Kochia [Kochia scoparia (L.) Schrad.] is an annual broadleaf weed species native to Eurasia and introduced as an ornamental to the Americas by immigrants in the mid- to late 1800s. Although sometimes categorized in the genus Bassia, there is no compelling reason for this classification. This naturalized species is a common and economically important weed in crop production systems and ruderal areas in semiarid to arid regions of North America, and has expanded northward in the Canadian Prairies during the past 30 yr. Although primarily self-pollinated, substantial pollen-mediated gene flow and efficient seed dispersal aids both short- and long-distance spread. The weed is morphologically highly variable, and its growth and development are markedly affected by environmental conditions. Kochia, a C4 species, is highly competitive in cropping systems because of its ability to germinate at low soil temperatures and emerge early, grow rapidly, tolerate heat, drought and salinity, and exert allelopathic effects on neighboring species. Moreover, herbicidal control has been compromised to some extent by the widespread evolution of herbicide resistance in the species. Kochia is used as a forage, is palatable to livestock with nutritional value similar to that of alfalfa (Medicago sativa), but can be toxic if it comprises the majority of the diet. Although kochia pollen is an allergen, the seed is a source of phytochemicals including mosquito pheromones and saponins that are potentially beneficial to human health; kochia also is beneficial in phytoremediation of soils contaminated by hydrocarbons or pesticides. Key words: Kochia, Kochia scoparia, Bassia scoparia, herbicide resistance, soil salinity tolerance, weed biology
The critical period of weed control is the portion of the life cycle of a crop during which it must be kept weed-free to prevent yield loss due to weed interference. The advent of herbicide-resistant canola (Brassica napus L.) varieties in western Canada has meant that there are now more options for postemergence weed control in canola, and this has prompted increased interest in identifying the optimum timing for weed control in this crop. A critical period experiment was conducted at three locations in southern Manitoba in 1998 and 1999, and it consisted of two sets of treatments. In the first set of treatments, the crop was kept weed-free for increasing lengths of time to determine when emerging weeds would no longer reduce crop yield. In the second set of treatments, weeds were permitted to grow in the crop for increasing lengths of time to determine when weeds emerging with the crop began irrevocably to reduce crop yield. Results of the experiments indicated that canola must be kept weed-free in most cases until the four-leaf stage of the crop (17–38 days after crop emergence [DAE]) and, in one early-seeded experiment, until the six-leaf stage of the crop (41 DAE), in order to prevent >10% yield loss. After the four- to six-leaf stage of the canola crop, few weeds emerged, and late-emerging weeds accumulated little shoot biomass. Weeds needed to be removed by the four-leaf stage of the crop (17–38 DAE) to prevent >10% yield loss due to weed interference. In all but the early-seeded experiment, the critical weed-free period and the critical time of weed removal overlapped, such that a single weed removal at the four-leaf stage of the crop would have been sufficient to prevent >10% yield loss. This information will be useful for providing weed control recommendations to canola producers.
Gene flow among herbicide-resistant (HR) canola varieties can lead to the development of multiple HR canola plants, creating volunteer canola management challenges for producers. In western Canada, escaped populations of HR canola are ubiquitous outside of cultivated fields, yet the extent of gene flow resulting in herbicide resistance trait stacking in individuals within these populations remains unknown. The objectives of this study were to document the presence of single and multiple herbicide resistance traits and assess the extent of gene flow within escaped canola populations. Seed was collected from 16 escaped canola populations along the verges of fields and roadways in four agricultural regions in southern Manitoba from 2004 to 2006. Glyphosate resistance was found in 14 (88%) of these populations, glufosinate resistance in 13 (81%) populations, and imidazolinone resistance in five (31%) populations. Multiple herbicide resistance was observed at levels consistent with previously published canola outcrossing rates in 10 (62%) of the tested populations. In 2005 and 2006, maternal plants from two escaped populations were tested using trait indicator test strips for glyphosate and glufosinate resistance to confirm outcrossing events. In 2005, two of 13 tested maternal plants with single herbicide resistance traits produced progeny with both glyphosate and glufosinate resistance. In 2006, of 21 tested plants, 10 single HR maternal plants produced multiple HR progeny, and five nonresistant maternal plants produced resistant offspring. This is the first report indicating that intraspecific gene flow results in stacking of herbicide resistance traits in individuals within escaped canola populations, confirming that multiple HR canola volunteers are not confined to agricultural fields. Results of this study suggest that escaped populations of crop plants can contribute to the spread of genetically engineered novel traits, which has important implications for containment, especially for highly controversial pharmaceutical and industrial traits in crop plants.
The objective of this study was to survey pedigreed canola (Brassica napus L.) seedlots for contaminating herbicide resistance traits because of complaints from farmers regarding glyphosate [N‐(phosphonomethyl)glycine]‐resistant canola volunteers occurring unexpectedly in their fields at densities and in patterns that suggested that pollen‐mediated gene flow from neighboring fields in previous years was not the source of contamination. Twenty‐seven unique, commercial certified canola seedlot samples were collected. Glyphosate‐resistant seedlot samples were not collected. Canola samples were planted in the field, and when the canola had two to four true leaves, glyphosate, glufosinate [2‐amino‐4‐(hydroxymethylphosphinyl)butanoic acid], and thifensulfuron {methyl 3‐[[[[(4‐methoxy‐6‐methyl‐1,3,5‐triazin‐2‐yl)amino]carbonyl]amino]sulfonyl]‐2‐thiophenecarboxylate} herbicides were applied. Surviving canola plants were counted. Of the 27 seedlots, 14 had contamination levels above 0.25% and therefore failed the 99.75% cultivar purity guideline for certified canola seed. Three seedlots had glyphosate resistance contamination levels in excess of 2.0%. Unexpected contamination (even at 0.25%) can cause problems for producers that practice direct seeding and depend on glyphosate for nonselective, broad‐spectrum weed control. To avoid unexpected problems and costs, it is important that farmers are cognizant of the high probability that pedigreed canola seedlots are cross‐contaminated with the various herbicide resistance traits.
A seed bioassay was developed and tested for the rapid identification of aryloxyphenoxypropionate (APP) and cyclohexanedione (CHD) resistance in wild oat. Two susceptible (S) genotypes, UM5 and Dumont, were treated with fenoxaprop-P and sethoxydim over a range of dosages on filter paper and agar. The former is a wild oat line and the latter a tame oat cultivar. Within 5 d, shoot and root development of both genotypes were completely inhibited by 10 μM fenoxaprop-P and 5 μM sethoxydim. These dosages were then tested to determine if they were suitable for distinguishing between resistant (R) and susceptible (S) plants. Agar medium was preferred over filter paper because of the ease of preparation and maintenance. Four known R wild oat populations were included in the tests. Those with high levels of resistance produced significantly longer coleoptiles and roots than S genotypes, but those with moderate or low levels of resistance could not be separated statistically from S biotypes based on quantitative measurements. However, after exposing the germinating, treated seeds to light for 24 to 48 h, all the R populations produced green coleoptiles and initiated a first leaf, unlike the S genotypes which did not turn green or produce any new growth. This procedure proved useful in discriminating between R and S genotypes and in ranking populations in terms of relative levels of resistance.
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