even though future climate changes will be highly spatially variable, some model climate projections suggest that precipitation will increase at high latitudes, and will decrease in the tropical and subtropical land regions(IPCC.2007). crop yields are projected to fall in the tropics and subtropics by 10 to 20% by 2050 due to combination of warming and drying. but in some places yield losses could be more severe (Jones and Thornton, 2003). Thus in these areas crop productivity will be a critical constraint for livestock systems.