Long-term Central Waikato summer-autumn rainfall and pasture growth trends. Are conditions for pasture growth changing over time?

Farmers in the upper North Island are concerned about the poor productivity of their perennial ryegrass pastures beyond 3 years and suggest this is linked to a trend towards drier conditions for pasture growth during summer and autumn. To explore how conditions for pasture growth and survival have changed, trends in rainfall (measured and interpolated; c. 1954 to 2020), frequency of soil moisture deficit stress on pasture growth (1972-2020), and pasture growth rates (simulated 1977-2020, and measured c. 1979-2020) were compiled for summer-autumn months (November to April) for two Waikato locations: DairyNZ Ruakura/Scott Farm near Hamilton; and a commercial farm at Paratu Road between Morrinsville and Matamata. Significant negative linear trends were observed for interpolated rainfall (Paratu Road only), total stress days (both sites) and simulated pasture growth (both sites). No significant trend in measured pasture growth was observed for either site, suggesting adaptive management has cushioned the physical effects of harsher climatic conditions. The suite of adaptive management practices adopted on the Paratu Road farm is described. Analysis of decadal patterns indicated greater variability in rainfall in the decade 2011-2020 (coefficient of variation ranging from 32% to 36% depending on site and data source, versus ~ 22% in earlier decades) and a significantly higher number of stress days in the decade 2011-2020 compared with the 1970s and 1980s at Paratu Road. Further adaptation strategies will be needed to mitigate these most-recent trends, should they continue as predicted by climate change forecasts.