Interspecific variability in seed germination and seedling emergence dynamics of cover crop species

International audience

إنجليزي. Cover cropping holds the potential for spatio-temporal diversification of cropping systems, which can help address food security and enhance environmental sustainability (Quintarelli et al. 2022). Yet, the global adoption of cover crops (CCs) by farmers is limited. One of the reasons of this poor CC adoption is difficulties to ensure a good CC establishment (Lamichhane and Alletto 2022; Feng et al. 2024). A better understanding of seed and seedling traits of CC species can guide their choice and sowing dates for an improved CC establishment.We conducted laboratory experiments to characterize seed germination and seedling growth dynamics of five CC species namely crimson clover (Trifolium incarnatum L.), faba bean (Vicia faba L.), fodder radish (Raphanus sativus L.), phacelia (Phacelia tanacetifolia Benth.), and sudan grass sorghum (Sorghum drummondii (Steud.) Millsp. & Chase.). The measured variables were the speed of germination, seedling elongation dynamics (shoot and root growth), and seedling mortality under clods, the latter being an indicator of CC seedlings ability to overcome mechanical obstacles in the seedbed during emergence. Observed results on seed germination, seedling heterotrophic growth, and seedling mortality under clods (with different clod sizes, spatial distribution in the seedbed) were fitted to a Gompertz function, a Weibull function, and a probability function, respectively. Crimson clover, fodder radish, sudan grass sorghum and faba bean showed high germination rates (97, 91, 99 and 99%, respectively) while phacelia had much lower germination rates (62%). The dynamics of seed germination, seedling shoot and root growth dynamics and seedling mortality were well-modelled by the adjustments used. Thermal time (TT) required to reach 50% seed germination was 14, 15, 20, 37 and 55 ℃d for crimson clover, fodder radish, sudan grass sorghum, faba bean and phacelia, respectively. Concerning shoot elongation dynamics, the final shoot length of crimson clover was significantly lower than that of the other four species (p < 0.05). The final root length of crimson clover and fodder radish was significantly lower than that of the other three species (p < 0.05). TT required to reach 50% of the maximum length differed significantly among species (p < 0.05) only for root with the highest values for faba bean (62 ℃d) followed by crimson clover (37 ℃d), fodder radish (35 ℃d), sudan grass sorghum (30 ℃d) and phacelia (10 ℃d). Significant differences in seedling mortality rates were found among species (p < 0.05) only for bigger clod sizes when the clods were not buried into the seedbed, with the highest mortality rates for crimson clover followed by sudan grass sorghum, phacelia, fodder radish and faba bean. Likewise, significant differences (p < 0.05) in seedling mortality rates among the species were observed when the clods were buried into the seedbed, with crimson clover showing the highest mortality rates followed by fodder radish, phacelia, sudan grass sorghum and faba bean. This is the first study to determine reference values for heterotrophic shoot and root growth dynamics and seedling tolerance to a mechanical stress of the seedbed of the five CC species mentioned above. These results will be used to parametrize the SIMPLE emergence model (Durr et al. 2001) followed by the assessment of its prediction quality, based on available data from historical field experiments in France. Furthermore, simulations will be performed taking into account both the baseline and future climate scenarios across contrasted soil and climatic conditions to determine the seedling emergence rates of the five CCs in relation to sowing depth, seedbed structure, sowing dates etc.