During the period from 2000 to 2013, average canola yields from Canadian farms increased from 1330 to 2025 kg ha^–1, or 54 kg ha^–1 year^–1. The objective of this review was to propose likely reasons behind this increase by examining genotypic, environmental and agronomic factors. During this period, hybrid canola cultivars with herbicide tolerance (HY-HT) expanded from 80% to >95% of the area sown to canola. Genetic gain from switching from open-pollinated cultivars to HY-HT cultivars was estimated to account for 32 kg ha^–1 year^–1. When some key environmental factors were examined, there were no significant linear changes in growing season temperature, although the linear increase in April and May precipitation was significant and likely responsible for an increase of 12 kg ha^–1 year^–1. When coupled with the yield increase from changes in atmospheric CO₂ (3 kg ha^–1 year^–1), the environment was estimated to account for ∼15 kg ha^–1 year^–1. Ignoring all main-factor interactions, changes due to management accounted for the remainder, or 7 kg ha^–1 year^–1. The expanded use of HY-HT varieties has resulted in better weed control, and an increase in the use of minimum tillage, leading to greater water-use efficiency and higher yield. It is likely that many of the effects of changes in management were hidden in the interaction with genotype and environment main effects. It is difficult to estimate these interactions without designing experiments to do so. The design and implementation of experiments to understand the interaction among main factors should be a priority. Future yield targets of 25 Mt canola by 2025 will require an increase in yield per ha beyond the current rate, or an increase in the land seeded to canola, or a combination of the two factors. Continued progress with canola yield depends on active plant-breeding programs, agronomic research using new varieties, favourable environmental conditions, and high world commodity prices.

The inland Pacific Northwestern USA (iPNW) wheat-producing region has a diversity of environments and soils, yet it lacks crop diversity and is one of the few semi-arid wheat-growing regions without significant integration of oilseeds. Four major agroecological zones, primarily characterised by water availability, feature distinctly different fallowed and annually cropped systems, each presenting different challenges and opportunities to integrate winter and spring canola. Although major interests in regional energy crops and rotational diversification spurred feasibility research on iPNW canola food, feed and fuel production in the 1970s, commercial canola adaptation has lagged behind other semi-arid wheat regions for various socioeconomic, ecophysiological and agronomic reasons. New federal crop insurance policies will reduce economic risks in new crop adaptation, and oilseed processing facilities are creating new local markets. Although canola management largely relies on wheat farm equipment, agronomic approaches require strategic adjustments to account for physiological differences between canola and cereals including seed size, seedling morphology and responses to temperature extremes. Climate change predictions for the region threaten to exacerbate current hot and dry summers and research aims to develop and adapt flexible winter and spring canola-based systems to regional water and temperature stressors in each zone. Adaptation will require novel planting, fertilisation and weed control strategies to successfully establish improved winter canola cultivars in hot dry summers that survive cold winters, and spring canola cultivars direct-seeded in cool wet springs. The adaptation of winter and spring canola will somewhat mirror the rotational placement of winter and spring cereals within each zone. Economic analysis of oilseed break crop benefits such as weed and disease control will help to demonstrate the medium-term economic benefits of crop diversification to support the growth of a regional canola industry in the iPNW.

Innovation has been integral in the development of the current Australian canola (Brassica napus L.) industry. From the initial introduction of poorly adapted Canadian germplasm, Australian breeders have developed high yielding, high quality, disease-resistant canola cultivars. The Australian canola industry has transitioned from being reliant on imports to becoming one of the world’s major exporters of canola. This review details the progressive innovations in the Australian canola breeding programs from the initial introduction of rapeseed to more recent developments including herbicide resistance, hybrid cultivars, speciality oil types and genetically modified canola.

Blackleg disease caused by the fungus Leptosphaeria maculans is the most important disease of canola worldwide. The impact of this disease on the development of the Australian canola industry, particularly over the last 20 years, is discussed. Deployment of a range of disease control measures has resulted in a thriving canola industry with production now approaching 4 million tonnes annually. Discoveries about disease mechanisms and key plant and fungal genes are described. Analysis of the L. maculans genome sequence has enabled an understanding of how fungal populations can evolve rapidly to overcome disease resistance bred into canola cultivars.

Timing of life history events (phenology) is a key driver for the adaptation of grain crops to their environments. Anthesis (flowering) date is the critical phenological stage that has been most extensively studied. Maximum crop yield is achieved by maximising the duration of the pre-anthesis biomass accumulation phase and hence yield potential, while minimising the risk of water stress and temperature stress (heat and cold) during flowering and grain-filling stages. In this article, we review our understanding of phenology of the valuable oilseed crop canola (oilseed rape, Brassica napus L.) from the perspectives of biophysical modelling and genetics. In conjunction, we review the genomic resources for canola and how they could be used to develop models that can accurately predict flowering date in any given set of environmental conditions. Finally, we discuss how molecular marker tools can help canola breeders to continue to improve canola productivity in the light of climate changes and to broaden its adaptation into new agricultural areas.

High yield is a major objective in canola-breeding programs. We analysed the genetic determinants controlling variation in grain yield in a doubled-haploid (DH) breeding population derived from a single BC₁F₁ plant from the cross Skipton/Ag-Spectrum//Skipton (designated as the SAgS population). DH lines were evaluated for flowering time and yield in two replicated trials and exhibited significant genetic variation for both traits. Yield showed negative correlation with flowering time; lines that flowered earlier had higher yield than late-flowering lines. A genetic linkage map comprising 7716 DArTseq markers was constructed for the SAgS population, and a ‘bin’ map based on 508 discrete single-position (non-co-segregating) marker loci was used for quantitative trait locus (QTL) analysis. We identified 20 QTLs (LOD ≥2) associated with variation in flowering time and grain yield. Two QTLs (Qy.wwai-A7/Qdtf.wwai-A7/Qfs.wwai-A7 and Qy.wwai-C3a/Qfs.wwai-C3a) appeared repeatedly across experiments, accounting for 4.9–19% of the genotypic variation in flowering time and yield and were located on chromosomes A07 and C03. We identified 22 putative candidate genes for flowering time as well as grain yield, and all were located in a range of 935 bp to 2.97 Mb from markers underlying QTLs. This research provides useful information to be used for breeding high-yielding canola varieties by combining favourable alleles for early flowering and higher grain yield at loci on chromosomes A07, C03 and possibly on A06.

Canola breeding in Australia began in the early 1970s with the first cultivars being released in the late 1970s. Thirty-four non-herbicide-tolerant canola cultivars, released in Australia between 1978 and 2012, were evaluated for improvements in yield, quality, blackleg resistance and adaptation to Australian environments. The cultivars were sown at three sites in 2008 and one site in 2014. In addition, blackleg susceptibility was assessed in two independent blackleg experiments in 2008. Yield improvement averaged 21.8 kg ha^–1 year^–1 (1.25% year^–1) but ranged from 8 to 39.1 kg ha^–1 year^–1 at the lowest to the highest yielding sites, respectively. Although the yield gain shown by our study was for conventional canola only, the different herbicide-tolerant types are derived by incorporating the herbicide tolerance genes into Australian germplasm and so the rate of genetic gain would be expected to be similar for all herbicide tolerance types. Oil and protein concentrations have increased by 0.09% year^–1 and 0.05% year^–1, respectively, whereas glucosinolate concentration was reduced to between 7 and 16 μmoles per gram of meal by the mid-1990s. Cultivars released before 2002 all had low to moderate resistance to the blackleg isolates present in the fields during the experimental period but more recent releases had improved survival under heavy blackleg pressure due to the incorporation of additional or different resistance genes. The data suggests that at least 25% of the yield improvement achieved by the breeding programs over 30 years was associated with improved blackleg resistance and the remainder with gains in other aspects of potential grain yield. The private breeding companies in Australia will need to continue to produce cultivars with high yield potential and deploy blackleg resistance genes wisely in order to maintain the yield improvements required to remain competitive in global markets.

If predictions are correct, heat stress during reproduction will become a yield limiting factor in many world crops and breeding heat stress tolerance a major goal. The objective of our paper was to highlight a novel system to investigate the influence of temperature (T) on pollen germination using a thermal gradient PCR programmed to establish differential Ts across 12 wells of a PCR plate. Seven cultivars of Brassica napus L. were grown through flowering in a cool growth cabinet (20/15°C day/night) or a heat stress cabinet (HST, 27/22°C day/night). Pollen from each cultivar × cabinet combination was aspirated from 6 opened flowers, and suspended in germination media. Drops of the pollen suspension were floated on media in each well, and the PCR T was set to 30°C with a gradient of ± 10°C, creating a range from ∼20 to 40°C from left to right. After an 8 h treatment, the pollen germination (pg, %) and pollen tube growth score (ptg, 1–5) were evaluated using a microscope. There were significant differences among cultivars for pg and ptg score and significant differences among well T for pg and ptg score. Pollen tubes grew best at T from 20 to 23°C. Well T exceeding 33°C reduced pg and ptg score, although 3 of the 8 cultivars had good pg even at 36°C. HST >29°C, in a growth cabinet, generally resulted in B. napus raceme sterility, although our experiment showed that pollen was still capable of germinating up to 33°C, indicating that pollen germination may not be the only reason for heat stress susceptibility.

Australian canola growers have new technology options including hybrid and herbicide technologies, which have offered yield and profitability advantages in other canola-growing regions of the world. This study compared the yield and gross margins of hybrid and open-pollinated (OP) canola from different herbicide tolerance groups: triazine-tolerant, Roundup Ready, Clearfield and conventional across a wide range of environments in south-western Australia, and in the National Variety Trial network in southern Australia to investigate the relative advantages of these technologies. There were significant differences in yield responsiveness between hybrid and OP canola, the magnitude of which was determined by the growing-season rainfall/available water to the crop. Hybrid out-yielded OP canola in favourable environments where rainfall was high and the growing season was long. However, in areas of low rainfall where yield potential was low, hybrids showed little yield advantage over OP. In contrast, there were no differences in yield response between the four herbicide tolerance groups across the rainfall zones. The economic analysis showed that the break-even yield for hybrids versus OP canola was 1.25 t/ha for triazine-tolerant canola, 0.7 t/ha for Roundup Ready canola, and 1.7 t/ha for hybrid Clearfield canola. The gross margin analysis suggested that hybrid triazine-tolerant, Clearfield and Roundup Ready canola was more profitable than the OP system in the medium (growing-season rainfall of 265–330 mm) and high (330 mm) rainfall environments, but not profitable in the lower (<265 mm) rainfall area because the cost associated with hybrid seed outweighed the small yield benefit. The sensitivity analysis indicated that ± 10% changes in canola price and seed cost shifted the break-even yield by ± 0.1 t/ha. Our study makes a case for Australian canola breeders to maintain OP canola varieties, rather than shifting their focus entirely to hybrids, to underpin continued productivity and profitability in lower rainfall areas.

The canola (Brassica napus L.) module in the Agricultural Production Systems Simulator (APSIM) was developed in the late 1990s. There has been no peer-reviewed account of the scientific underpinnings of the module, despite considerable testing across a wide range of environments in the Australian grains industry and numerous applications of the model to address agronomic and crop adaptation issues. This paper presents a summary of the parameters in the module and reviews the physiological evidence justifying their values and module performance, and reflects on areas of module improvement and application.

APSIM-Canola simulates crop development, growth, yield and nitrogen (N) accumulation in response to temperature, photoperiod, radiation, soil water and N supply, with a daily time-step, using well-accepted approaches. The module has been validated on more than 250 data points across Australia, China, and Germany and typical root mean squared deviations for days to flowering are ∼5 days and for grain yield are ∼0.4 t ha^–1.

Testing on vernalisation-responsive winter types and in high yielding situations has indicated that more research is required to define phenology parameters and yield forming processes in high yielding environments. There is a need to develop better predictive routines for grain oil content that take account of the dynamics of grain filling and interactions with environmental conditions, and improve upon current regression-type approaches. Further testing of N responses is required. Physiological characterisation of new cultivar types, such as hybrids, Indian mustard (Brassica juncea), and new herbicide tolerance types is required to make the module more applicable to contemporary canola production systems. A lack of understanding of the effects of high and low temperature extremes on reproductive processes is currently limiting the use of the module outside conventional sowing dates and agro-climatic zones.

Implementation of the BBCH coding system for winter oilseed rape (OSR) phenology simulation can allow detailed description of crop ontogeny necessary for crop management and crop growth modelling. We developed such a BBCH model using an existing approach (Habekotté 1997).

The new model describes winter OSR development by a combination of differential and conversion equations based on the structure of the BRASNAP-PH model (Habekotté 1997). Six phenological phases were reproduced daily according to the BBCH codes (00–89): emergence (00–09), leaf development (10–19), stem elongation (30–39), inflorescence emergence (50–59), flowering (60–69) and pod development-maturation period (70–89). The model takes into account temperature (including vernalisation) and photoperiod as the main environmental forces affecting crop phenology. The macro stages of leaf development and shooting were reproduced considering the rates of leaf appearance and internode extension. Model calibration and validation were performed using an extensive database of phenological observations collected from several experimental sites across France (n = 144), Germany (n = 839) and Italy (n = 577). The stability of the parameterisation was checked by a cross-calibration procedure.

Applied to the independent datasets used for validation and cross-validation, the model was able to predict the whole-crop cycle with a root mean square error (RMSE) of 2.8 and 3.2 BBCH stages, respectively. Particularly accurate predictions of winter OSR development were obtained with the Italian datasets (RMSE: 2.1 and 2.3 BBCH stages for validation and cross-validation, respectively). Considering the phenological phases separately, emergence, leaf development, flowering and the pod development–maturation period were simulated with RMSE of 1.0, 2.4, 2.9 and 3.2 BBCH stages, respectively (validation datasets). Slightly higher uncertainty emerged in the prediction of stem elongation and inflorescence emergence phases (RMSE: 3.5 and 4.1 BBCH stages, validation datasets).

The model reproduced winter OSR development with a sufficient degree of accuracy for a wide range of years, locations, sowing dates and genotypes, resulting in an efficient and widely applicable prediction tool with relevant practical purposes in the crop management scheduling.

In the High Rainfall Zone (HRZ) of southern Australia, long-season winter canola types have been commercially available only since 2011. Experiments in this region show that these varieties can provide improvements in grain yield over spring types of >20% because of their ability to make better use of the longer growing season. However, within this longer crop duration, the optimum length and timings of the critical growth phases to maximise grain production are unknown. Data from eight field experiments conducted between 2010 and 2014 at Hamilton, in the HRZ of south-western Victoria, were analysed to determine whether different phases within the crop’s life cycle vary in their contribution to grain yield and, if so, how this is influenced by climatic conditions. The dataset provided 536 genotype–environment–management combinations including 60 varieties ranging in total crop duration from 186 to 236 days. Over the 5 years, seasons were highly variable with annual rainfall ranging between 479 and 981 mm and spring rainfall (September–November) between 84 and 199 mm. The range of crop maturity types (i.e. winter and spring types) and environmental conditions provided a wide spread in growth, development and grain yield. The analysis showed a positive association between longer duration from flowering to maturity and grain yield, and showed that the duration was influenced by both environmental and genetic factors. Pre-flowering reserves made an important contribution to grain yield, and remobilisation of reserves from the pre-flowering period was greatest for winter types, presumably due to less favourable conditions for growth during grain-filling. Optimising flowering to produce sufficient pre-flowering reserves for remobilisation while ensuring that environmental conditions post-flowering are such that the grain-filling duration is maximised may provide a strategy to increase yields in this environment.

The better performance of hybrid canola compared with open-pollinated triazine-tolerant canola can be associated with greater biomass and harvest index. We compared several hybrid and open-pollinated canola cultivars in field conditions to (i) quantitatively analyse yield formation and identify the key drivers of yield formation process; (ii) investigate biomass accumulation and partitioning and evaluate the relative importance of biomass, harvest index and yield components. Six elite varieties, two from each of the three types (triazine-tolerant (TT), hybrid TT, and hybrid imidazolinone-tolerant (IT) or conventional (CV) (hybrid IT/CV)) of canola, were grown under the optimum crop management in the 3 years from 2009 to 2011 in the high-rainfall zone of south-western Australia. Leaf area, specific leaf area, light interception, biomass, seed yield and yield components were measured at key growth stages to determine biomass accumulation, crop growth rate (CGR), radiation-use efficiency and to investigate the relationship between yield, biomass, CGR, specific leaf area, yield components and harvest index. Hybrid IT/CV canola grew more vigorously with thicker leaves and greater leaf area, allocated more biomass into leaves, intercepted more radiation, produced higher biomass in the vegetative stage and maintained its biomass superiority throughout the whole crop cycle. It had radiation-use efficiency of 1.74 g MJ m^–2 photosynthetic active radiation, 28% higher (P < 0.001) than TT canola (1.41 g MJ m^–2 photosynthetic active radiation) and 16% higher (P < 0.001) than hybrid TT canola (1.52 g MJ m^–2 photosynthetic active radiation). The average CGR for hybrid IT/CV canola (12.1 g m^–2 day^–1) was 32% higher than that of TT canola (9.2 g m^–2 day^–1) from budding to the beginning of pod filling. Hybrid IT/CV canola produced 38% higher seed yield than TT canola in favourable growing conditions (2009, 2011). However, there was no yield difference between the hybrid IT/CV, hybrid TT, and TT canola in the drought year (2010). The number of pods m^–2 and seeds m^–2 was highly associated with biomass at vegetative, budding, flowering, podding and maturity and CGR from budding to podding. High yield in hybrid canola was attributed mainly to higher biomass from each phenological phase from the vegetative stage to maturity and not to improved harvest index.

Optimising the sowing date of canola (Brassica napus L.) in specific environments is an important determinant of yield worldwide. In eastern Australia, late April to early May has traditionally been considered the optimum sowing window for spring canola, with significant reduction in yield and oil in later sown crops. Recent and projected changes in climate, new vigorous hybrids, and improved fallow management and seeding equipment have stimulated a re-evaluation of early-April sowing to capture physiological advantages of greater biomass production and earlier flowering under contemporary conditions. Early–mid-April sowing generated the highest or equal highest yield and oil content in eight of nine field experiments conducted from 2002 to 2012 in south-eastern Australia. Declines in seed yield (–6.0% to –6.5%), oil content (–0.5% to –1.5%) and water-use efficiency (–3.8% to –5.5%) per week delay in sowing after early April reflected levels reported in previous studies with sowings from late April. Interactions with cultivar phenology were evident at some sites depending on seasonal conditions. There was no consistent difference in performance between hybrid and non-hybrid cultivars at the earliest sowing dates. Despite low temperatures thought to damage early pods at some sites (<−2°C), frost damage did not significantly compromise the yield of the early-sown crops, presumably because of greater impact of heat and water-stress in the later sown crops. A validated APSIM-Canola simulation study using 50 years of weather data at selected sites predicted highest potential yields from early-April sowing. However, the application of a frost-heat sensitivity index to account for impacts of temperature stress during the reproductive phase predicted lower yields and higher yield variability from early-April sowing. The frost–heat-limited yields predicted optimum sowing times of mid-April at southern sites, and late April to early May at the northern sites with lower median yield and higher yield variability in crops sown in early April. The experimental and simulation data are potentially compatible given that the experiments occurred during the decade of the Millennium drought in south-eastern Australia (2002–10), with dry and hot spring conditions favouring earlier sowing. However, the study reveals the need for more accurate and validated prediction of the frost and heat impacts on field-grown canola if simulation models are to provide more accurate prediction of attainable yield as new combinations of cultivar and sowing dates are explored.

In 24 experiments conducted across a range of agricultural environments in Western Australia between 2010 and 2014 canola (Brassica napus L.) grain yield response to crop density was adequately described by an asymptotic model (where yield approaches but never quite reaches a ceiling at very high density) in 101 out of 112 individual responses; in the other 11 yield reached a maximum and declined slightly at higher densities. Seed oil was more likely to increase than decrease with increasing density but the effect was always small; less than 1% oil over the range of densities tested. Increasing density also suppressed annual ryegrass (Lolium rigidum (L.) Gaud.) head numbers in six experiments where it was measured, especially at densities below 20 plants/m².

Economic optimum densities ranged from 7 to 180 plants/m², with a median of 32.2. Mean optima in low and medium rainfall zones (growing season rainfall <300 mm) were about 25, 30, and 75 plants/m² respectively for glyphosate-tolerant (Roundup Ready), hybrid triazine-tolerant (TT), and open-pollinated TT cultivars, assuming open-pollinated TT cultivars were grown from farm-saved seed. There was little difference between optimum densities for hybrid and open-pollinated glyphosate-tolerant cultivars, and optima in the high rainfall zone were about 10 plants/m² higher than in low and medium rainfall zones. Yield at optimum density was greater than 90% of maximum yield in 74% of cases. The economic penalty for not achieving the optimum density with hybrids was usually small if the deviation was less than 10 plants/m², and with open-pollinated TT cultivars was small even 50-60 plants/m² below the optimum. The penalty was usually greater for deviations below than above the optimum in medium and high yield potential environments (yield potential >1000 kg/ha).

Predicted optima were more sensitive to seed cost and field establishment (the proportion of viable seeds that become established) than grain price or seed size over the range of values expected in Western Australian agriculture. Field establishment varied from 0.3 to 1 and was higher at low target densities and for hybrid compared with open-pollinated cultivars, with a median of 0.585 at a target density of 40 plants/m². We identified improving field establishment of canola as an important research priority.

April sowing of canola is considered optimal for grain yield in many regions of Australia; however, there is often insufficient rainfall in April to sow seed into moisture at the ideal depth of 15–30 mm. We report a series of experiments that investigated the seed characteristics (cultivar type and seed size) that would facilitate successful canola emergence from relatively deep sowing (>30 mm). Ten canola cultivar by sowing depth experiments, each with three hybrid and three open-pollinated cultivars, found hybrid cultivars were able to maintain higher emergence rates and grain yield compared with open-pollinated cultivars from deep sowing. Further investigations in the glasshouse showed that the emergence advantage of the hybrid cultivars was largely due to their inherently large seed size, as increased seed size also improved emergence of open-pollinated canola. Early biomass accumulation also improved with larger seeds. In a field experiment, larger seed size of both hybrid and open-pollinated canola increased early biomass accumulation and final grain yield. Simulation modelling in New South Wales demonstrated the importance of timely sowing of canola, as delayed sowing caused a larger reduction in grain yield than reduced plant population. Although ‘moisture-seeking’ (placing seed into moist soil below a layer of dry soil) reduced the emergence rate of canola, the reduction could be offset by planting large seed (>2 mm diameter). This practice of ‘moisture-seeking’ large-seeded canola should be considered as a strategy to improve the timeliness of establishment and subsequent grain yield of canola when rainfall for crop establishment is marginal yet there is moisture available deeper in the seedbed.

The expansion of canola production in Australia coincided with an increase in cropping intensity and a reduction in pastures and tillage. These changes mean that nitrogen (N) is often recognised as the most limiting nutrient in canola production, and is the largest single input cost for many growers. Canola responds to added N by producing larger plants that results in a longer leaf area duration, building a larger photosynthetic canopy for seed filling. Although the crop can compensate for poor early growth, a larger canopy is able to compete more effectively against weeds and helps reserve water for crop transpiration rather than soil evaporation. Nitrogen uptake is most rapid during stem elongation, and the N acquired can be remobilised to developing pods and then to seeds. Unlike wheat, N uptake can continue until drought or high temperatures prevent further assimilate supply to the reproductive apex. Data from Australian experiments that measured N uptake over the whole growth period showed that each tonne of seed required ∼80 kg N to be taken up, and this forms the basis of a budgeting approach for determining N supply. Typically, added N reduces seed oil concentration at a rate of between –0.03 and –0.13%/kg N. Despite this decline due to added N, oil yield usually increases and the overall value of the crop also increases. Nitrogen has little impact on oil quality or seed glucosinate concentration.

The efficiency and effectiveness of N management depends first on selecting a rate appropriate to the water-limited yield potential. Most growers estimate the N rate required using an N budget based on supplying 80 kg N/t less indigenous N supply. The budgeted N can be split over two, three or even more applications with little loss in agronomic efficiency. Splitting application enables growers to make decisions about N when there is more certainty about seasonal conditions. Urea is the most common N source used, and unless there are particular loss processes that are likely to occur, it is cheap and effective.

Suggested areas for future N research on canola are to develop tools that can assess in-crop N status, an evaluation of late season N product rate and timing particularly on seed oil concentration, N management for grazed canola, and the development of guidelines to identify, and then address, particular N loss pathways using enhanced efficiency fertilisers.

A rotational field experiment was established in the year 2002 at the experimental farm Etzdorf in the Hercynian dry region of central Germany. Since 2005 field measured datasets were used to determine the effect of different preceding crop combinations and different nitrogen (N) fertilisation treatments on the seed yield, oil content, oil yield and N-use efficiency of oilseed rape (Brassica napus L.). The preceding crop combinations compared were winter wheat (Triticum aestivum L.)-winter wheat (WW), WW-oilseed rape (OSR), OSR-OSR and an OSR monoculture. In addition to the preceding crop combination, N fertiliser treatments with either 120 kg N ha^–1 or 180 kg N ha^–1 were established in the year 2013.

Overall the results demonstrated that seed yield, oil yield and N-use efficiency all declined with an increased cropping intensity for the period 2005–2012. Higher N rates in the 2013–2014 seasons increased seed yield and oil yield when OSR followed WW-WW pre-crops. OSR monoculture had lowest yield independent of applied N. Seed yield declined from 4.61 t ha^–1 (OSR following WW-WW) to 4.28 t ha^–1 in the OSR monoculture with 120 kg N ha^–1, and from 4.81 t ha^–1 (following WW-WW) to 4.42 t ha^–1 in the OSR monoculture with 180 kg N ha^–1. Higher N rates generally reduced N-use efficiency, with highest N-efficiency for WW-WW-OSR (38.4 kg kg^–1), and lowest for continuous OSR receiving 180 kg N ha^–1 (24.5 kg kg^–1).

These results emphasise the importance of crop rotation to maintain seed yield and oil yield of oilseed rape, and to maximise the response to applied N. A reduced N rate increased N-use efficiency and reduced the risk of high N surpluses without a significant/equivalent decrease of the seed yield when the rotation was optimised.

Canola (Brassica napus L.) is widely grown throughout all rainfall zones in south-western Australia. Yields are low by world standards, and variable in low-rainfall (<350 mm annual rainfall) and medium-rainfall (350–450 mm) zones, so that minimising production costs is a major consideration for growers in these areas. One of the major input costs is nitrogen (N) fertiliser. Fifteen N rate × application time × canola plant-type experiments were conducted in the low- and medium-rainfall zones between 2012 and 2014. In most experiments, five rates of N were tested, of ranges 0–75, 0–100, or 0–150 kg N/ha. Nitrogen was applied at four different times (seeding, or 4, 8 or 12 weeks after sowing) or split between these timings. Each experiment compared triazine-tolerant (TT), open-pollinated (OP) canola with Roundup Ready (RR) hybrid canola, and one experiment included TT hybrid and RR OP canola types. On average, RR hybrid produced 250 kg/ha, or 23% more seed and 2.2% more oil than TT OP canola, and the average gross margin of RR hybrid was AU$65/ha more than TT OP. However, seed yield and gross margin differences between RR hybrid and TT OP canola were reduced when seed yields were <1400 kg/ha.

Canola growth (dry matter) and seed yield responded positively to N fertiliser in most experiments, with 90% of maximum seed yield achieved at an average of 46 kg N/ha (s.e. 6). However, 90% of maximum gross margin was achieved at a lower average N rate of 17 kg N/ha, due primarily to the relatively small yield increase compared with the reduction in concentration of oil in the seed with N applied. Because canola growers of south-western Australia are now paid an uncapped premium for canola grain with oil concentration >42%, decreases in oil percentage have a significant financial effect, and recommended rates of N should be lower than those calculated to optimise seed yield. In 80% of cases, the first 10 kg N/ha applied provided a return on investment in N >$1.50 for every $1 invested. The next 20 kg N/ha applied provided a return on investment of $1.25 for every $1 invested 80% of the time, and further increases would most likely break even. The timing of N application had a minor effect on yield, oil and financial returns, but delaying N application would allow farmers to reduce risk under poor conditions by reducing or eliminating further inputs. Overall, our work demonstrates that a conservative approach to N supply mindful of the combined impacts of N on yield and oil is necessary in south-western Australia and that split and delayed applications are a viable risk-management strategy.