Non-Peer ReviewedA field study was conducted to evaluate the effectiveness of several herbicides for long-term control of Canada thistle growing in winter wheat. Miscellaneous herbicides were applied at the five leaf stage of winter wheat in the presence of Canada thistle (30 plants per square meter). All herbicides gave effective control of Canada thistle top growth but did not increase yields compared to the weedy check. When Canada thistle control was evaluated one year after herbicide application, three of the nine treatments significantly reduced regrowth: 2,4-D amine at 840 g/ha, clopyralid + MCPA ester at 100 + 400 g/ha, and Clopyralid at 200 g/ha. These treatments reduced Canada thistle numbers by 58, 67, and 83 %, respectively. All herbicide treatments significantly reduced Canada thistle dry matter production one year after application as measured by shoot dry weight per square meter. Clopyralid at 100 g/ha in mixture
with either 2,4-D or MCPA ester at 400 g/ha, and 2,4-D amine alone at 840 g/ha reduced dry weight of thistle shoots equally. Clopyralid at 200 g/ha was the most effective treatment tested; Canada thistle shoot dry weight per square meter was reduced by 93 % one year following treatment

Ashford, R.

Paquette, S.

English

eCommons@USASK

Canada thistle regrowth control one year after herbicide application in winter wheat

University of Saskatchewan Research Archive

CANADA THISTLE REGROWTH CONTROL ONE YEAR AFTER HERBICIDE APPLICATION IN WINTER WHEAT S. Paquette and R. Ashford Crop Science and Plant Ecology Department University of Saskatchewan ABSTRACT A field study was conducted to evaluate the effectiveness of several herbicides for long-term control of Canada thistle growing in winter wheat. Miscellaneous herbicides were applied at the five leaf stage of winter wheat in the presence of Canada thistle (30 plants per square meter). All herbicides gave effective control of Canada thistle topgrowth but did not increase yields compared to the weedy check. When Canada thistle control was evaluated one year after herbicide application, three of the nine treatments significantly reduced regrowth: 2,4-D amine at 840 g/ha, clopyralid + MCPA ester at 100 + 400 g/ha 1 and Clopyralid at 200 g/ha. These treatments reduced Canada thistle numbers by 58%, 67%, and 83% respectively. All herbicide treatments significantly reduced Canada thistle dry matter production one year after application as measured by shoot dry weight per square meter. Clopyralid at 100 g/ha in mixture with either 2,4-D or MCPA ester at 400 g/ha, and 2,4-D amine alone at 840 g/ha reduced dry weight of thistle shoots equally. Clopyralid at 200 g/ha was the most effective treatment tested; Canada thistle shoot dry weight per square meter was reduced by 93% one year following treatment. INTRODUCTION Canada thistle is the worst perennial weed problem in western Canada (Friesen, 1968). Calculations by Hunter and Smith 192 (1972) from a survey by Alex (1966) indicated that nearly 9 million hectares of cultivated land in the prairie provinces were infested with this weed. Canada thistle is also highly competitive in cereal crops (Cameron, 1938; Hodgson, 1968; Kirkland, 1977). Compared with a wild oat plant in barley, a Canada thistle shoot may be 3.4 times as competitive as wild oats (O'Sullivan et al, 1982). One report, however, suggests that Canada thistle may cause less serious cereal crop losses that are generally reported (Peschken, 1980). The success of Canada thistle is due mainly to vegetative production of aerial shoots from buds produced on roots (Moore, 1975). Established Canada thistle stands are characterized by an extensive system of branching horizontal roots from which new shoots develop. Thus, to effectively control this weed with herbicides, it is generally assumed that herbicides must translocate to roots in sufficient quantities to be toxic (Moore, 1975, Devine and Vanden Born, 1985). Chemicals such as phenoxies and benzoics suppress topgrowth, but generally fail to kill the root buds unless applications are repeated (Carson and Bandeen, 1975). Topgrowth control of Canada thistle is beneficial to the extent that it reduces crop competition from the weed, and also prevents spread of the weed by preventing seed production. However, it would be more desirable if a high degree of regrowth control could be obtained with a single in crop spraying operation. Canada thistle is a particularly serious problem in cereal crops grown in western Canada since there are no herbicides currently registered in Canada which provide effective selective control of both shoots and roots of Canada thistle in 193 these crops (0 1 Sullivan et al, 1982). There is a needu therefore, to develop herbicides that are not only more effective on Canada thistle, but also safe to apply on cereals. The compound clopyralid is a selective postemergence herbicide that is being developed in Canada for control of broadleaf weeds, including Canada thistle, in cereal crops. Clopyralid, as yet unregistered for use in cereals; is chemically similar to picloram although less persistent (Haagsma, 1975). Mixtures of clopyralid with phenoxy herbicides are currently being developed for control of phenoxy-tolerant weeds such as Canada thistle in cereal crops. The purpose of this study was to compare clopyralid and clopyralid-phenoxy mixtures with currently recommended herbicide treatments for control of Canada thistle in winter wheat. These kind of data are needed to determine whether effective control of Canada -thistle can be achieved with a single herbicide application in winter wheat. MATERIALS AND METHODS A field study to determine the long-term efficacy of miscellaneous herbicides on Canada thistle growing in winter wheat was initiated in fall 1983. The study was located at Clair, 200 km east of Saskatoon, on a loam soil naturally infested with Canada thistle. Norstar winter wheat was seeded on September kg/ha of kg/ha of 14, 1983 at a rate of 100 kg/ha in 20 em 11-51-0 fertilizer was applied with the seed 34-0-0 was broadcast in the spring when 194 rows. and the 56 224 crop commenced growth. The design of the experiment was randomized complete block with four replications. Herbicide treatments are listed in table 1 and all rates were active ingredient per hectare. Plot size was 2.5m by 7.0m and the same plots were used throughout the experiment. Herbicides were applied with a hand-held plot operated to deliver 108 1/ha of spray solution at sprayer 290 kPa pressure. All treatments were applied on June 11, 1984 to winter wheat at the five leaf stage (35 em tall) and Canada thistle in the vegetative stage (20 em tall). Canada thistle shoots were counted in 0.5 square meter quadrats on August 9, 1984 and plots were harvested with a combine to obtain winter wheat yield data in 1984. Plots were not recropped, but Canada thistle regrowth was measured in summer of 1985. Number and dry weight of Canada thistle shoots per square meter were measured June 27, 1985. Data obtained were subjected to analysis of variance and Duncan's Multiple Range Test at the 5% level where applicable. RESULTS AND DISCUSSION The Canada thistle infestation was significantly reduced in all herbicide treated plots in the first year (Table 1 ). However, some treatments were significantly more effective at reducing thistle numbers than others. Bromoxynil + MCPA ester provided the lowest level of control, and clopyralid the highest. Other herbicides gave control levels between these two extremes; the mixtures of phenoxy herbicides with dicamba or clopyralid generally resulted in slightly greater control of topgrowth than phenoxies alone. Yield of winter wheat, however, was not 195 affected by herbicide treatment. Although the reason for the herbicides' failure to increase yields is not known, thistles may have exerted most of their competitive effects before control was achieved, or roots may have continued to compete despite the apparent high level of topkill following herbicide application. Table ·1. Effects of spring applied herbicides on winter wheat yield and Canada thistle numbers in the year of application. Treatment Rate Winter Canada Wheat* Thistle (g/ha) Yiel~ Numbira (g/m ) (#/m ) Weedy check 355 31 a 2,4-D amine 840 326 13.5bc 2,4-D ester 840 351 8 cd MCPA ester 840 343 1 3 b-d Dicamba+MCPA salt 100+400 321 11 • Sed Dicamba+2,4-D amine 100+400 356 7.5cd Bromoxynil+MCPA ester 280+280 338 21.5b Clopyralid 200 338 4 d Clopyralid+2,4-D ester 100+400 337 7 cd Clopyralid+MCPA ester 100+400 362 1 0 cd * Overall F-test for difference in winter wheat the yield was n.ot significant at the 5% level. aValues followed in columns by the same letter are not significantly different at the 5% level according to Duncan~s multiple range test. One year after treatment, most herbicides did not reduce weed infestation significantly; however, significant improvement in control was obtained with three treatments (Table 2). 2,4-D amine at 0.84 kg/ha, clopyralid + MCPA ester at 100 + 400 g/ha, and clopyralid at 200 g/ha, respectively, reduced Canada thistle shoot numbers by 58%, 67%, and 83%. Shoot counts alone do not fully describe the effects of the treatments on 196 Canada thistle because even though shoot numbers did not always decline significantly, there was always a significant decline in the dry weight of shoots per square meter on the treated plots. Among the herbicides tested, clopyralid at 200 g/ha was the most effective at reducing dry weight of Canada thistle per square meter. Table 2. Effects of spring applied herbicides on Canada thistle shoot number and dry weight one year after application in winter wheat. Treatment Weedy check 2,4-D amine 2,4-D ester MCPA ester Dicamba+MCPA salt Dicamba+2,4-D amine Bromoxynil+MCPA ester Clopyralid Clopyralid+2,4-D ester Clopyralid+MCPA ester * Rate (g/ha) 840 840 840 100+400 100+400 280+280 200 100+400 100+400 Canada Shoot * Counts 42 a 17.5bc 33.5ab 28 ab 28.5ab 30.3ab 26.8ab 7.3c 24 a-c 13.8bc Thistle Shoot Dry Weig2t (g/m ) 148a 47d 102b 80c 116b 109b 76c 11 e 43d 36d Values followed in columns by the same letter are not significantly different at the 5% level according to Duncan~s multiple range test. In summary, single applications of recommended selective herbicide treatments generally did not control Canada thistle one year after treatment in this study. Phenoxy herbicides either alone or in mixtures with dicamba or bromoxynil failed to significantly reduce Canada thistle shoot numbers, with the exception of the amine formulation of 2,4-D. However, similar applications with clopyralid and clopyralid-phenoxy mixtures, at 197 anticipated use rates in cereals, provided a measure of control in some cases one year after treatment. Clopyralid alone and in mixture with MCPA, but not with 2,4-D, resulted in a significant decline in thistle shoot numbers. The level of control obtained with clopyralid alone and in mixture with MCPA was equal to that obtained with 2,4-D amine. In addition, all herbicide treatments did not injure winter wheat. These results indicate that single applications of clopyralid alone and in mixture with MCPA may improve long-term control of Canada thistle in winter wheat obtained with similar applications of most recommended treatments. LITERATURE CITED Alex,J.F. 1966. Survey of weeds of cultivated land in the prairie provinces. Can. Dep. Agric., Exp. Farm Res. Branch, Regina, Sask. 68pp. Cameron,J. 1938. Report of the Dominion Experimental Substation. Regina, Sask, 1931,1936. In Experimental Farm Reports 1930-1936. Dominion of Canada, Department of Agriculture, Dominion Experimental Farms. 21pp. Carson,A.G. AND J.D. Bandeen. 1975. Chemical control of Canada thistle. Weed Sci. 23:116-118. Devine, M.D. and W.H. Vanden Born. 1985. Absorption, translocation, and foliar activity of clopyralid and chlorsulfuron in Canada thistle (Cirsium arvense) and perennial sowthistle (Sonchus arvensis). Weed Sci. 33:524-530. Friesen, H.A. 1968. Trends in Canadian research to control Canada thistle. Proc. N. E. Weed Control Conf. Res. Rep. 27-36. Haagsma,T. 1975. Dowco 290 herbicide- A coming new selective herbicide. Down Earth 30:1-2. Hodgson,J.M. 1968. The nature, ecology and control of Canada thistle. u.s. Dep. Agric. Tech. Bull. No. 1386. 32pp. Hunter,J.H. and L.W. Smith. 1972. Environment and herbicide effects on Canada thistle ecotypes. Weed Sci. 20:163-167. 198 Kirkland,K.J. 1977. Glyphosate for the control of Canada thistle on summer fallow. Can. J. Plant Sci. 57:1015-1017. Moore,R.J. 1975. The biology of Canadian weeds. 13. Cirsium arvense (L.) Scop. Can J. Plant Sci. 55:1033-1048. O'Sullivan,P.A., v.c. Kossatz, G.M. Weissv and D.A. Dew. 1982. An approach to estimating yield loss of barley due to Canada thistle. Can. J. Plant Sci. 62:725-731. Peschken,D.P., J.H. Hunter and A.G. Thomas. 1980. Damage in dollars caused by Canada thistle in wheat in Saskatchewan. Proc. Can. Thistle Syrnp. Regina, Sask. pp. 37-43. 199 

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Canada thistle regrowth control one year after herbicide application in winter wheat

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