Livestock production in New Zealand is based primarily on pasture and forage for all classes of ruminants (Waghorn and Clark, 2004). Increasingly, sheep and beef cattle are being confined to dry hill country and some un-irrigated plains as the irrigated land is converted to dairy and cropping. Dryland farming on the east coast of both main islands is subject to significant climate variability. Rainfall is the main climatic factor constraining pasture growth, with spring and summer rainfall accounting for 60% of the variation in pasture production in New Zealand (Radcliffe and Baars, 1987). Baars and Waller (1979) identified both rainfall and temperature as influencing pasture production, with temperature playing an important role in pasture growth in winter and early spring.