Trade-offs between forage nutrition and ruminant carrying capacity in response to fertiliser application – Findings from the Park Grass long-term experiment (1860–2020)

Rothamsted Research’s Park Grass Experiment, established in 1856, is the longest-running grassland study globally. Naturally regenerating grassland swards are grown in plots with varying applications of fertiliser including ammonium sulphate and sodium nitrate (at varying application rates), organic fertiliser, minerals (K, Mg, Na, P), and lime, which is mown twice a year. As the world’s most widely produced crop, grass is predominantly used to feed ruminants, however, the nutritional properties and carrying capacities of these plots have not previously been quantified. The objective of this study was to characterise the nutritional profile of forage gathered from the Park Grass plots from 1860 to 2020 and the ruminant carrying capacity that the plots would support. The study further aimed to explore the trade-offs between productivity, forage nutritional quality, and biodiversity. Dried PGE herbage samples were taken from the Rothamsted sample archive at decade intervals from 1860 to 2020, representing a range of plot treatments. Proximate analysis and XRF elemental analysis were performed, and the data was used to estimate ruminant carrying capacity of plots based on metabolisable energy and crude protein requirements for production. Fertiliser applications increased carrying capacity due to yield improvements but reduced crude protein while increasing cellulose and hemicellulose. Increased growth appeared to have a dilution effect on some essential minerals, particularly Ca, Mg, Mn, and P. Sodium nitrate produced higher carrying capacities per unit of nitrogen compared to ammonium sulphate or organic manure. The findings highlight trade-offs in improved grasslands between forage quality, quantity, biodiversity, and management inputs. Results show that fertiliser applications enhance carrying capacity by increasing forage yield but potentially at the cost of reduced nutritional quality and species diversity. This study also provides the first comprehensive nutritional analysis of the Park Grass plots, revealing how historical fertiliser treatments influenced forage quality and ruminant carrying capacity over 160 years. Studying the trade-offs and gradients within grassland systems is essential for understanding the balance between productivity and biodiversity. This study also contributes to the rich dataset available on the Park Grass Experiment, providing future opportunities and insight, whilst also highlighting the importance of long-term experimental studies in the agricultural and environmental sciences • The study presents the first forage nutritional data from the long-running Park Grass experiment, spanning 1860–2020. • Fertiliser application may increase carrying capacity, but there are trade-offs with forage nutrition. • Liming appeared to enable greater species richness. • Results highlight the importance of long term experiments in agriculture.