Grazing versus mowing: productivity and forage quality of mixtures and cultivars of perennial ryegrass
Perennial ryegrass (Lolium perenne L.) is an important grass species in temporary grasslands, where it is often sown as a mixture of multiple cultivars. In variety testing, however, candidate cultivars are evaluated as single-cultivar stands under standardised mechanical defoliation. In practice, swards are mown, grazed, or both, where grazing adds treading, selective defoliation, and heterogeneous nutrient return. Whether cultivar rankings obtained under mechanical defoliation also hold under grazing, and whether mixtures perform differently than their component cultivars, are unresolved. We compared six early-heading perennial ryegrass cultivars (three diploid, three tetraploid), and three 50:50 two-cultivar mixtures. The plots were grown under three defoliation treatments (i.e., regular cutting, simulated grazing, and grazing by dairy cows), over three years at two sites. Here we show that defoliation treatment and year affected dry matter yield (DMY) and forage quality stronger than the cultivar grown. The effect of ploidy depended on the defoliation treatment. Tetraploid cultivars had slightly higher DMY under regular cutting and simulated grazing and higher metabolisable energy throughout, whereas diploid cultivars had slightly higher DMY under grazing. Simulated grazing matched the forage quality of grazed swards, including cultivar ranking, but not the cultivar ranking for DMY. Forage quality rankings can therefore be assessed through simulated grazing, whereas DMY rankings of cultivars intended for grazing require evaluation under grazing. Diploid-tetraploid mixtures were intermediate in forage quality and, under grazing, slightly higher in DMY when compared to their individual components. Evaluating cultivars and mixtures under grazing would better reflect their relative performance in the field.