Agriculture & Agri-Food Canada, 107 Science Place, Saskatoon, Canada S7N 0X2
In the mid-1990's, canola growers in Saskatchewan began to express concerns about below-average yields despite good growing conditions. In response, growers were surveyed and asked to identify possible factors contributing to low yields (41-90 % of normal) in 1994 and 1995. High temperatures during flowering and early seed-set were identified as the main causal factor, followed by plant diseases, insect damage and weeds as the primary factors responsible for low yields. To corroborate the reports, regions of the province which suffered low yields were identified and a retrospective examination of the predominant factors responsible for yields was conducted. The results indicated that the causal factors could not be clearly identified. Temperatures were not above average during the period in question. Insect pests may have played a minor role in reducing yields in the areas affected by low canola yield. Producers received early warnings of the major insect pest infestations with the result that heavy infestations should not have gone undetected. Diseases such as blackleg, sclerotinia stem rot and alternaria black spot were likely not responsible because these diseases have very obvious symptoms of damage in cases where they are responsible for significant yield loss. Weeds were not identified as a major factor; however, herbicide residues may have been an issue in some cases. Yield data did not correlate well with specific cultivars, but soil macro-nutrients may have been a factor in reduced canola yields. The issue is being further investigated by an agrologist with support from agronomists, entomologists, plant pathologists and weed scientists.
KEYWORDS: Causal Factors, Insects, Weeds, Diseases, Environment, Agronomy
Canola production in the province of Saskatchewan accounts for approximately half of the five million hectares seeded to canola and mustard in western Canada. In the mid-1990's the industry began to hear concerns from Saskatchewan producers about below-average yields in spite of seemingly good growing conditions. This paper documents regions of the province experiencing low canola yields, reviews the findings of a multi-disciplinary team investigating the phenomenon and discusses options for further study.
In response to reports of low canola yields in Saskatchewan for the cropping seasons of 1994 and 1995, producers were asked by researchers to participate in a survey in 1996 (Brandt 1996). The two-part survey asked producers to: (i) identify areas of the province where yields were low and the cause could be identified with some degree of confidence; and (ii) identify areas where yields were below expectations and where causal factors could not be clearly identified. Of the 133 responses received, producers identified drought stress (n=17), insect damage (n=12), and heat stress at flowering (n=9) as the primary factors responsible for low yields (41-90 % of normal) in 38 canola fields (Brandt (1996). Of greater interest, producers were asked to indicate factors that they felt were most commonly associated with low yields in cases where it was unexpected. An aggregate score was calculated by weighting the number of cases where a small yield loss (2X) was reported and where a substantial yield loss was reported (3X). High temperatures at flowering received the highest aggregate score, followed by plant disease and insect damage as the causal factors responsible for low yields.
To document which regions of the province suffered low yields in 1994 and 1995, 10-year yield averages were calculated for each (n=391) rural municipality (RM); yields in 1994 and 1995 were then expressed as a percentage of this 10-year average. Geographic information systems (GIS) software was used to analyse this spatial data set (POTMAPtm module, SPANStm; Tydac Technologies). In 1994, canola yields were 25-50% below average in five RM=s and more than 50% below average in two RM=s. In 1995 the extent of the perceived problem increased dramatically, when 47 RM=s were 25-50% below average and one RM at more than 50% below average.
Environmental conditions. Canola cultivars have been selected to perform well within an optimal range of environmental conditions. Reduced yield potential may be the result of fewer plants per unit area, fewer pods per plant, fewer seeds per pod or smaller seed. All of these parameters can vary in response to environmental stress, depending upon the stage of growth at which the stress occurs. Moisture or temperature stress early in the growing season may reduce germination and plant emergence. However, most canola cultivars are able to compensate quite effectively for reduced plant densities later if environmental conditions improve (McGregor 1987). High temperatures during flowering or early seed set was implicated by the majority of producers who responded to the survey. This could not be corroborated from the weather data for those regions in either 1994 or 1995. In 1994 and 1995, there were only three days and two days near or slightly above 30oC during the critical developmental period (July 15 to 31), respectively.
Insects. Current practices for canola production in Canada require inputs to ensure plant protection from several insect pests. These pests include flea beetles, diamondback moths, bertha armyworms, root maggots, grasshoppers and lygus bugs (Elliott et al. 1997). Flea beetles, Phyllotreta species, are the most serious pest of canola in the central and eastern regions of Saskatchewan. Diamondback moth, Plutella xylostella (L.), is a more sporadic pest of canola and mustard than are flea beetles. Infestations result primarily from migrant moths carried on winds from the southern USA in May and June. Bertha armyworm, Mamestra configurata Walker, is also a sporadic pest; in most years, populations are moderated by weather conditions, fall tillage and biological agents. Root maggots, Delia species, are considered a significant pest of canola primarily in the western region of the prairies. Grasshoppers (Acrididae) can be a problem in canola during seedling establishment in areas where canola production overlap with grasshopper infestations. Plant bugs, Lygus species, are an emerging pest of canola and can severely damage yield and quality by feeding on the developing pods.
Of the six insect pest groups listed above, only bertha armyworm had a major impact on canola production in Saskatchewan in both 1994 and 1995 (Elliott et al. 1997). Approximately 150,000 and 650,000 hectares were treated with insecticides to control bertha armyworm in 1994 and 1995, respectively. Analysis of relative canola yield and moth counts for 1995, indicated that there were 11 RM=s where the areas of low canola yield overlapped with very high moth populations. Diamondback moth had a significant impact on canola production costs in Saskatchewan in 1995 but not in 1994. In 1995, insecticides were applied to about 1.3 million hectares, however, most of that was applied in areas other than those that were experiencing unexpected low canola yields. The other insect pests were below economic levels in most canola growing areas in both 1994 and 1995.
Plant diseases. Diseases of canola are widespread throughout the canola-growing area. They attack canola at all stages of growth, from seeding to harvest (Duczek 1997). Blackleg, caused by the fungus Leptosphaeria maculans (Desm.) Ces. et de Not., is widespread throughout Saskatchewan and can cause significant yield loss in canola. Sclerotinia stem rot, caused by Sclerotinia sclerotiorum (lib.) de Bary, commonly reduces seed yield in canola by 5-10%. Alternaria black spot, caused by Alternaria brassicae (Berk.) Sacc. and A. raphani Groves & Skolko, can cause yield loss and also reduce seed weight, decrease seed viability and seed germination. White rust and staghead, caused by the fungus Albugo candida (Pers.) Kuntze, caused significant losses prior to the adoption of resistant canola varieties. Root rot and foot rot, caused by a complex of soil-borne fungi such as Rhizoctonia solani Kühn, Fusarium, species, and Pythium spp., are often responsible for poor stand establishment. Prevalence of the organism causing Sclerotinia stem rot was high in 1994 over a large area but the incidence of disease was low. Prevalence of the organism causing blackleg was also high in much of the same region but the incidence of disease lesions was generally lower. In 1995, the prevalence of Sclerotinia stem rot was low but the incidence of blackleg was highest in the northeast where surveys found cankers on 22% of basal stems (Kaminski et al. 1996).
Weeds. Weeds can be an important limiting factor in canola production in Saskatchewan. Growth of canola is relatively slow in the early stages. As a result, canola does not compete well with weeds until it is adequately established. Weeds emerging one to two weeks after canola are usually not economically important. The extent of yield loss caused by weeds can be estimated using relationships quantified for specific weed species in canola. Estimates have been developed for Canada thistle, Cirsium arvense (L.) Scop., wild mustard, Brassica kaber (DC.) L.C. Wheeler, perennial sow-thistle, Sonchus arvensis L. , tartary buckwheat, Fagopyrum tartaricum (L.) Gaertn., wild oats, Avena fatua L. and for volunteer wheat and barley. Surveys have shown that four of the five perennial weed species found in canola have increased in frequency and rank over the last 10 years; cleavers, Galium spurium L., has also increased significantly as a weed associated with canola production (Thomas et al. 1998).
Agronomy. Many production factors are involved in determining the establishment of a canola crop with high yielding potential, beginning with soil preparation. For spring seeding, the objective is to provide an environment conducive to rapid seed germination, seedling root development and emergence. Early (May) seeding generally results in higher yields than later seeding. Crop establishment is also determined by seeding rate, seeding depth, row spacing, and the type of seeding equipment used. These production factors do not significantly influence input costs for the producer but they can have a major impact on crop yield. The quality of the seed is also of importance. Beyond good germination, seed quality will also influence seedling vigour. The survey did not identify any major deviation from standard agronomic practices in the areas suffering from low yields
Climatic conditions across the region are highly variable over years and ecoregions. For that reason, an extensive regional cultivar testing system is used to evaluate cultivar performance across ecoregions and years. While the system is not foolproof, it is effective in identifying cultivars that are poorly adapted. In the survey conducted during 1996, there was no evidence that one particular cultivar or group of related cultivars was associated with low yield.
Soil nutrients. Canola is one of the most macronutient-demanding crops grown in western Canada. Nutrients such as nitrogen, phosphorus, potassium and sulphur are in heavy demand by a vigorously growing canola crop. Karamanos (1997) reported that 80% and 25% of the soil samples tested in 1995 were deficient in soil nitrogen and sulphur levels, respectively. This corresponds to approximately 800,000 ha and 250,000 ha of canola that may have been deficient in soil nitrogen and sulphur levels, respectively in 1995. In addition, Janzen and Bettany (1984) have shown that canola production is influenced by a ratio of these two macronutrients. They concluded that a ratio of 7:1, nitrogen and sulphur, was the optimum balance for optimum canola production. Karamanos (1997) also reported that about 60% of the soils samples tested in the province in 1995 did not match the optimum ratio of nitrogen to sulphur.
Insect pests may have played a minor role in reducing yields in the areas affected by low canola yield. Low flea beetle numbers were observed throughout most of the province during the two years in question suggesting that flea beetles had relatively little effect on canola production. There was some overlap in regions where diamondback moth and bertha armyworm were being controlled and where canola yields were below average. However, producers were made aware of diamondback moth and bertha armyworm infestations throughout the region and infestations should not have gone undetected. Based on field surveys, the impact of root maggots, lygus bugs and grasshoppers was probably not economically important.
Diseases such as blackleg, sclerotinia stem rot, alternaria black spot and white rust are likely not responsible for unexpected low yields in canola crops. This is because these diseases have very obvious symptoms of damage in cases where they are responsible for significant yield loss. Symptoms such as premature ripening, severe lodging, and various types of foliar and stem discolouration would likely not have gone undetected. However, there may be other pathogens such as root diseases or bacterial infections that reduce the number of viable florets.
Weed surveys indicate that there is a growing problem with perennial weed species in canola. However, weed management is commonly practised by the majority of producers and severe weed infestations should not have gone undetected. Herbicide residues are being reported in some regions of the province and this may have been an issue. However, as part of the registration process, extensive testing of residual effects of herbicides on rotational crops is conducted. If this were a problem, this process should provide some indication, unless many growers are not using them in accordance with label recommendations. In the grower survey, there was no indication that this was a problem.
Changes in seeding equipment have allowed growers to seed more quickly, allowing them to complete these operations earlier in the season. As a result, it is unlikely timing of seeding was an issue for most cases where yield was below expectations. Problems with crop establishment did not appear to be widespread. Also, surveys could not correlate low yields with varieties/species.
There is evidence to suggest that soil macronutrients may have been a factor in reduced canola yields. Micronutrients such as copper, zinc, iron and manganese were not considered to be a significant source of canola yield loss in Saskatchewan (Karamanos 1997) A recent trend associated with reduced tillage is to place N and S at depths nearer the depth of seeding, often during the seeding operation. The impact of such placement on crop establishment can vary depending on soil conditions and the rates of nutrients applied. There is reduced risk of excessive drying of the seedbed, but that has to be balanced against greater risk of fertilizer damage to seedlings.
The study confirmed that, in these instances, the factors responsible for the unexpected low yields were not apparent. An agrologist has been assigned to examine the issues with support from agronomists, entomologists, plant pathologists and weed scientists. The investigation will begin with an analysis of extensive crop production data sets together with weather data sets to identify causal factors and develop hypotheses which can be tested in controlled experiments. Field monitoring sites will be established to obtain crop growth and yield data under various agronomic and environmental conditions. A crop stress index will be developed to detect episodes of yield-limiting stress conditions beyond which crop damage occurs.
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