Adding forbs and legumes to a grass-clover mixture suppressed weeds and maintained herbage yield and crude protein content across slurry application rates

Grassland leys are valuable for improving the performance of agricultural production systems. Multispecies mixtures have been proposed as a means of improving yields and reducing weed pressure in temporary grasslands, but the effect is influenced by both the length of the grass phase and fertilization. Based on a long-term organic dairy crop rotation experiment with four years of grass-clover in a six-course rotation, we tested the benefits of multispecies grassland leys (3- or 12-species) on herbage yield, crude protein content and weed suppression in one- to four-year-old leys with different cattle slurry inputs (0–300 kg total-N ha⁻¹) over two years. Herbage yield and crude protein content were maintained as species diversity was increased, but the proportion of weeds was reduced. Weed suppression was most pronounced in the four-year-old swards, where a reduction from 18.5 % to 11.0 % was observed. Irrespective of the mixture, increasing the slurry application rate increased herbage yield, reduced the proportion of legumes and increased the proportion of grasses. The proportion of forbs in the 12-species mixture remained stable across N rates. The most abundant additional species in the 12-species mixture were plantain, chicory, caraway, and for longer leys, lucerne. Nevertheless, ryegrass and white clover accounted for 54–84 % of the herbage yield in the 12-species mixtures across the combinations of year, N rate and sward age. This indicates that establishing very diverse mixtures in productive grassland leys for cutting may be challenging and highlights the need for studies focusing on increasing the evenness of diverse mixtures.