 weeks after weed emergence were 68-340/m(2) at three sites and 535-610/m(2) at two sites when averaged over tillage treatments. Averaged over herbicide doses, stubble-cultivation reduced the plant density of annual broadleaved weeds by 6-32 % at three sites and increased the density by 25 % at one site. At the remaining site, the density was not significantly influenced. Stubble-cultivation reduced the populations of two perennial and seven annual weed species, while one species was stimulated and nine species showed null, or inconsistent, responses. In the spring of 1997, i.e. one year after the last herbicide application, the densities of weed seedlings in 1/8, 1/4 and 1/2-doses were 34, 46 and 56% lower, respectively, than in the untreated controls. Stubble-cultivation increased crop yields at four sites by 200 kg/ha as a mean over herbicide doses. At these four sites, averaged over 1993-1995, herbicides increased yields in plots that were not stubble-cultivated by 7, 8 and 10 % in the 1/8, 1/4 and 1/2 of a full dose, respectively, relative to the untreated control. In 1996, herbicides increased yields at only two sites. It is concluded that a fruitful way for weed management with a low input of agrochemicals is to combine the use of herbicides at reduced rates with repeated stubble-cultivation.