Kröbel, R., Campbell, C. A., Zentner, R. P., Lemke, R., Desjardins, R. L. and Karimi-Zindashty, Y. 2012. Effect of N, P and cropping frequency on nitrogen use efficiencies of spring wheat in the Canadian semi-arid prairie. Can. J. Plant Sci. 92: 141-154. Knowledge of nitrogen use efficiency (NUE) is useful for determining fertilizer requirements. We used balance and difference methods to determine the effect of N and P fertilizer on nitrogen use efficiencies for continuous wheat (Triticum aestivum L.) (Cont W) and fallow-wheat-wheat (F-W-W) in a 39-yr crop rotation study conducted on a Brown Chernozem at Swift Current in semi-arid southwestern Saskatchewan. In the balance method, NUE was estimated as yield (Y), or grain N content (GN), divided by either fertilizer N (FN), or FN soil test N (SN), or FN SN growing season net N mineralization (N_min). Most reasonable results [calculating NUE as either (Y or GN)/(FN SN N_min)] were unaffected by fertilizer or rotation and averaged 10.9 kg grain kg^-1 available N and 0.3 kg grain N kg^-1 available N, respectively, for the different fertilizer treatments of Cont W and F-W-W. Using the difference method, where check values are deducted from treatment values in the numerator, Cont W had greater NUE than F-W-W (roughly 2:1). Variations in NUE were not easily explained in the rotation experiment because of the confounding effect of concurrent increases in available moisture and FN availability in the last decade. However, results from a semi-controlled lysimeter experiment at Swift Current showed that irrigation increased NUE, while increasing FN decreased NUE curvilinearly. Of the methods used to assess NUE, the simplest (Y/FN) was the least accurate. However, data needed for more accurate estimates are less likely to be available to the farming community.