Management of Weeds within Tillage Systems: What have we learned from Prairie Weed Surveys?

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1 Management of Weeds within Tillage Systems: What have we learned from Prairie Weed Surveys? Summary Julia Y. Leeson and A. Gordon Thomas Agriculture and Agri-Food Canada, Saskatoon Research Centre, Saskatoon, SK, S7N X2. Weed surveys conducted in the Prairie Provinces from 21 to 23 indicate that most producers successfully managed their weeds regardless of tillage used. Differences in weed populations among tillage s are as much a function of other management decisions as they are to differences practices. For example, more use of herbicides in the s would be expected to contribute to lower weed densities in these s. Conversely, producers using s tended to include a higher diversity of crops in their rotations, leading to a larger presence of volunteer crops. Higher numbers of narrow-leaved hawk s-beard and cleavers was noted for. Introduction The annual cropping area using minimum- and s has been increasing steadily in the Canadian Prairie Provinces, especially during the in the last decade 5. A number of studies conducted to determine the influence of tillage s on weed species diversity and density have produced inconsistent results 7. Recently, weed survey data have also been used to correlate tillage s with management practices and weeds in Saskatchewan 3. Here, similar methodology is used with survey data from all three Prairie Provinces. The effect of the different tillage s on weed species density and abundance is assessed based on data collected in fields. Prairie Weed Surveys Provincial weed surveys provide important clues on the status of weed control under a wide variety of actual field conditions. The most recent weed surveys were conducted in 21, 22 and 23 in Alberta, Manitoba and Saskatchewan, respectively 4. Weeds were counted in randomly selected fields in July or August after all weed management practices had been imposed. Therefore, the weeds surveyed were the end result of all agronomic management decisions (e.g., crop rotation, time and type of tillage, rate and placement of fertilizer, and selection, rate, timing, effectiveness of herbicide used etc.) made by the producers. All annual cereal, oilseed and pulse crops with more than 142, seeded ha within a province were included in the provincial survey. In conjunction with the survey, producers were asked to fill out a management questionnaire detailing practices used on the surveyed field questionnaires were returned with complete information about tillage, fertilizer and seeding practices. Definition of Tillage Systems Tillage is defined as the amount of soil disturbance imposed on a field during a given crop year. This would include tillage during the preceding fallow period (if applicable), after harvest in the preceding year, in the spring prior to seeding, during the seeding operation and after seeding (Table 1). The does not include any preseeding tillage; however, it may include the use of packers, harrows and harrow-packers. Also, heavy harrows (equivalent to 25% soil surface disturbance) can be used once at any time during the crop year, usually immediately after harvest or prior to seeding. At the time of seeding, smooth coulters, on-row packing, tine harrows and harrow packers are allowed but rotary harrows and fluted coulters are not allowed. Harrowing after seeding is allowed, causing up to 2% disturbance. The does not use any tillage on fallow in the previous year. A single separate fertilizer banding application before seeding is allowed if it caused less than 33% soil disturbance. When a Volume

2 separate fertilizer banding application is made, the seeding operation is only allowed to cause up to 33% soil disturbance, which is a function of opener width and row spacing. If there are no separate fertilizer banding or manure applications that involve soil disturbance, the seeding operation may cause up to 4% soil disturbance. Both scenarios are grouped together and defined as a. Systems with high levels of soil disturbance during the seeding operation, but with soil disturbance equivalent to the maximum allowed for at other times would technically qualify as minimum-till. However, they are placed in a separate high-disturbance seeding category for this paper (Table 1). This category includes all direct-seeded s where the soil disturbance at seeding exceeds 4% such as seeding with discers or air seeders equipped with full cut sweeps. Table 1. Description of tillage operations allowed with their respective acceptable level of soil disturbance (expressed as a percentage) for various categories of tillage s. Highdisturbance (NT) Time of tillage Scenario 1 Scenario 2 (HDNT) (MT) (HT) Summer none none none tillage and No restriction fallow chemical Fall <25% <33% <33% <33% No restriction (after harvest) disturbance disturbance disturbance disturbance or or or and Spring <25% <33% <33% <7% No restriction (before seeding) disturbance disturbance disturbance disturbance At seeding <4% <33% No restriction No restriction No restriction disturbance disturbance After seeding <2% disturbance <2% disturbance <2% disturbance No restriction No restriction In the minimum-till, fall tillage may include unrestricted used of heavy harrows as well as harrows, packers and anhydrous knives (Table 1). Spring tillage may also include any combination of one pass with a cultivator, rodweeder and/or rotary harrow. If the preceding crop is fallow, weed control must include the use of herbicides rather than relying entirely on tillage. Systems with more intensive tillage are classified as high till (Table 1). This term is used to avoid the regional connotations associated with conventional tillage. While higher levels of tillage are considered conventional in some areas of the Prairies, actual rather than relative tillage intensity is more likely to affect weed populations. Geographic Distribution of Tillage Systems was the most common in all areas in Saskatchewan; conversely, the high-till was most common in all ecoregions in Alberta and Manitoba with the exception of the Fescue Grasslands in southwestern Alberta (Figure 1). Within Alberta and Manitoba, was more commonly practiced in the drier southern areas. The use of the high-disturbance was least common in the northern areas of all provinces. The minimum-till was rarely practiced in Manitoba and the Peace Lowland and Aspen Parkland Ecoregions of Alberta. Previous studies in the Prairie Provinces have shown that ecoregion is a more important determinant of weed communities than management 1. Therefore, the data presented in this paper have been weighted to remove the effect of regional differences in the adoption of tillage s Volume 2 29

3 PS&C % area Figure 1. Percentage of area in each ecoregion using each tillage. Crop Management Practices associated with Tillage Systems Use of herbicides and the timing of herbicide applications were related to tillage (Figure 2). Systems with higher tillage intensity were less likely to use herbicides in each of the various application windows during the crop production cycle. Approximately 13% of area with the high-till did not use any herbicides, regardless of application window. With, less than 1% of the production area did not receive at least one herbicide application Pre-harvest After harvest Before crop After crop Any time emergence emergence Application Window Figure 2. Percentage of area in each tillage using herbicides in four application windows during the crop year. Diversity of crops grown was also related to tillage (Figure 3). Summer fallow was more common in s with higher tillage intensity. Summer fallow was part of the crop rotation in 57% of the area in the high-till and only 26% of the area in the. Hay was most often included in the high-till (9%). In these cases, tillage may have been thought to be necessary to break the perennial crop. Conversely, oilseed and pulse crops were more often included in crop rotations where was practiced. The association of pulse crops and conservation seeding s has previously been shown2. Overall, 27% of the high-till area had only one crop type (cereals) planted in six years while only 8% of area had comparably low crop diversity (Figure 4). Only 16% of the area in the high-till had three or more crop types included in the rotation, as compared to 3% in the highdisturbance and 36% in s. Volume

4 Summer Cereal Oilseed Pulse Hay fallow Figure 3. Percentage of area in each tillage using summer fallow and various crop types in a six-year period Number of crop types in six years Figure 4. Percentage of area in each tillage using a diversity of crop types in a six-year period. Influence of Tillage Systems on Weeds Weed Density Most producers, regardless of tillage, were able to successfully control their weed populations based on measured values of weed densities (Figure 5). In all tillage s, weeds were not detected in a few fields during surveys. These weed-free fields accounted for at least 3% of the area in each and 6% of the area in the. At least 25% of the area in all s had residual weed densities of less than 3 plants per square metre, and approximately 5% of the area in each of the s had less than 1 plants per square metre. In general, these densities would not be expected to have a large impact on yield; however, this is dependant on the weed stage relative to the crop stage and weed species as well as competitive ability of the crop. The had the lowest weed densities followed by the high-disturbance and the minimum- and hightill s had the highest weed densities. The differences between and high-till were larger at higher weed densities with areas under showing a smaller percentage of high weed densities (Figure 5). Only 18% of the area in the had weed densities greater than 3 plants per square metre compared to 26% of the area in the high-till. The larger proportion of area in the high-till with high weed densities may be attributable to the larger percentage of area in this not using herbicides compared to the (Figure 2) Volume 2 29

5 Weed density (m -2 ) 3 to 6 1 to 3 3 to 1 Less than 3 Highdisturbance Minimumtill Figure 5. Percentage of area s with selected weed densities. Species Associated with Tillage Systems In the context of this paper, abundance is an index used to rank weeds based on the density of the weed in individual fields, percentage of the field occupied by the weed and the number of fields with the weed present 6. Green foxtail was the most abundant species recorded in all tillage s and therefore ranked first (Table 2). Wild buckwheat and wild oat ranked either second or third depending on the tillage. Four additional species, Canada thistle, dandelion, lamb s-quarters and stinkweed, were ranked among the top ten most abundant species in both the high- and s. Of all weed species, volunteer crops have largest difference in rank between the and high-till (Table 2). The positive value of this difference indicates that these species were found more often in the. Volunteer wheat ranked fifth in the and 23rd in the high-till. Volunteer canola ranked higher in two s than in the high-till and minimum-till s. Similarly volunteer flax ranked much lower in the high-till than the other s. The higher abundance of volunteer flax and canola in the may be attributable to the more frequent inclusion of oilseeds in this (Figure 3). While cereals crops were more commonly planted in the high-till, it is difficult to identify volunteer wheat in a wheat crop, probably contributing to the lower relative abundance of volunteer wheat in the high-till. Most of the remaining species show no clear association with tillage ; however, a few exceptions exist. In the, narrow-leaved hawk s-beard placed 2th, which is five ranks higher than in the high-till (Table 2). This species was also more prevalent in the high-disturbance. Cleavers also ranked higher in the notill and high-disturbance s than high-till s. Species that were more abundant in the high-till than the included Russian thistle, bluebur, wild mustard, shepherd s-purse and redroot pigweed. Although it is generally expected that the lack of physical disturbance in the fall associated with s would favour winter annuals, the survey data do not support this concept. Winter annuals such as narrow-leaved hawk sbeard are associated with, while other winter annual species including shepherd s-purse and bluebur are associated with high-till s. Other management practices such as fall applied herbicides may be able to adequately control these species, mitigating the effects of. Volume

6 Table 2. Abundance ranking of the common weed species. Species order is based on difference in rank between the and high-till s. Common name Scientific name NT HDNT MT HT HT-NT Difference in Rank Abundance Rank Volunteer wheat Triticum aestivum L Volunteer flax Linum usitatissimum L Volunteer canola Brassica napus L Narrow-leaved hawk's-beard Crepis tectorum L Cleavers Galium aparine L Barnyard grass Echinochloa crusgalli (L.) P. Beauv. & E. muricata var. microstachya Wiegand Canada thistle Cirsium arvense (L.) Scop Dandelion Taraxacum officinale Weber in F.H. Wigg. Green foxtail Setaria viridis (L.) P. Beauv Lamb's-quarters Chenopodium album L Perennial sow-thistle Sonchus arvensis L Wild buckwheat Polygonum convolvulus L Wild oat Avena fatua L Hemp-nettle Galeopsis tetrahit L Pale smartweed Polygonum lapathifolium L Quack grass Elytrigia repens (L.) Desv. ex B. D. Jacks Spiny annual sow-thistle Sonchus asper (L.) Hill Field horsetail Equisetum arvense L Kochia Kochia scoparia (L.) Schrad Chickweed Stellaria media (L.) Vill Stinkweed Thlaspi arvense L Redroot pigweed Amaranthus retroflexus L Shepherd's-purse Capsella bursa-pastoris (L.) Medik. Wild mustard Sinapis arvensis L Bluebur Lappula squarrosa (Retz.) Dumort. Russian thistle Salsola kali L. subsp. ruthenica (Iljin) Soó Conclusion The choice of tillage is part of an entire management. This fact is emphasized in this paper by examining the residual weed populations that represent the cumulative effect of all management decisions, not just the tillage used. In general, the observed weed densities and species were similar for all tillage s, a good indication that most producers were able to successfully manage their weeds. The timing of the weed counts, after all 36 Volume 2 29

7 weed management control options had been imposed, enabled the producers to treat any individual weed problems prior to weed assessments providing a good measure of the overall success of weed management. Differences in residual weed populations between tillage s may be partially attributable to management decisions other than tillage. For example, more common use of herbicides in the may be expected to contribute to lower weed densities in this. Conversely, producers using more easily noticeable volunteers. Weed species, other than volunteers, that were more abundant in than high-till s included narrow-leaved hawk s-beard and cleavers. Producers should be aware that these species can be more troublesome in s, particularly if the weeds are not controlled in the early stages of field infestation. More work on the biology and ecology of individual species is necessary to fully understand the reasons for the association of particular species with particular tillage s. The relatively recent adoption of s by Prairie producers may also have contributed to the similarities in weed communities observed among tillage s in the survey data. It is expected that observable changes in weed communities would require many years to develop, particularly in well managed fields where the annual contribution to the weed seedbank is low. Future surveys are necessary to determine if gradual changes in weed communities are occurring. References 1. Dale, M. R. T., A. G. Thomas, and E. A. John Environmental factors including management practices as correlates of weed community composition in spring seeded crops. Can. J. Bot. 7: Haak, D. 23. Crop residue levels and seeding s in Saskatchewan. Results of a PFRA survey, [Online] Available: [4 January 28]. 3. Leeson, J. Y. and A. G. Thomas. 28. Impacts of direct seeding weed dynamics. Pages in Fueling the Farm. Proceedings of the 2 th Annual Meeting and Conference of the Saskatchewan Soil Conservation Association, Regina, Saskatchewan. 4. Leeson, J. Y., A. G. Thomas, L. M. Hall, C. A. Brenzil, T. Andrews, K. R. Brown, and R. C. Van Acker. 25. Prairie weed surveys of cereal, oilseed and pulse crops from the 197s to the 2s. Weed Survey Series Publication 5-1CD. Agriculture and Agri-Food Canada, Saskatoon Research Centre, Saskatoon, Saskatchewan. 5. Statistics Canada Census of Agriculture, Farm Data and Farm Operator Data, catalogue no XWE27. [Online] Available: [19 December 28]. 6. Thomas, A. G Weed survey used in Saskatchewan for cereal and oilseed crops. Weed Sci. 33: Thomas, A. G., D. A. Derksen, R. E. Blackshaw, R. C. Van Acker, A. Légère, P. R. Watson, and G. C. Turnbull. 24. A multistudy approach to understanding weed population shifts in medium- to long-term tillage s. Weed Sci. 52: Volume