Cover crops could ease the shift from perennial seed to annual crops

Transitioning from perennial forage seed to annual crops can be challenging, especially when the original crop lingers or weeds emerge post-termination. A study in Canada, initiated in 2024, examined five cover crop combinations following herbicide termination of a creeping red fescue stand. The five treatments included hairy vetch, sweet clover, fall rye, tillage radish, and field pea, with no additional fertilizer applied.

The research found that cover crops significantly suppressed weeds, primarily through competition. Mowing was used to prevent seed set. Residues were left to decompose before planting canola in 2025, with a wheat crop following in 2026. Hairy vetch emerged as the top performer, generating 7.5 tonnes of dry matter per hectare in its first year.

Combinations of hairy vetch with sweet clover and fall rye delivered the highest biomass, with 7.0 and 6.6 tonnes of dry matter per hectare, respectively. These combinations created dense plantings that effectively suppressed weeds.

However, field pea produced less biomass and was associated with greater weed pressure in the following season. The study also noted that biennial sweet clover, rather than the intended annual type, was planted, and fall rye persisted into the second year. These factors, combined with a severe drought affecting canola establishment in 2025, limit the economic interpretation of the results.

Sweet clover paired with tillage radish produced the highest canola-equivalent return, followed by hairy vetch with sweet clover. Both treatments maintained relatively low weed levels. While legume residues can supply nitrogen, they may also increase nitrous oxide emissions. Hairy vetch had the lowest emissions relative to its biomass production, whereas the sweet clover and tillage radish combination had the highest emission intensity.

In summary, the study suggests that cover crops, particularly hairy vetch and sweet clover mixtures, can effectively terminate perennial fescue stands, suppress weeds, enhance soil fertility, and generate economic returns. However, there are trade-offs between biomass productivity and greenhouse gas emissions, specifically nitrous oxide (N₂O).

Biennial legumes like sweet clover show promise, offering both profitability and ecological benefits under challenging climatic conditions. These findings may be particularly relevant to Australian pasture seed producers, though local validation is necessary.