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Water use efficiency (WUE) has often been analyzed for semiarid environments, but fallow-containing cropping systems were assessed inappropriately. Further, these short-term studies are unlikely to correctly assess weather variability impacts in such environments. We assessed the impact of fertilizer N and P on water use efficiency (WUE) and precipitation use efficiency (PUE) of spring wheat (Triticum aestivum L.) from a 39-yr long-term crop rotation study in semi-arid southwestern Saskatchewan. In the rotation experiment, continuous wheat (Cont W) with N + P or P fertilizer only, and fallow-wheat-wheat (F-W-W) with N + P, P only, or N only were studied. We calculated WUE using: (i) Yield (Y)/[water use (WU)/potential water use (PET)]; (ii) Y/WU; (iii) Y/WU with a fallow phase element added; and (iv) Y/harvest-to-harvest precipitation (PUE). The WUEs in the rotation experiment were generally greater for treatments with N + P fertilizer, and greatest after an increase of N application coupled with favourable soil water conditions in the final decades of this study. In cases (i) and (ii), WUE for F-W-W was greater than for the Cont W-treatment. In case (iii), the WUEs were 5.7, 4.5, 3.9, 3.6, and 3.6 kg ha-1 mm-1 water for Cont W (N + P), Cont W (P), F-W-W (N + P), F-W-W (P), and F-W-W (N), respectively. For PUE [case (iv)] the values were 4.0, 3.1, 3.4, 3.0, and 2.9, respectively. We concluded that case (ii) was most appropriate for continuous cropping and case (iii) for systems including fallow, while case (iv) was usable in general.
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