Overview
- Fast-growing, dense cover crops may help suppress weeds and regrowth after perennial seed stands are terminated.
- Hairy vetch produced the most biomass, while sweet clover treatments showed potential economic and weed-control benefits.
- The findings are preliminary and Canadian, so Australian testing is needed before commercial recommendations can be made.
Transitioning an ageing perennial forage seed stand into annual cropping can be difficult, particularly when the original crop persists or weeds establish after termination. Preliminary Canadian research suggests cover crops could help by suppressing regrowth and weeds, retaining biomass and cycling nutrients.
Testing cover crops during transition
The three-year study began in 2024 after a creeping red fescue stand was terminated with herbicide. Researchers compared five treatments containing combinations of hairy vetch, sweet clover, fall rye, tillage radish and field pea.
No fertiliser was applied. Cover crop competition provided the main weed suppression, with mowing used to prevent seed set. Residues were retained to decompose before canola was planted in 2025. Wheat will follow in 2026.
Vigorous cover helped suppress weeds
Hairy vetch produced the most first-year biomass at 7.5 tonnes of dry matter per hectare. Hairy vetch with sweet clover produced 7.0 t DM/ha, followed by hairy vetch with fall rye at 6.6 t DM/ha.
These treatments formed dense canopies and showed the strongest potential to suppress weeds. Field pea produced less biomass and was associated with greater weed pressure in the following season.
These results highlight the importance of rapid establishment and canopy closure when choosing a cover crop to compete with weeds and regrowth.
Benefits came with trade-offs
Severe drought affected canola establishment in 2025. Biennial sweet clover was also planted instead of the intended annual type, while fall rye persisted into the second year. The economic results therefore need cautious interpretation.
Sweet clover with tillage radish produced the highest canola-equivalent return, followed by hairy vetch with sweet clover. These treatments also maintained relatively low weed levels. Legume residues may supply nitrogen but can increase nitrous oxide emissions as they decompose. Hairy vetch produced some higher emission peaks, although it had the lowest emissions relative to biomass production. The sweet clover and tillage radish treatment had the highest emission intensity.
Overall, the study demonstrates that cover crops, particularly hairy vetch and sweet clover mixtures, can play a significant role in terminating perennial fescue stands while providing weed suppression, soil fertility benefits, and economic returns. However, trade-offs exist between biomass productivity and greenhouse gas, nitrous oxide (N₂O) emissions. Biennial legumes such as sweet clover appear promising, offering both profitability and ecological services under challenging climatic conditions. The findings may interest Australian pasture seed however would require local validation.
