24. Late Summer Grass Production
TH Sparks and JM Potts

Late summer hay yields (t/ha) at Park Grass, Rothamsted, Herts shown in relation to the mean maximum temperature (EC) for July and August at Rothamsted Meteorological Station

Specific Indicator
There is a classic long-term experiment, known as Park Grass, at Rothamsted Experimental Station in Hertfordshire, in which grass yields have been recorded since1856. Yields in late summer are depressed in hot, dry summers. Yields have been recorded, with constant treatments, using standard measurement methods, since 1965. This unique record is included as a specific indicator: the summer hay yields at Rothamsted (Park Grass).
[Source: Institute of Arable Crops Research, Rothamsted, Harpenden, Hertfordshire. AL5 2JQ]

Relevance
Grassland covers half of the UK land surface, or about two-thirds of the farmed area. The productivity of grassland is important economically for the UK livestock industry and it affects the survival of native species. It represents a basic unit of productivity on which many human and wildlife systems depend.
The Park Grass experiment was originally established (in 1856) to investigate the effects of fertilisers on hay yields and thus provides an unbroken record of an agricultural experiment in which the effects of climate and other factors can be investigated. Park Grass is the oldest grassland experiment in Great Britain. The soils have never been mechanically ploughed and it has been pasture since at least the 1750s.

Sensitivity to climatic and other factors
For most of the year, grass growth responds positively to both temperature and rainfall. (Yields are also enhanced by applying fertilisers and pest control measures and by reseeding with improved cultivars.) However, yields in late summer can be negatively related to temperature, because high temperatures are associated with dry soils.
Annual late summer hay yields at Park Grass were examined with respect to temperature and rainfall recorded at the meteorological station at Rothamsted. Yields were strongly inversely related to mean maximum temperatures in July-August, but were positively related to summer rainfall.

Change over time
In the Park Grass experiment, there has been a tendency towards warmer summers and lower summer yields since 1965. The three lowest yielding years were 1976, 1990 and 1995, all years with hot dry summers.
The relationship with temperature suggests that a 1EC increase in maximum temperature in July-August is associated with a 0.33 tonne loss of yield. Thus, in the event of climate warming, with no change in rainfall, we would expect to see a substantial reduction in grass productivity in late summer. Drier summers would also reduce yields, whereas warmer wetter conditions might maintain production.