Recommended sowing rates for new perennial ryegrass pastures are around 20 kilograms of seed per hectare (kg seed/ha) for diploid cultivars and 28kg seed/ha for tetraploid cultivars (due to the tetraploid’s larger seed size). Decades of research have shown little advantage of moving away from those standard rates. However, some schools of thought suggest sowing rates should be higher to ensure good establishment, or lower to prevent seedling competition that compromises their ability to survive in the first summer after sowing.
DairyNZ researchers compared the establishment and survival of seedlings from four perennial ryegrass cultivars sown in autumn 2011. The cultivars were sown at five sowing rates (equivalent to 6, 12, 18, 24 and 30kg diploid seed/ha) and three sites (Jordan Valley in Northland, Newstead in the Waikato, and Lincoln in Canterbury). Fifty seedlings of each sowing rate X cultivar combination were tagged seven weeks after emergence and checked for tiller number and survival over the following 12 months.
What were the results?
As expected, there was an inverse relationship between sowing rate and almost all aspects of plant size, such as the number of tillers and roots per plant, and total root and shoot biomass per plant. However, the weight of individual tillers was the same across all sowing rate treatments.
Thus, competition between plants pivots around a more-or-less constant tiller weight. Lower sowing rates result in plants with a relatively small number of multitillered plants, while higher sowing rates result in many plants with smaller tillers. Importantly, pastures grown from all sowing rates quickly reached the same total tiller density (although the lowest, 6kg/ha sowing rate, tended to lag).
Most seedling death occurred in the first two months after emergence, with higher death rates at high sowing rates. After that point, about 20 percent of plants died (mostly in summer/ autumn) but there was no difference between sowing rates in mortality. Plants that accumulated 15 tillers or more by late winter/early spring had near 100 percent survival over the following summer/autumn. Generally, plants that did not reach this threshold died.
The message is clear: sowing rates of 18kg seed/ha for diploids and 25kg seed/ha for tetraploids are enough to ensure best possible establishment and survival into the second year of pasture life. However, success is still reliant on good management for seed bed preparation, weed control and care during establishment (until plants reach the 15-tiller threshold).
Lee, J. M., E. R. Thom, K. Wynn, D. Waugh, L. Rossi, and D. F. Chapman. 2017. High perennial ryegrass seeding rates reduce plant size and survival during the first year after sowing: Does this have implications for pasture sward persistence? Grass and Forage Science 72(3): 382-400.
This article was originally published in Technical Series September 2018