The use of cover crops in crop rotations provides a wide range of ecosystem services including protection of the soil from nutrient loss. The objective of this study was to determine the suitability of winter cover crop species for Estonian conditions. Field trials with cover crop species winter rye (Secale cereale L), winter turnip rape (Brassica rapa spp. oleifera L.), forage radish (Raphanus sativus L. var. longipinnatus), hairy vetch (Vicia villosa Roth) and berseem clover (Trifolium alexandrinum L.) were carried out during the period 2016–2018 at the Estonian Crop Research Institute. Biomass production, nitrogen (N), phosphorous (P), potassium (K), calcium (Ca) and magnesium (Mg) accumulation were evaluated. The results under northern European conditions indicated that together with winter rye and winter turnip rape, hairy vetch survived the winters. By the spring, hairy vetch had produced the most biomass and accumulated the highest amount of N, compared with the other species. Winter turnip rape showed the greatest uptake of P, K and Ca in the spring. In both years, forage radish presented the highest biomass and nutrient accumulation in the autumn. After winterkill and decomposition in the spring, the forage radish left holes in the ground, which could be a beneficial characteristic, particularly in no-tillage farming systems. Berseem clover did not produce remarkable biomass in the autumn and was killed by the first frosts and was therefore considered unsuitable as an overwintering cover crop for northern latitudes.

Using cover crops in fallow periods of crop production is an important management tool for reducing nitrate leaching and therefore improving nitrogen availability for subsequent crops. We estimated the short-term effect of five cover crop species on the yield of successive spring barley (Hordeum vulgare L.) for two years in Estonia. The cover crop species used in the study were winter rye (Secale cereale L.), winter turnip rape (Brassica rapa spp. oleifera L.), forage radish (Raphanus sativus L. var. longipinnatus), hairy vetch (Vicia villosa Roth), and berseem clover (Trifolium alexandrinum L.). The results indicated that out of the five tested cover crops, forage radish and hairy vetch increased the yield of subsequent spring barley, whereas the other cover crops had no effect on barley yield. All cover crop species had low C:N ratios (11–17), suggesting that nitrogen (N) was available for barley early in the spring.

Summer cover crops can improve soil fertility and contribute to nitrogen (N) supply in temperate cropping systems, yet the effects of mixture composition and sowing timing remain insufficiently documented. This study evaluated biomass production and N accumulation of five multispecies cover crop mixtures grown in Estonia during 2024–2025 under two sowing dates per year. Aboveground biomass, botanical composition, and carbon (C) and nitrogen concentrations were measured to assess productivity, species contributions, and residue quality. Earlier sowing was generally associated with higher biomass and N accumulation, with first-sown mixtures producing, on average, 38.7% more biomass than later-sown mixtures. Mixture performance was strongly shaped by species composition and competitive hierarchies. Total N accumulation of the cover crop mixtures ranged from 42 to 275 kg N ha⁻¹ depending on mixture composition and sowing time, with mixtures dominated by common vetch (Vicia sativa L.) achieving the highest values. Oat (Avena sativa) dominated and contributed substantially to biomass in mixtures lacking competitive legumes, whereas sunflower (Helianthus annuus) and maize (Zea mays) performed less well under delayed sowing. Low-growing species such as Persian clover (Trifolium resupinatum) produced little biomass when grown with highly competitive species. Legumes exhibited lower C:N ratios than non-legumes, while mixture-level values remained moderate, suggesting residue quality with potential for favourable decomposition and nutrient release in summer cover crop systems under temperate conditions.