Impact of different weed management on the interference between Oxalis pes-caprae and Sulla coronaria in a Mediterranean environment

Oxalis pes-caprae L. (soursob), an invasive perennial herb native to South Africa, has become a dominant and problematic weed across Mediterranean agroecosystems. This study evaluated the effectiveness of weed treatments and agroecological alternatives on its vegetative regeneration capacity and investigated the interference dynamics between soursob and a forage legume rotation, Sulla coronaria (L.) Medik (sulla) in a cereal farming system of Sardinia (Italy). Sulla ( cv. Bellante) was sown in autumn (October 2024) following triticale ( x Triticosecale Wittmack) in a randomized complete block design with three replications. Plots had previously undergone eight weed treatments: cover-cropping, mowing, sulfonylurea and phenoxy herbicides at two rates, timely destructive soil tillage, and an untreated control. Over two growing seasons, sulla and soursob plant densities, aboveground dry matter and nitrogen yields, and derived competitive indices were measured. Achieving the highest weed suppression, sulfonylurea herbicide and soil tillage significantly reduced soursob density (11- and 6-fold), biomass (8.5- and 4-fold, respectively), and nitrogen accumulation (about 3-fold) compared to untreated control. Conversely, sulla plant density and growth were enhanced. The same trend ( p < 0.001) but with low absolute values was recorded in the second growing season, likely resulting from the combination of the residual effect of treatments and stand age of sulla. A significant multivariate separation among treatments confirms that the competitive mechanism from sulla shifted from density (first year crop cycle) to plant vigour dependence (second year crop cycle) with 5-fold individual weight reached. Although soursob was not completely removed and even increased in some treatments, its competitive role was strongly reduced in the best-performing treatments due to the suppressive effect of the sulla canopy. The initial weed management strategy applied before crop rotation and subsequent sulla biomass were significant determinants of the final soursob density. Overall, results pointed out that sulla does not only coexist with soursob but actively contributes to limiting its spreading. Throughout its biennial cycle, successfully established sulla provides distinct interference mechanisms, serving in meantime as valuable forage crop and effective biological barrier against soursob.