Yield increase in Zone I was mainly ascribed to increase in Tmean, where temperature was generally less than the optimum temperature for wheat (Table 2). The mean of optimum temperatures for wheat was 22.0◦C, 4.9◦C, 10.6◦C,21.0◦C and 20.7◦C for sowing to emergence, vernalization, terminal spikelet, anthesis and grain-filling phase, respectively (Porter and Gawith, 1999).