Therfield Heath (SSSI) is a fine example of undisturbed chalk grassland in East Anglia which supports rare grasses and flowers, butterflies and insects. It adjoins Thrift Farm and the farm benefits from grazing rights (stints) on the Heath.
Grazing continues throughout the part of the year the sheep do not have lambs at foot.
The rare plants and insects are dependent on short grass which is a result of regular sheep grazing. They would disappear if management by sheep was discontinued and more undesirable species would take over.