How cover crop residue management and herbicide rate affect weed management and yield of tomato (Solanum lycopersicon L.) crop

In the conventional cropping systems, increased costs and resource pollution are attributed to the intensive use of chemical inputs. The adoption of cover crops could be a part of a suitable strategy for improving the sustainability of the agro-ecosystems due to their ability to affect nutrient and weed management. A 2-yr field experiments were conducted in Gorgan, North of Iran, with the aim of assessing the effect of cover crop residue management and herbicide rates on weed management and the yield of tomato crop. The treatments consisted in: (a) three winter soil management: two cover crops [annual medic (Medicago scutellata L.) and barley (Hordeum vulgare L.)] and no covered soil; (b) two soil tillage (no-tillage, where cover crop residues were left in strips on the soil surface, and conventional tillage, where cover crop residues were green manured at 30 cm of soil depth); and (c) three pre-emergence herbicide rates (no-herbicide application, half rate recommended or full rate recommended ). Cover crops were sown in early September and mechanically suppressed in March about 2 weeks before tomato transplanting. At cover crop suppression, annual medic showed the highest aboveground biomass [569 g m−2 of dry matter (DM)], while barley showed the lowest weed content (32 g m−2 of DM). At tomato harvesting, weed density and aboveground biomass ranged from 6.9 to 61.5 plants m−2 and from 33.6 and 1157.0 g m−2 of DM, respectively. Cover crop residues placed on soil surface suppressed weeds more effectively than incorporated residues, especially in barley, mainly due to the physical barrier of residues which reduced the stimulation of weed germination and establishment. As expected, herbicide rate decreased both weed density and biomass, even if the adoption of annual medic and barley cover crops before the tomato cultivation could allow a possible reduction of herbicide rate while maintaining similar fruit yield. Tomato yield was higher in annual medic than barley and no cover regardless of tillage management (on average 62.3, 51.8 and 50.1 t ha−1 of fresh matter, respectively) probably due to an abundant availability of soil nitrogen throughout the tomato cultivation. This was confirmed by high and constant values of tomato N status grown in annual medic and evaluated using SPAD chlorophyll meter. Although further research of cover crop residue management is required to obtain a better understanding on herbicide rate reduction, these preliminary results could be extended to other vegetable crops which have similar requirements of tomato.