Yield and water use of temperate pastures in summer dry environments

The water use efficiencies (WUE) of a range of temperate pasture species were calculated from measurements on several different dryland and irrigated pastures in Canterbury. The annual WUE ranged from 6.7 kg DM/ ha/mm for a dryland cocksfoot pasture to 40 kg DM/ha/ mm for a dryland lucerne crop grown on a Wakanui silt loam soil. The lucerne crop extracted 328 mm of water to at least 2.3 m depth. Its deep root enabled high recovery of rainfall stored in the soil profile. By comparison a perennial ryegrass pasture only extracted 243 mm of water to 1.5 m depth on the same soil type and it had an annual WUE of 18 kg DM/ha/mm. Where practical, species with deep roots should be sown on deep free draining soils to extract all available soil water. On a stony Lismore soil, perennial ryegrass extracted 129 mm of water to a depth of 1.5 m. On a more stony, shallower soil, at the same location, lucerne extracted 131 mm to a depth of 2.3 m. Both pastures had similar annual DM yields and an annual WUE of 16 kg DM/ha/mm. Within the year WUE of the ryegrass pasture ranged from 3 to 22 kg DM/ha/mm. This seasonal variability reflected how soil moisture deficit, soil evaporation and drainage affected pasture growth. During spring, when moisture was non-limiting, clover monocultures and binary mixtures had higher WUEs than pure grass swards due to higher herbage nitrogen (N). Furthermore, a cocksfoot monoculture had an annual WUE of 38 kg DM/ha/mm when fertilised with N but it was only 17 kg DM/ha/mm when unfertilised. These results suggest WUE can be maximised annually and seasonally by growing monocultures of legumes, such as lucerne, adopting grazing management to enhance clover production or strategic application of N fertiliser to maximize growth when soil moisture is available.Financial assistance for PhD scholarships was provided for H.E. Brown by FoRST Bright Futures and for A. Mills by Meat & Wool NZ, AGMARDT, the Cocksfoot Growers Association and Sinclair Cummings Trust