reduction in primary tillage depth and secondary tillage intensity for irrigated canola production in a loam soil in central iran

the introduction of canola (oilseed rape; brassica napus l.) as a new source of vegetable oil production in iran prompts evaluation of the performance of this crop under different tillage systems. a field experiment was conducted to determine the impact of depth and intensity of tillage on soil physical properties, crop establishment and yield of irrigated winter canola in a loam soil (typic haplargids) near isfahan in central iran. in a split-plot design, three primary tillage treatments consisted of moldboard plowing to 20 cm (mp20), two passes of a cultivator first to 10 and then to 15 cm (2tc15), and one single pass of cultivator to 10 cm (tc10), were combined with two seedbed preparation treatments (four passes with a disk harrow as opposed to a single pass with a rotary tiller). results showed that the soil bulk density in the 0-5 and 5-10 cm layers were not significantly affected by primary tillage treatments. soil penetration resistance (pr) in the 0-10 cm layer was significantly higher in 2tc15 compared to moldboard plowed soil; however, no significant effect of primary tillage was detected on pr in the 10-20 cm depth. a single pass by a rotary tiller was as effective in seedbed preparation as four passes of a disk harrow, as assessed by bulk density, penetration resistance and the percentage of emergence. the number of plants per square meter at final emergence and at harvest was statistically similar for both the seedbed preparation methods. mean canola total dry matter biomass was 10,020, 9,860 and 10,410, kg ha-1 and dry grain yield was 2,340, 2,410 and 2,880 kg ha-1 under mp20, 2tc15 and tc10, respectively. however, the effects on mean crop yield were non-significant (p 0.05). the mean oil content of the 2tc15 (40.1%) was significantly lower than the mp20 and tc10 treatments, 43.7 and 42.3%, respectively. lack of yield response to tillage treatment may have been the result of achieving a good seedbed (aggregate mean diameter of less than 15 mm) under all tillage methods, which help to obtain sufficient plant establishment. these results indicate that the yield of irrigated winter canola is not sensitive to reduction in the depth of primary tillage or intensity of secondary tillage. with reduced tillage, an optimum plant per unit area can also be achieved. overall, tc10 combined with a single pass of a rotary tiller was considered to be agronomically desirable, due to the absence of grain yield difference compared with both the mp20 or 2tc15 systems and reduced tilling depth.