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.[Show full abstract]
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