at 500, 2000, 3000, and 2500 kg ha Ϫ1 for Sun95, Con95, Con96, and Sun96, respectively. Planting date for Sun95 and Con95 Spring canola (Brassica napus L. var. napus ) is becoming a significant oilseed crop adapted to the western USA. Often N and S limit and for Con96 was 2 May. The cultivar Westar was planted crop growth. Field experiments were established to study the effects at the rate of 7 kg ha Ϫ1 .of N and S fertilization on seed yield, oil content, and N, P, K, and Four N rates (0, 84, 168, and 252 kg N ha Ϫ1 ) and three S S uptake of spring canola. Four N rates in combination with three S rates (0, 22, and 45 kg S ha Ϫ1 ), organized into a randomized rates were evaluated on two irrigated and three rainfed locations in complete block design with a 4 ϫ 3 factorial treatment arrangethe western triangle area of Montana near Conrad. Seed yields ranged ment and four blocks, were applied to plot areas. Plot size from 0.1 to 3.8 Mg ha Ϫ1 . Seed yield and oil content N responses were was six rows wide (30 cm row spacing) and 6 m long. Each closely related to available N (fertilizer N plus soil NO 3 -N in 90 cm research site received 15 kg P ha Ϫ1 as triple superphosphate of soil). Seed oil content varied from 370 to 510 g kg Ϫ1 and was applied with the seed and 34 kg K ha Ϫ1 as KCl broadcast while depressed by increasing N. Optimum seed and oil yield occurred at planting. Nitrogen fertilizer as urea and ammonium sulfate about 200 kg N ha Ϫ1 . Two of the experimental sites responded to S. and S fertilizer as potassium sulfate or ammonium sulfate were About 20 kg S ha Ϫ1 was adequate for optimum seed and oil yields. also applied broadcast while planting. At the optimal N and S levels, total plant N, P, K, and S uptake Irrigated plot sites received enough supplemental water to averaged 140, 25, 170, and 60 kg ha Ϫ1 , respectively. Of the total N, maintain the soil profile (0-90 cm) at about 50% of the avail-P, K, and S accumulation, about 40% of the N, 30% of the P, and able water holding capacity. Growing season precipitation or 85% of the K and S remained in the postharvest residue.precipitation plus irrigation was 24 cm for Sun95, 50 cm for Far95 (flood-irrigated twice at an estimated 15 cm of water per irrigation), 35 cm for Con95, 11 cm for Sun96, and 35 S ince fertilizer programs are a major expense and cm for Con96. Con95 required no supplemental irrigation; profit margins are very small in canola production, however, Con96 required six irrigations, totaling 24 cm of supplemental water.

found a regression relationship between canola seed yield and precipitation from 21 June to 20 August and Canola (Brassica napus L.) yield is often limited by heat and water mean daily temperature from 15 June to 15 August. The stress. Early seeding may avoid the heat and water stress at critical regression equations indicated that for each millimeter growth stages but will encounter low soil temperatures and frequent increase in precipitation, the yield of canola increased frosts. Three experiments were performed at two locations in Montana by 5.9 kg ha Ϫ1 . Also, for each degree rise in mean daily from 2002 to 2004 to determine (i) early spring seeding effect on seed temperature, there was a corresponding yield reduction yield and oil content and optimum seeding rates for early seeding, (ii) base temperature (T b ) for germination and heat requirement for of 188 kg ha Ϫ1 . In dryland cropping systems, water is emergence, and (iii) suitable cultivars for early spring seeding. Late-the most limiting factor for crop production. In a review March-seeded canola yielded 0 to 5% greater than mid-April seeding. paper, Johnston et al. (2002) suggested that a minimumDelaying seeding from mid-April to mid-May resulted in 43 to 63% of 127 mm of water is required for canola seed producyield reduction. Oil content was 12 to 22 g kg Ϫ1 greater for mid-May tion in the northern Great Plains. After the minimum seeding than mid-April seeding in 3 out of 5 site-year combinations.water requirement is met, canola produces 6.9 to 7.2 A seeding rate of 32 to 65 seeds m Ϫ2 was found sufficient to produce kg ha Ϫ1 of seed for every millimeter of precipitation optimum yields. Oil content tended to decrease 10 to 20 g kg Ϫ1 when consumed. Canola has a tap root system that can extract seeding rate increased from 11 to 97 seeds m Ϫ2 . The T b for germination water from a soil depth of 1.1 to 1.7 m (Nielsen, 1997). In was less than 4؇C, and the growing degree days for 50% emergence shallow soils that have a limited water-holding capacity, (GDD 50 ) were 42 to 81. Yield was negatively correlated (r ϭ Ϫ0.46 such as the Judith clay loam (fine-loamy, carbonatic to Ϫ0.65) to the days to 50% flowering, and biomass measured at Typic Calciborolls) in central Montana, canola may have 60 d after planting was negatively correlated to the chlorophyll fluores-to rely on frequent rainfall to sustain growth and procence ratio (F v /F m ) after cold stress (r ϭ Ϫ0.58). The optimal seeding duce seed during the latter part of the growing season.period for the region is between late March and mid-April. Several genotypes were found to have favorable characteristics for early Precipitation timing and amount vary greatly year to seeding.year and location to location in Montana. Consequently, growers in the region encounter highly unstable canola yields.

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