Data search and collection

Recent peer-reviewed publications were compiled by searching Google Scholar and the University of Lethbridge Library database by using the following keywords: “canola”, “North America”, “NUE”, “WUE”, “water”, “nitrogen”, “sulfur”, “irrigation”, and “seed yield”. To avoid any bias, previous publications were selected according to the following criteria: (i) the field experiment must have been conducted in North America; (ii) the cropping system in the field experiment must contain spring Argentine canola; (iii) the study included one or more of the following management-related factors: N fertilizer rates, S fertilizer rates, irrigation, or soil moisture management; (iv) studies reported seed yields; (v) treatments must have been replicated and randomized; and (vi) if one study reported different years or site-year observations within the same experiment, each year or site-year observations were considered as separate observations (Van Groenigen et al. 2013). Accordingly, a total of 24 publications were incorporated into our dataset. The dataset consisted of 355 measurements for NUE from 12 peer-reviewed publications from 2008 to 2020, 276 measurements for SUE from 4 peer-reviewed publications from 2002 to 2020 and 99 measurements for WUE from 8 peer-reviewed publications from 2004 to 2020. The research locations included in this review are shown in Fig. 1. Soil information and weather data from these locations are provided in Table S1.

Fig. 1.

Research locations included in this review of factors affecting nutrient and water use efficiencies in spring canola in North America (Google Maps).

Data processing

NUE was calculated as

(1)

where NUE is the nitrogen use efficiency (kg kg⁻¹), Y is the crop yield (kg ha⁻¹), and N is the amount of N (kg N ha⁻¹) applied as fertilizer.

WUE was calculated as

(2)

where WUE is the water use efficiency (kg mm⁻¹), Y is the crop yield (kg ha⁻¹), and W is the total amount of irrigation water plus rainfall (mm), representing total water input.

SUE was calculated as

(3)

where SUE is the sulfur use efficiency (kg kg⁻¹), Y is the crop yield (kg ha⁻¹), and S is the amount of S available in soil plus S applied as fertilizer (kg S ha⁻¹).

NUE values were analyzed in the following categories: source of N [urea vs. controlled-release urea (CRU)], application (urea spring band vs. combinations of N source) (CRU and a blended mixture) and timing (spring-banded, fall-banded, and split application), N rate, species (B. napus vs. other species) × N rate, year × N rate, variety × N rate, S rate × N rate, stubble management (stubble retained or incorporated) × N rate, and fertigation stage (no fertigation vs. other).

WUE values were grouped into nine categories: irrigation at different growth stages (rain-fed vs. partially or fully irrigated), stubble height (no stubble at seeding vs. stubble maintained at different heights and seeding), irrigation rate (rain-fed vs. different amounts of irrigation), time of stubble management (no stubble in spring vs. stubble at different heights either in spring or fall), irrigation (rain-fed vs. irrigated), row spacing × stubble height (30 cm apart with 15 cm stubble height vs. wider spacings with taller (>15 cm) stubble heights), species × water regime (B. napus, well-watered vs. other species, rain-fed or no precipitation), species × seeding date (B. napus seeded early spring vs. B. napus/B. campestris seeded late spring or late fall), and species × stubble management (B. napus on fallow vs. B. napus on stubble or B. campestris on fallow or stubble).

SUE values were grouped into four categories: N rate × S rate, application timing-growth stage × S rate (incorporated at seeding vs. side-banded and seed row placed at seeding and top-dressed or foliar applied at bolting and early flowering), S rate (no S applied vs. different rates of S applied), and timing × source × S rate (spring-applied ammonium sulfate at 10 kg S ha^–1 vs. spring/fall-applied S in various fertilizer forms applied at two rates (10 or 20 kg S ha^–1).

Rates of N fertilizer application and NUE

Brassica oilseed crops respond to N fertilizer positively even if applied at rates as high as 180 kg N ha^–1, but the amount of N fertilizer required for maximum yield of oilseed species differs, depending on environmental conditions (Brandt et al. 2002; Gan et al. 2008). Assefa et al. (2018) also concluded that canola yield plateaus when available N reaches 100 to 200 kg ha^–1, depending on the environment. Ma and Zheng (2016) suggested that the optimum rate of N fertilizer for canola production is ∼150 kg ha^–1 in humid regions, such as eastern Canada and higher yield and (or) NUE and can be obtained when N fertilizer is side-dressed, under normal weather conditions. Lafond et al. (2008) reported that in the western Canadian prairies, applying 50% of N in-season can efficiently match N requirements and reduce the risk of N leaching (Ma and Zheng 2016). This suggests that N application rates may not yet be optimized for canola production.

A field study conducted at 11 sites in Saskatchewan from 2003 to 2005 showed that there was a general trend of decreasing NUE with increasing N fertilizer rate in all five of the oilseed species studied, including B. napus (Gan et al. 2008). Maximum NUE was obtained at N fertilizer rates up to 100 kg N ha-¹ less than the rates required to maximize seed yield (130 kg N ha^–1). Gan et al. (2008) reported that at any given fertilizer rate, NUE was high when soil N supply was low, although the trend for NUE and fertilizer NUE was similar. Furthermore, the study reported that the magnitude of decrease in NUE with increasing rates of N fertilizer was interactively influenced by soil N supply and rainfall during the months of vigorous vegetative growth and flowering period (June and July). The crop N uptake response to fertilizer varied among oilseed species; however, all oilseed species showed a similar N uptake response at sites with low (100 mm) rainfall in these months (Gan et al. 2008). Under high rainfall (250 mm) conditions, rapa canola was the poorest user of fertilizer and soil N, while juncea canola and mustard were better at using N under higher moisture conditions (Gan et al. 2008). Therefore, Gan et al. (2008) recommended optimizing rates of N fertilizer for different canola species under different environmental conditions, soil N supply, and rainfall to improve NUE in canola production.

Timing, source, and placement of N fertilizer application and NUE

Optimization of the timing of N applications is a must for developing best nutrient management practices to improve NUE (Ma and Zheng 2016). Canola yield is associated with growing-season dry matter (DM) accumulation (Karamanos et al. 2005). In a canola crop, the greatest DM accumulation can be seen during the flowering period. In addition, the effect of N fertilizer on canola yield partially depends on the capacity of the crop to mobilize N from senescing vegetative organs to seeds. The seed yield and the DM content could be affected by the timing of N application, however contradictory results were reported (Ma and Zheng 2016). A study under humid eastern Canadian conditions indicated that side dressed N application at the 6-leaf stage is more effective in improving crop N uptake and provides better N economy in comparison with an equal amount of N received entirely at the preplant stage. Therefore, split application is more productive because it provides N at suitable stages during crop growth (Ma and Herath 2016). Ma and Herath (2016) also reported that the rates and timing of fertilizer N required to maximize the yield of canola depend on environmental conditions. In contrast, studies conducted in western Canada under arid and semi-arid conditions by Grant et al. (2012) showed that split applications of N were no more effective in both DM and yield than all N applied at seeding, likely as a result of different soil moisture conditions. Therefore, a better understanding of the underlying changes in seasonal DM and N accumulation and utilization in seed formation may be necessary to improve both the seed yields and NUE (Ma and Zheng 2016).

An N source-placement study conducted by Malhi et al. (2010) included two sources of N (urea and controlled release urea; CRU) applied alone and in combination in a blend, and different placement methods (spring-banded, fall-banded, or split application (half of fertilizer spring banded and half broadcasted at tillering)). Spring banding CRU improved NUE compared with spring-banded urea applications, but only marginally. Their results also showed that fall-applied urea produced the lowest NUE (19.4 kg kg^–1 N), and spring-banded CRU produced the highest (24.5 kg kg^–1 N) NUE, with a narrow range. In general, irrespective of the difference in N source, spring-applied N treatments had higher NUE than the fall-applied N. This may be due to volatilization, immobilization or leaching of CRU under warm fall conditions or early spring weather conditions. The authors concluded that for boreal soils of the Canadian prairies, spring-banded CRU is as effective as urea, and in some years more effective, in increasing crop yield and N recovery. The study showed that urea/CRU blends and split applications of urea had similar NUE to that of spring-banded CRU (Malhi et al. 2010). Split applications have the advantage of providing an available source of N early in the season and reducing the risk of N losses (Malhi et al. 2010).

Water supply and NUE

The canola crop root system has a high surface area characterized by long root hairs, which play an important role in nutrient acquisition and transport in water-limited soils (Pan et al. 2016). For instance, dry spring conditions can leave soil N “stranded” due to impaired root growth, thus restricting available N uptake (Pan et al. 2007). Previous studies showed that water supply during the flowering period is critical for oilseed production as low soil moisture content during this crop growth stage can negatively impact plant N uptake, thus reducing the photosynthetic activity of leaves and mobilization of the assimilate (Morrison and Stewart 2002; Gan et al. 2004, 2008). Another study showed that, with increasing available water and fertilization, spring canola becomes more efficient at accumulating both grain mass per unit grain N and grain N per unit of available N supply (NUE) (Maaz et al. 2016). In addition, NUE component analysis indicated that water-enhanced yields were correlated with higher N uptake and utilization efficiencies, which in turn were attributed to higher grain NUtE, followed by higher N retention. Most canola production occurs in arid and semi-arid regions where irrigation infrastructure is absent, therefore, canola cultivars should be screened for improving WUE and grain N accumulation in environments with limited water supply (Maaz et al. 2016).

Except for one study conducted by Smith et al. (2019), the NUE studies that we considered in our analysis did not include irrigation treatments, where the water deficit conditions would be minimized. However, in some studies, stubble management treatments were included in combination with N rate with the objective of conserving available soil water to support the crop productivity and improve the NUE. No positive impact on NUE was reported in the studies that were considered, and soil water deficit conditions may have played an important role in the mediocre response to N treatments.

Genotypic variation and NUE

Genotypic variation in NUE has been reported among oilseed canola cultivars, which suggests that uptake and distribution of N in canola is an inherited characteristic. Significant genotypic variations in yield under N limiting conditions and in yield responses to high inputs of fertilizer N were reported for spring canola. Furthermore, it was indicated that the genotypic differences in NUtE are more apparent under limited N than under optimum N supply. The yield response of canola cultivars under limited N level may depend partially on their inherent ability to remobilize N from senescing leaves and translocate them to developing seeds (Svecnjak and Rengel 2006). In addition, the extensiveness of the canola root system correlates with genotypic variation in NUE (Maaz et al. 2016). A previous study reported that the N uptake efficiency (NUpE) of canola seed is predominantly determined by root growth instead of the N uptake rate per unit of root surface (Kamh et al. 2005). Several studies have indicated the genetic diversity of N-related traits in both spring and winter cultivars under field and controlled conditions (Svečnjak and Rengel 2006; Kessel et al. 2012; Ulas et al. 2013; Lee et al. 2015). Combining the genetic diversity of the spring and winter gene pools using backcrosses may enhance genetic variation for NUE improvement (Bouchet et al. 2016). Moreover, genetic variation in the activity of different nitrate and ammonium transporters could be relevant to improve NUpE in canola (Xu et al. 2012).

Sulfur use efficiency of canola

Canola is sensitive to S concentrations in plant tissue, as S is an essential component of amino acids, cysteine and methionine, and oilseed crops in the Brassicaceae family contain high levels of sulfur-containing secondary metabolites, called glucosinolates. Canola requires about four times more S than wheat or maize, and an adequate supply of S is important for optimum growth and yield (Haneklaus et al. 2007; Abdallah et al. 2010). In canola, a low S supply suppresses the development of reproductive organs and leads to silique abortion, decreased seed yield, and decreased oil content (Ngezimana and Agenbag 2014). Therefore, oilseed crops have a greater demand for S compared with cereal crops (Haneklaus et al. 2007). Field trials carried out over the years across the Canadian prairies determined that in canola, optimum yield can be achieved at rates of application of 15-30 kg S ha⁻¹ (Malhi and Gill 2006, 2007; Karamanos et al. 2007; Malhi et al. 2007).

Both N and S are important constituents of protein and adequate supplies of both nutrients are required to optimize crop yield (Grant et al. 2012). Inadequate S combined with an excessive amount of N can lead to a nutrient imbalance that can restrict protein synthesis and reduce canola growth and yield (Malhi and Gill 2002). The interactive effects of N and S on canola yields have demonstrated that N fertilizers stimulate plant S uptake and yield responses to applied S only occurred when N was applied (Ngezimana and Agenbag 2014). Therefore, N and S additions must be in balance for optimum crop yield (Jackson 2000; Malhi et al. 2007; Malhi and Gill 2007). In western Canada, it is recommended that S be applied in a fertilizer mixture having an N to S ratio of 5:1 to 7:1 (Karamanos et al. 2007).

In contrast, field crops in eastern Canada were rarely fertilized with S as anthropogenic sources of S from airborne pollution (acid rain) and inherent soil S reserves have historically been considered sufficient to meet crop S requirements (Ma et al. 2019). Environment Canada measured SO₄-S deposition levels ranging from 5.6 to 11.2 kg S ha^–1 in 1990, but only approximately 3.4 kg S ha^–1 in recent years (OMAFRA 2018). Consistent with this diminished S availability, Ma et al. (2019) reported that fertilizer S application greatly improved canola seed yields at 6 out of 9 site-years, and the highest NUE was observed in the N150 + S20 kg ha^–1 treatment, suggesting the importance of S supplement when high N rates are applied for canola production in eastern Canada.

Sulfur fertilizer source, and timing and method of application

Various S-containing fertilizers are utilized, including gypsum (CaSO₄·2 H₂O) and potassium sulfate (K₂SO₄), but in the northern great plains, the most widely used S sources are ammonium sulfate ((NH₄)₂SO₄), ammonium thiosulfate ((NH₄)₂S₂O₃), and various elemental S forms. Before S can be utilized by the plant, it must be converted to SO₄- S (SO₄) ions as plants absorb S primarily through the roots from the soil solution in the form of (SO₄^–2) (Grant et al. 2012). Ammonium sulfate, gypsum, and potassium sulfate supply S in the SO₄ form, while the S₂O₃ in ammonium thiosulfate rapidly oxidizes to plant available SO₄. Therefore, applications can be made in both spring and autumn because the fertilizer is available for crop uptake upon dissolution and movement into the soil solution in the rooting zone (Malhi 1998, 2005; Grant et al. 2004). Autumn applications of sulfate forms of S, however, are vulnerable to leaching losses under high moisture conditions in sandy soils thereby reducing fertilizer use efficiency (Malhi 1998, 2005; Grant et al. 2004; Malhi et al. 2009).

Elemental S fertilizers must be oxidized to the sulfate form before they are available for crop uptake, and the oxidation is mediated by soil microorganisms. In many environments, the oxidation rate of S is simply not rapid enough to release sufficient available S to optimize yields of Brassica species in the year of application, or possibly for several years. In general, the soils on the semi-arid Canadian prairies do not reach temperatures above 10 °C until mid-May and microbially mediated S oxidation may be slow due unfavourable soil conditions resulting in reduced availability of SO₄-S to the respective crops (Malhi and Leach 2003; Wen et al. 2003; Grant et al. 2004; Karamanos and Poisson 2004). This may explain the poor or negative response to elemental S by the canola crop.

Malhi and Leach (2003) showed that applying a higher rate of elemental S sometimes increased SUE, and more importantly, that SUE for elemental S improved significantly when elemental S was fall applied compared with spring applied. In fact, the SUE of fall-applied elemental S came closer to that observed with spring applied sulfate-S products (Malhi and Leach 2003). Elemental S should be managed in a manner that increases particle dispersion and contact with the soil microorganisms to accelerate the oxidation process (Grant et al. 2012). Recent work with micronized elemental sulfur shows potential for use with canola (Bremer et al. 2021).

Canola has a high demand for S during flowering and seed set stages (Malhi et al. 2007). If deficiencies are observed during the growing season, application of S source as late as rosette to early bolting can be beneficial. However, yield will generally be lower than if the S had been available from the start of crop growth (Grant et al. 2012). Malhi and Gill (2002) applied sulfur as potassium sulfate at different stages of growth either as a top-dressed treatment or as a foliar application. Their results showed that the response to S and SUE was greatest when applied at seeding, followed by bolting, followed by flowering and foliar applications that were better than top-dressed applications at some sites. In addition, ammonium sulfate and ammonium thiosulfate may have residual benefits on S availability for several years after application, depending on the environmental conditions. For example, in the Canadian prairies under low leaching conditions, applications of 20–30 kg S ha⁻¹ as ammonium sulfate reduced S deficiency in crops for 2–4 years after application (Grant et al. 2003b, 2004; Malhi and Leach 2003; Karamanos and Poisson 2004). Residual benefits from sulfate fertilizers may be due to carry over of SO₄- S as ammonium sulfate has been shown to enhance soil SO₄-S (Malhi et al. 2009).

Under adequate soil moisture conditions, a range of application methods can be used to supply readily available S for plant growth and the response to the method of placement for S source varies with the soil and weather conditions. In a study conducted in the black and gray soils of the northern Canadian prairies, similar canola yields were obtained by applying ammonium sulfate as a surface broadcast, in-soil banded or seed-placed, under reduced or conventional tillage (Grant et al. 2004). However, a 3-year study conducted in northern Saskatchewan reported a higher seed yield with placement of ammonium sulfate in the seed row and (or) side-banded compared with broadcast and incorporated S in one year, while in the remaining two years all treatments had comparable yields (Malhi and Gill 2002). This may have occurred because the S bands were better accessed by the roots under drier conditions compared with the shallow placement in surface soil.

Genotypic variation and SUE

Most of the canola currently produced is from hybrid cultivars that have a higher yield potential compared with open-pollinated cultivars (Carew and Smith 2006). Previous studies conducted across western Canada indicated that, hybrid cultivars produced higher yields at a given S level than did open-pollinated cultivars (Karamanos et al. 2005; Brandt et al. 2007). That is, the optimum yield can be achieved with S levels similar to those required to achieve sub-optimal yield of open-pollinated cultivars, showing that the hybrid cultivars may be better able than the open-pollinated cultivars to extract S from the soil (Karamanos et al. 2005, 2007; Malhi and Gill 2006).

Effect of field management practices on SUE

Our analysis shows that adjusting the N rate with the S rate resulted in a higher SUE compared with other management practices, such as only adjusting S rate, applying S in spring or fall, and applying S at different growth stages either as a top-dressed or foliar treatment. An N rate × S rate study was conducted in eastern Canada, where soil moisture was not as limiting a factor as in the Canadian prairies. This multiple-year (2012–2014) study composed of 12 treatment combinations with three levels of S (0, 20, and 40 kg ha^–1) and four levels of N (50, 100, 150 and 200 kg N ha^–1) rates. The results showed that the growing season had a significant effect on the SUE of different treatments (Ma et al. 2020). These results indicate that the effect on SUE among years was not consistent when applied N rates were considered; however, it confirmed that the highest SUE rate corresponded with 20 kg ha^–1 for a given level of N and additional S did not always improve SUE. The lowest SUE was noted from a 3-year (2000–2002) study that included two rates of S (20 and 40 kg S ha^–1) applied in fall or spring using elemental S and SO₄-S as sources. As expected, the results suggested that S applied in the sulfate form produced a higher SUE than the treatments with elemental S in all years (Malhi and Leach 2003).

Improving WUE of canola

In the Brown and Dark Brown soil zones, which comprise most of the semi-arid Northern Great Plains, the potential evaporative demand for water usually exceeds the water available to the crop, representing the greatest limitation to crop production in this semi-arid region (Cutforth et al. 2002). Therefore, improving WUE, especially in the drier regions of the prairies, is an important consideration for increasing yield (Hu et al. 2015). In the present analysis, the impacts of several factors, including canola species, seeding date, irrigation and irrigation timing, stubble height, and timing of stubble incorporation, on the WUE of canola were considered.

Canola has a deep taproot system, and it can extract water from a soil depth down to 1.7 m (Din et al. 2011; Zhu et al. 2016). Out of total soil water extraction, canola can withdraw 45% of water from below 0.6 m (0.6–1.7 m) depth. Also, the short growing season of canola further reduces its irrigation requirement (Katuwal et al. 2020).

Oilseed yield is expected to increase with water use, up to a maximum yield potential (Anastasi et al. 2010). The rate of yield increase, relative to increased water use, represents a measure of water productivity.

One of the greatest challenges for agriculture is to develop technology or agronomic approaches to improve WUE. WUE is partially a function of canola adaptation to environmental conditions. Therefore, favourable agronomic approaches are of great importance. Canola has been shown to have WUE values ranging from 8.3 to 11.4 kg ha^–1 mm^–1 in the sub-humid regions of Canada (Faraji et al. 2009). The growing season on the Canadian prairies generally extends from May to August with most of the precipitation during June and July. After seeding, canola emerges, normally in May, and then grows rapidly through June and early July. The rapid increase in leaf surface area, along with high temperature can increase evapotranspiration and create a moisture deficit during the growing season. A moisture deficit can arise depending on the amount of spring soil moisture and the local level of precipitation, and under limited soil moisture, the crop can be harmed due to water stress during growth and development (Bullock et al. 2010). In addition, canola prefers cooler temperatures, especially during the flowering period and high temperatures during the flowering stage can also lead to a reduction in yield (Cardillo et al. 2015). The effect of irrigation rate and the ability to irrigate (rain-fed vs. irrigated) improved WUE in a positive way, but these practices are not available to all producers. In years with insufficient rainfall, yield increased with irrigation treatments and WUE increased as well (Hergert et al. 2016). In years with adequate rainfall, improvements in yield and WUE between treatments were not always evident.

Irrigation was compared with that of rain-fed canola production by Katuwal et al. (2020). Treatments that provided irrigation from emergence to harvest had the highest WUE followed by full season irrigation (seeding to harvest), whereas withholding irrigation at the reproductive stage resulted in the poorest WUE (Katuwal et al. 2020). These results confirm that a satisfactory supply of water at the reproductive stage of canola is key for improved WUE. As the overall aboveground biomass and leaf area index reach a peak during the reproductive stage (Katuwal et al. 2018), it likely increases total water loss through surface transpiration and increases ET in canola. In addition to the increased ET requirement, the reproductive stage is characterized by many water-sensitive processes such as floral retention, floral bud development into pods and assimilate supply from leaves and pods for seed setting which determines yield in oilseed crops (Eck et al. 1987; Sweeney et al. 2003; Katuwal et al. 2020). Katuwal et al. (2020) reported that skipping irrigation during the vegetative stages and irrigating only during the reproductive stage could maximize water productivity without significantly changing seed yield in canola.

Angadi et al. (2004) measured WUE in relation to canola seeding date. They reported that early spring seeding resulted in greater WUE than early fall planting, which was better than late spring planting with both Brassica rapa and B. napus. The canopy of spring canola is established under cool conditions with modest evaporative demand. Therefore, evaporative losses can somewhat be avoided. Aiken et al. (2011), stated that delaying initial irrigation can reduce evaporation from the soil surface before canopy closure and increase the crop transpiration fraction of ET. Deficit irrigation is a method of utilizing limited irrigation water resources, in which crops are supplied with water below their evapotranspiration requirements during less critical growth stages (Yang et al. 2017). A previous study conducted in the US Southern Great Plains showed that adopting spring canola cultivar L140 and skipping irrigation during the vegetative stage (seeding to bolting) could enhance WUE in water-limited semi-arid regions like the US Southern Great Plains (Katuwal et al. 2020). The WUE of spring canola can be enhanced by minimizing evaporative losses from soil by delaying initial irrigation, seeking rapid canopy closure, or planting an early spring oilseed that forms the canopy under conditions of low evaporative demand. In addition, increasing harvest index can improve WUE and this can be favoured by planting optimal populations, selecting appropriate planting dates, varieties, or hybrids, and avoiding water deficits for vigorous growth and during floral development and seed fill. Furthermore, developing varieties and hybrids that maintain crop productivity and yield under soil water deficit conditions can increase WUE.

The second most effective practice for improving WUE was maintaining stubble height of the crop preceding the canola crop after harvest. This research consisted of leaving wheat stubble standing at various heights, including extra tall (45 cm), tall (30 cm), and short (15 cm) heights from the soil surface. The effect of the stubble height on WUE was compared with that of no stubble (cultivated after harvest). The results suggested that, in general, compared with cultivated stubble, the tallest stubble (45 cm) had the highest WUE followed by 30 and 15 cm standing stubble (Cutforth et al. 2011). Cardillo et al. (2015) reported that, depending on stubble height, stubble management changed the micro-climate near the soil surface by reducing wind speed, solar radiation, and soil temperatures throughout the life cycle of canola. Tall stubble was found to reduce wind speed, soil drying, and evapotranspiration compared with shorter stubble (Cardillo et al. 2015). Their results suggest that tall stubble may have increased seed yield and WUE by providing a favourable micro-climate for increased water conservation. The treatment with no stubble had the lowest WUE. Combining row spacing with stubble height did not have a large impact on WUE.

The third most effective practice for improving WUE was the time of stubble incorporation, which included stubble maintained at 30 cm in fall and seeded to canola in spring, stubble maintained at 15 cm or 30 cm in height and seeded to canola in spring, cultivation in fall and seeded to canola in spring, stubble maintained at 30 cm and seeded to canola with an extra 34 kg N ha^–1 added in spring, and stubble cut at 30 cm tall in fall and then cultivated in spring and seeded to canola. Among these treatments, stubble maintained at 30 cm with extra N produced the highest WUE compared with stubble cut at 30 cm tall in fall then cultivated in spring and seeded, suggesting that both soil moisture and added N had a synergistic positive effect on plant growth and yield, resulting in an improved WUE (Cutforth et al. 2006). These results agree with those of Miller et al. (2003) and Cutforth et al. (2002), who reported that canola and mustard grown in the semi-arid prairie can respond to higher levels of N under favourable moisture conditions. In spite of the potential to conserve soil moisture, the positive response of canola varied with the species studied. A study by Angadi et al. (2008) reported a negative impact on WUE from planting into stubble, and this was attributed mainly to the poor yield response to any stubble by both B. napus and B. campestris, compared with fallow (Angadi et al. 2008). Angadi et al. (2008) also showed that WUE for B. rapa was greater than for B. napus under drought conditions, but that B. napus WUE was higher under rain-fed and irrigated conditions.