Canola (Brassica napus L.) is an oil seed crop that may have production potential in the central Great Plains. A market is readily available due to the existence of processing facilities that currently handle sunflower oil production and consumer demand for low saturated fat oil. Producers would be able to use their existing wheat production equipment for tillage, spraying, planting, and harvesting of canola. Sims et al. (1993) reported that canola yields in Montana increased greatly with increased availability of water, but that increased water lowered mean oil content. Canola production in Alberta is reported to be about 1008 kg/ha for 203 mm of water use, and to increase by 59.5 kg/ha for each additional 10 mm of water used (Anonymous 1985). Shafii et al. (1992) reported that four winter canola cultivars grown in 1988 in Kansas yielded from 1170 to 1550 kg/ha with oil contents ranging from 37.7% to 40.0%. They provided no precipitation or water use data. Francois (1994) reported that the oil content of irrigated canola (cv. Westar) grown in Brawley, California averaged 40% in a 2-year study. He also reported that the long-term average oil content for 'Westar' grown in Canada was 43%. Wright et al. (1988) reported that severe evironmental stresses during the rapeseed growing season caused intense competition for assimilates, pod abortion, and seed loss.
Evaluations of the response of crops to varying water availability and water stress can easily be accomplished by calculating the Crop Water Stress Index from crop temperatures obtained with an infrared thermometer (Gardner et al. 1992a, b). This calculation requires knowledge of the relationship between crop temperature, air temperature, and vapor pressure deficit for a non-water-stressed crop (the non-water-stressed baseline). This relationship has not been determined for canola.
The objectives of this study were to determine: (1) a water use/seed yield production function for spring canola; (2) the sensitivity of yield components, oil content, and leaf area development to water deficits at various growth stages; (3) canola rooting depth; (4) canola production potential from the long-term precipitation record at Akron, Colorado; and (5) a non-water-stressed baseline for future water stress evaluations of canola.
Irrigations were applied to the plot area with a gradient line-source solid-set irrigation system, with full irrigation next to the irrigation line, and linearly declining water application as distance increased from the line. Four replications of four irrigation levels existed along the line-source system, with a soil water measurement site and irrigation catch gage at each of the 16 locations. Irrigations were applied weekly to replace evapotranspiration losses from the measurment sites closest to the irrigation line. These were considered the fully irrigated, non-water-stressed plots.
Canopy temperatures were measured on six dates from June 21 to July 27, 1993 and five dates from June 9 to July 5, 1994. Measurements were taken every 45 min from 1000 to 1700 MDT on the fully irrigated plots from the southeast and southwest corners of the plots following the methods described by Gardner et al. (1992a, b). These data provided a range of temperature and vapor pressure deficit conditions from which to construct the non-water-stressed baseline for canola.
Plots were harvested for seed yield on Aug. 6, 1993, and July 18 and 27, 1994. Two harvest dates were used in 1994 due to differences in development rate associated with the gradient application of water.
Following emergence, plots were thinned to a stand of about 1,092,000 plants/ha. Leaf area was measured periodically during the growing season with the LAI-2000 Plant Canopy Analyzer (Li-Cor, Inc., Lincoln, Nebraska). Prior to planting, plots were fertilized with 67 kg/ha N. Plots were hand-weeded as needed throughout the experiment. Final seed yields were taken on July 29 and Aug. 4, 1993 and July 11, 1994.
The change in soil water content between the beginning and ending soil water readings is shown in Fig. 2a (rainout shelter plots, trt. 2) and Fig. 2b (solid set irrigation plots, low end of the irrigation gradient). The data show that water extraction by canola occurred from depths down to 180 cm, but 92% to 95% of growing season water use comes from growing season precipitation and water extracted from the 0-120 cm soil layer. Under the extreme water deficit condition of trt. 2 in the rainout shelter (no water applied during the last 5 weeks of development), canola was able to extract water out of the soil down to a volumetric water content of 0.08 m3/m3.
Water stress during the vegetative growth stage (trt. 4) limited early leaf area development, but plants recovered and produced more leaf area as water became available later in the growing season (Fig. 3). Water stress during the grain-filling stage (trt. 2) resulted in a more rapid loss of leaf area than water stress occurring during other growth stages. Water stress during the reproductive growth stage (trt. 3) was the most restrictive to leaf area development, with maximum leaf area development 64 to 68% of that observed when water stress did not occur until the grain-filling period (trt. 2) (Fig. 4).
In neither 1993 nor 1994 was there a statistically significant effect of water stress timing on seed yield, although the trend in 1993 was for the lowest yield to occur when water stress occurred during the grain-filling period (trt. 2) (Table 2). This was a result of fewer branches/plant and pods/branch, and smaller seeds. The seed yields ranged from 629 kg/ha when water stress occurred during grain-filling to 1018 kg/ha when water stress occurred during the vegetative period. Yields were much lower for all four treatments in 1994, for which we have no explanation. Plants showed no visual signs of insect or disease problems. There was no trend for any particular treatment to result in higher or lower yields than the other treatments. Water stress during grain-filling (trt. 4) did result in fewer branches/plant than the other treatments, as in 1993.
The highest water use in both years occurred with water stress during grain-filling (trt. 2). The larger leaf area that developed early in the growing season and maintained itself during the reproductive stage was the probable cause of this higher water use. This higher water use resulted in a statistically nonsignificant trend for lowest water use efficiency in trt. 2. Water use efficiencies from Experiment 1 (line source) ranged from 2.20 to 4.41 kg/ha/mm between the evapotranspiration range of 254 to 381 mm, similar to the values obtained from Experiment 2 (rainout shelter) in 1993 (1.58 to 3.08 kg/ha/mm). The low yields in Experiment 2 in 1994 resulted in extremely low water use efficiencies (0.81 to 1.04 kg/ha/mm).
There was a small reduction in oil content with water stress during grain-filling (trt. 2) (Fig. 5). The oil contents in Experiment 2 in the rainout shelter ranged from 34% to 39%, with higher contents in 1994. Oil contents in Experiment 2 under the solid set gradient irrigation were also higher in 1994 than in 1993. These data showed a strong trend for increasing oil content with increasing level of irrigation, with values ranging from 37% for the low irrigation level in 1993 to 44% for the high irrigation level in 1994.
In order to assess the long-term yield potential for canola in the central Great Plains, the precipitation records were examined for the 15-week growing season of Apr. 2 to July 15 over the 30-year period of 1965 to 1994 (Fig. 6). These data show that 50% of the years have growing season precipitation of less than 203 mm. Assuming, conservatively, that canola could extract 102 mm of soil water from the profile during the growing season, and applying the water use/seed yield production function given in Fig. 1, 50% of the years would have seed production less than 1133 kg/ha. The predicted range of seed production over the past 30 years was 314 to 2643 kg/ha, averaging 1142 kg/ha.
Fig. 7 shows the relationship between vapor pressure deficit and canopy temperature minus air temperature (the non-water-stressed baseline). The data over the two growing seasons shows a linear response over the vapor pressure deficit range of 0.5 to 4.6 kPa. Infrared thermometry can be used with the non-water-stressed baseline to reliably quantify water stress in canola in future studies of water stress effects on canola production.
|Treatment||Water withheldz||Water appliedz||No. irrigations||Weekly irrigation amount (mm)||Total water applied (mm)|
|1||--||V, R, GF||15||15.7||234|
|Irrig. trt.||No. branches/ plant||No. pods/ branch||No. seeds/ pod||1000 seed wt. (g)||Seed yield (kg/ha)||Evapo- |
|Water use efficiency (kg/ha/mm)|
|Fig. 1. Water use/seed yield production function for canola grown at Akron, Colorado, during 1993 and 1994 growing seasons.|
|Fig. 2. Soil profile volumetric water content at the beginning and end of the canola growing season in (a) the rainout shelter and (b) the solid set irrigation area.|
|Fig. 3. Seasonal development of leaf area index.|
|Fig. 4. Maximum leaf area index as affected by timing of water stress.|
|Fig. 5. Percent oil content for canola grown under four water stress timing treatments (rainout shelter) and four irrigation application levels (solid set).|
|Fig. 6. Probability of receiving at least a given amount of precipitation during the period of Apr. 2 through July 15 at Akron, Colorado.|
|Fig. 7. Non-water-stressed baseline for canola.|