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

Abstract

Abstract Herbage accumulation, botanical composition, tiller density and insect pest populations were monitored over 6 years for four perennial ryegrass functional types grazed by dairy cows in the Waikato. The aim was to identify genotypic and environmental factors contributing to ryegrass persistence failure in the upper North Island. Perennial ryegrass content of pastures declined as low as 60% of total herbage mass (HM) in summer of the first 3 years but recovered in autumn to at least 75%. However, following two successive severe summer/autumn droughts in Years 2 and 3, and subsequent grass grub larvae populations exceeding the damage threshold of 200/m2, by 4 years after sowing (autumn 2015), ryegrass had fallen to 20% of HM. This pattern was repeated in the following 2 years, and was not prevented by any combination of ryegrass functional type, endophyte, seeding rate, or best-practice dairy cattle grazing and soil nutrient management. The abiotic and biotic environmental stresses dominated all other factors. Keywords: pasture persistence, perennial ryegrass, drought, insects, black beetle, dairy systems