COVER CROPS FOR WEED MANAGEMENT IN RICE AND MAIZE

Extensive pesticide use is responsible for soil and water pollution. For this reason, current European policies are promoting a more sustainable and resilient agriculture, that implies the reduction of pesticide and fertilizer use. As weed control largely relies on herbicides, alternative solutions such as cover crops are needed to reduce chemical inputs. Cover crops are non-cash crops that are grown for multiple benefits, such as preventing soil erosion, enhancing soil fertility and suppressing weeds. Cover crops are usually grown in the intercropping period to cover the soil and terminated before seeding the cash crop. After their termination, the residue layer provides a “dead mulch” that reduces light interception and can impede weed emergence. Cover crops can also suppress weeds when they are grown together with the cash crop, providing a “living mulch” that competes with weeds for water, light and nutrients. In organic rice, winter cover crops are grown in the intercropping period and terminated to create a dead mulch that reduces weed emergence. Moreover, since rice fields are flooded, the cover crop biomass undergoes degradation processes that can lead to the release of potentially phytotoxic organic acids. For rice cropping system, the objectives of the thesis were: i) to evaluate the efficacy of the dead mulch in suppressing weeds in both simulated and field conditions; ii) to understand the role of flooding water in reducing weed germination during the cover crop residues decomposition; iii) to identify and quantify the organic acids released in the flooding water and to evaluate their phytotoxicity for rice and its major weeds. The trials reported in the present thesis showed that the dead mulch contributes the most in hindering weed emergence, even though in field conditions the efficacy of the mulch is also related to agronomic practices (e.g. tillage, crop rotation). Even though several organic acids are released during the decomposition of the cover crop, the effect of the flooding water seems negligible. In field conditions, the most abundant compounds were acetic and propionic acids, but their concentrations were fluctuating. Indeed, propionic acid was phytotoxic for Echinochloa crus-galli and reduced root growth in cultivated rice. In maize, cover crops have been extendedly used to create a dead mulch and manage weeds. However, the focus of this thesis was to test the suppressive ability of a spring-established living mulch. In field conditions, two cover crops, a grass (Secale cereale L., rye) and a legume (Trifolium alexandrinum L., Egyptian clover) were seeded in maize interrow, either one month before maize planting or the same day, and they were either mowed or not mowed once during the crop cycle. This technique was combined with the application of herbicide in bands along the maize rows, which reduced herbicide input by 80%. Seeding cover crops the same day as maize provided a better development and a large amount of biomass, especially with Egyptian clover, thanks to the adequate water supply provided by rainfall. Maize maintained adequate yield levels with both species and mowing cover crops did not reduce the crop-mulch competition. Egyptian clover was highly suppressive on weeds but did not excessively compete with maize. The lower competitiveness of legume cover crops was also confirmed in a pot trial, in which the suppressive ability of both grass and legume living cover crops was evaluated. In this trial, cover crops were overall able to reduce weed growth when growing in favourable climatic conditions, but grass cover crops also reduced maize development. However, when the temperatures are not favourable for cover crop growth, weeds can thrive and outcompete the living mulch. In both cropping systems, cover crops reduced the level of weed presence, representing a sustainable alternative to herbicide use for weed control