Trajectory and causes of decline in the botanical composition of dairy-grazed pasture in the Waikato

Herbage accumulation, botanical composition, tiller density and insect pest populations were monitored over six years for four perennial ryegrass functional types grazed by dairy cows in the Waikato to identify genotypic and environmental factors contributing to ryegrass persistence failure in the upper North Island. Perennial ryegrass content of pastures dipped as low as 60% of total herbage mass (HM) in summer of the first three years but recovered in autumn to at least 75%. However, following two successive severe summer/autumn droughts in years 2 and 3, and coinciding with grass grub larvae populations exceeding the damage threshold of 200/m2, ryegrass fell to 20% of HM in autumn of 2015, four years after sowing. This pattern was repeated in the two following years, and was not prevented by any combination of ryegrass functional type, endophyte, seeding rate, or best-practice grazing and soil nutrient management. The environment dominated all other factors.