Effects of Green Mulching With Cover Crops on Rice Growth, Weed Suppression and Organic Acid Release in Flooding Water

A common technique to manage weeds in organic rice in Italy involves winter cover crops (CC) that are grown in the intercropping period and terminated at rice seeding to create a mulching layer. The physical effect of the CC biomass and its degradation in flooded conditions with the release of potentially phytotoxic organic acids may impact both weeds and rice. Field trials were conducted in 2023/2024 to evaluate the impact of this technique on weed suppression and crop growth. The fields differed in the CC species, soil tillage and crop precession. Mulch-free control areas, where the CC biomass was removed, were included within each field to assess the effect of the flooding water alone. The organic acids released during biomass degradation were identified and quantified. Weed density was lower when the technique was combined with a crop precession different to rice and with ploughing prior to CC seeding (23.2 plants m−2) compared to fields where the preceding crop was rice and CC was seeded after minimum tillage (596.6 plants m−2). Overall, weed density was higher in the mulch-free control areas, while little differences were observed in rice growth compared to the rest of the field. Acetic and propionic acids were the most abundant; within the first week, acetic acid peak ranged between 158.2 and 233.9 mg L−1, while propionic acid ranged between 99.8 and 167.4 mg L−1. This technique can reduce weed density, but the efficacy relies upon the CC biomass, since weed control is mainly due to the presence of a mulching layer, and the agronomic practices, particularly ploughing and crop rotation.