Long-term measurement and modelling of net herbage accumulation in grazed pastures do not align with predictions under climate change

Abstract

A long-term phosphorus (P) fertiliser and sheep grazing experiment at the AgResearch Research Station at Ballantrae (1975-2022) provides a unique resource to examine long-term changes in net herbage accumulation (NHA) and animal production under conditions where soil P fertility have been non-limiting for more than 35 years. This paper examined historical NHA, soil phosphorus (P) and nitrogen (N) fertility from the high fertiliser (HF) farmlet, and pasture growth trends using the climate driven pasture growth module AgPasture within the Agricultural Production Systems Simulator (APSIM). On the medium slope of the HF farmlet, NHA in 2020-21 was only 87% of that measured on the same farmlet between 1982-88, even though P was non-limiting. The measured decline in NHA aligned with a reduction of on-site nominal sheep stocking rates since the late 1990’s. Prior to this paper, climate driven modelling has often predicted a likely positive outcome in NHA for this environment into the future. Understanding the apparent discrepancy between predictions into the future with what is happening on the ground today is discussed in the paper.