Nitrogen, Phosphorus and Sulphur Fertility of Hybrid Canola Cultivars.
Tee Boon Goh1, Don Flaten1, and Rigas Karamanos2. (1) Dept of Soil Science, Univ of Manitoba, Winnipeg, MB R3T 2N2, Canada, (2) Western Co-Op Fertilizers Ltd., "Box 2500, 11111 Barlow Tr., SE", Calgary, AB T2P 2N1, CANADA
The introduction of high yielding canola hybrids in the last few years has been extremely rapid. Consequently, very little work was initiated to assess the fertility requirements of these crops, since the normally accepted span of three years for fertility research in many cases exceeded the life of some of these cultivars. A series of experiments have been carried out over a six-year period (1999 to 2004) to primarily assess the nitrogen fertility of canola hybrid cultivars and at the same time ascertain whether the associated phosphate and sulphur fertility is influenced by nitrogen application. Although differences in individual canola hybrid cultivar behaviour can be significant in some cases, “hybrid” in this study does not pertain to the behavior of one specific cultivar, but to the group of hybrids tested. The study consisted of 27 site-years of comparison between hybrid and conventional cultivars and was carried out in the three Canadian Prairie Provinces in two stages: Stage 1: An exploratory program over the first three years (1999-2001) culminated in 17 site-years of comparison between hybrid and conventional canola cultivars. All experiments were carried out on soils with very “low to moderate” soil S levels. Stage 2: An additional three-year program (2002-2004) included 10 site-years of comparisons between hybrid (primarily Roundup-Ready) and conventional or Roundup-Ready open pollinated cultivars grown on soils with “adequate” soil S levels. The N and S status of each site was characterized by sampling a 5X10 grid to establish spatial variability of these nutrients within each site. All experiments included twelve rates of N (0, 20, 40, 60, 80, 100, 120, 140, 160, 180, 200 and 220 kg N ha-1) set in a split-split plot design with the canola cultivars as main plots and N rates as subplots. Sub-sub plots in stage 1 of the study consisted of three rates of either P (0, 8.7 and 17.4 kg P ha-1) or three (0, 20 and 40 kg S ha-1) and in stage 2 of rates of S applied based on the N rate so that, in addition to a control, N:S ratios of 12:1, 6:1 and 1.5:1 for each N rate were obtained. All plots in each test were seeded to hard red spring wheat the year following each experiment with addition of only 20 kg N ha-1 in order to establish whether differences in yields due to superior genetics of hybrids resulted in higher uptake of nutrients under the same nutrient regime compared to conventional cultivars. Under an identical nutrient regime, on average, hybrid varieties produced 14% higher yield compared to conventional cultivars, but did not reach maximum potential; this, on average, occurred at a higher N fertility regime at which hybrid varieties produce 33% higher yields than traditional or parent varieties. Seed yield of both hybrid and conventional canola cultivars was highly dependent on N fertility. Nitrogen requirements to obtain optimum yield of hybrid canola cultivars are higher than that of conventional canola cultivars; on average, maximum potential yield was obtained with an additional 30 kg of soil plus fertilizer N ha-1. A N:S interaction was obtained with conventional cultivars only when soils were deficient in S. Optimum yields of canola were derived so long as individual N and S requirements were fulfilled independently of the ratio at which these two nutrients were applied. Once an N or S deficiency was corrected, there appeared to be no need for balancing N and S application rates at any particular ratio, especially with canola hybrids. Most of S deficiencies were corrected with approximately 20 kg S ha-1, although more severe S deficiencies required levels of S application closer to 40 kg S ha-1. Spatial variability of S may necessitate a blanket application of 10 kg S ha-1 even on soils that test sufficient in S. No interaction between N and P were observed in any of the twelve site-years included in Stage 1 of this program. Responses to P were obtained in seven of the 12 site-years and, overall in these experiments carried out on P deficient soils, maximum yield was obtained with 17 kg P ha-1 independently of the rate of N. Hence, to maintain maximum yield, hybrids must be supplied with phosphate and sulphur at regimes that are similar to those of conventional varieties. Canola hybrids are more efficient “scavengers” of soil nutrients. A theory that is partly supported by reduced wheat yields obtained the year following in plots sown to hybrids under the same nutrient regime compared to conventional cultivars.