The older the better? Delayed complementarity, overyielding, and improved residue composition in ageing alfalfa-fescue mixtures

The introduction of ley in cropping systems can provide multiple services, including high quality forage production and C and N inputs into arable soils. Little is known about the dynamics of these services in degrading grassland and the extent to which trade-offs regarding the use of fixed N for either forage production or as a source of N in the rotation can be found.

A six-year field experiment compared the performance of an alfalfa-tall fescue (A-FE) mixture versus the single species alone (A, FE) in terms of forage production, forage quality and root biomass available as residues for the following crop. N mineralisation potential was then simulated for different grassland destruction scenarios using the STICS model.

The forage production, proportion of legumes, and forage quality all declined in the A-FE mixture after year 3 to reach less than 7 T.ha⁻¹ and 20% legume in year 6. The yield advantage of A-FE compared to monospecific grasslands increased over time and was strongly associated with greater species complementarity, resulting from higher N transfer to grasses and increased asynchrony in the seasonal growth of A and FE. Root residue quantities and the potential for N supply to the rotation were not reduced when a degraded legume proportion was reached.

The advantage of A-FE mixture over monospecific stands was greater integrated over a long period of time. No major risk of losing the N legacy effect by delaying mixture destruction was detected up to the first year after a sharp legume decline.