Effect of nitrogen and snow management on efficiency of water use by spring wheat grown annually on zero-tillage
Water and N fertility are major limitations to cereal production in southwestern Saskatchewan. Both factors interact to determine grain yield and thus water use efficiency. A 9-yr study to assess the effects of snow management and fertilizer N on yields of spring wheat (Triticum aestivum L.) grown annually under zero-tillage provided an opportunity to quantify water use efficiency and relate this to apparent water used by the crop (WU), years of cropping (Yr) assumed to be 10 or fewer, soil test N (SN), and rate of fertilizer N (FN). The results showed that the estimated initial yield point (IYP) (available water required to produce the first unit of grain) and the yield increase per unit of water used beyond IYP increased with available N. Within the range of available N encountered in this study, IYP ranged between 60 and about 100 mm, values similar to those reported in the literature. However, because IYP is obtained by extrapolation, these values are regarded as first approximations. The yield increase per unit of WU increased with FN, varying between 10 and 14.4 kg ha−1 mm−1 (avg. 12.7) when a simple linear regression model was used to relate yield to WU. When a more accurate multiple regression equation that related yield to WU, SN, FN, and time and placement of fertilizer N was used, the marginal increase in yield per unit increase in available water (∂y/∂WU) ranged between 5 and 23 kg ha−1 mm−1 for assumed combinations of SN ranging between 0 and 50 kg ha−1 and FN ranging between 0 and 100 kg ha−1. The multiple regression model is superior to linear models because it allows the separation of available soil and fertilizer N effects. Key words: Water use efficiency, initial yield point, regression, N fertilizer, soil N