Soybean yield response to different mechanical weed control methods in organic agriculture in Luxembourg

Soybean (Glycine max (L.) Merr.) is very sensitive to interferences with weeds during vegetative growth. Weed control is a key factor for the successful production of soybean in organic agriculture. In Luxemburg, where soybean is not yet an established crop, cultivation trials were implemented to test and evaluate different methods of mechanical weed control in soybean cultivation. Trials were conducted in 2018 and 2019 at three locations in Luxembourg. A flex-tine harrow that can harrow the whole ground surface and a hoe used as inter-row cultivator were the basis of the mechanical treatments. Complementary technologies for the hoe such as the finger weeder, and an alternative method involving intercropping of soybean with camelina were also integrated in the trials. The main objective of the study was to evaluate the efficiency of the tested methods regarding weed regulation and their effects on soybean growth and development and grain yield. In comparison to the weed-free control, both hoeing and harrowing led to yields losses; these losses were lower with hoeing (-2 %) than with harrowing (-34 %). Yields were similar between hoeing and hand weeding and were also similar between harrowing and the weed-free control. The weed control index based on the difference in weed cover compared with the non-weeded treatment was above 0.6 for hoeing, about 0.2 for harrowing and lower than 0.01 for intercropping. Intercropping with camelina was not suitable for Luxembourg, especially during the dry study years of 2018 and 2019 due too poor establishment. Intra-row weeding with finger weeder in addition to inter-row hoeing did not significantly enhance the single use of the hoe in the inter-row. The non-selectivity of harrowing generated more soybean plant losses (-28 %) and lower plant height (-8 %) than hoeing (-11 % and + 5 %, respectively) when compared with the weed-free control. Other yield components such as the number of pods per plant, number of beans per pod, thousand kernel mass, and the first pod height were not significantly affected by the weeding method. The efficiency of any treatment was low when weed infestation was already high after soybean emergence, although where weed infestation was low, any of the treatments were applicable. Encouraging rigorous weed management throughout the crop rotation is a key factor to maximize soybean yield.