Impact of different establishment methods for summer-sown cover crops in Southwestern Germany on their weed suppression properties

Early establishment of cover crops (CCs) is essential for attaining successful weed suppression in the autumn-tospring period. Due to reduced precipitation and extended periods of drought in the summer within temperate zones, the establishment of CCs poses challenges. Three methods of cover crop (CC) establishment were tested: 1) preharvest sowing 10 days before harvest of cash crop (PreHa); 2) no-till sowing directly after harvest of cash crop (NoTi) and 3) sowing with reduced tillage 10 days after harvest of cash crop (ReTi). The five CC species black oat (Avena strigosa Schreb.); common buckwheat (Fagopyrum esculentum Moench), phacelia (Phacelia tanacetifolia Benth.), fodder radish (Raphanus sativus var. oleiformis L.) and common vetch (Vicia sativa L.), along with a mixture comprising 20% of all five species, were included as second factor in a three-year field experiment (2020-2022) conducted in South West Germany. One treatment per establishment method was maintained without CCs as a control. Measurements of CC, volunteer cereal and weed density, soil coverage, and dry matter were carried out throughout the autumn-to-late autumn timeframe. On average across all years and CCs, the PreHa and NoTi establishment demonstrated the highest CC dry matter contents of 99 g m  2 and 119 g m  2, respectively, despite lower CC densities. This had a particularly notable impact on weed density in the ReTi variant, which, over the three-year average across all CC treatments, was 5 times higher than in NoTi and PreHa but exhibited a 18-27% smaller volunteer cereal density by comparison, thereby influencing the respective volunteer cereal and weed coverages. Meanwhile, weed and volunteer cereal dry matters were consistent across all establishment methods. The CC mixture exhibited enhanced resilience to fluctuating weather conditions, leading to an overall superior performance across the three-year experimental period and showing highest measured CC dry matters up to 272 g m  2 in 2022. However, the observed effects in the treatments varied considerably depending on the CC species, sowing date, and weather conditions. In summary, this study demonstrates that early CC sowing, whether directly after the cash crop harvest or 10 days before the cash crop is harvested, has a positive impact on CC biomass formation, especially leading to a reduced weed emergence. Additionally, under variable weather conditions, a diverse CC mixture can exhibit a more stable overall performance compared to CC monocultures.

Automated summary

This study conducted a field experiment to test different methods and species of cover crops. The results showed that early establishment of cover crops, specifically direct sowing or sowing 10 days before harvest, had a positive impact on biomass formation and weed suppression. The performance of cover crops varied depending on the species, sowing date, and weather conditions, but a diverse cover crop mixture showed more stable performance under variable weather conditions.