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2011/12  Fall Alfalfa Herbicide Trials for Control of Canada Thistle

MFSA 2012 Final Report
Kevin GulayB.Sc. (Agric)
MFSA Research Manager

Canada thistle (Cirsium arvense), a perennial weed, can be quite difficult for alfalfa seed producers to control. MFSA continues to search for better options to control Canada thistle in alfalfa seed crops. In the past MFSA has spring applied herbicides, looking for control of Canada thistle, in alfalfa seed crops. In the summer of 2011, the most successful treatment was a spring application of Embutox, followed by an application of Pardner 14 days later. This provided a Canada thistle control rating of 78% in June 2011. The alfalfa crop tolerance rating was 77%, but the yield was very similar to the untreated check. This study looks at the control of Canada thistle using fall herbicide applications.

Materials and Methods
Sites were selected in Ashern, Springstein, and Steep Rock to test herbicides for the control of Canada thistle in alfalfa seed crops. Areas of the field with very high Canada thistle populations were selected. Trials were prepared using a randomized block design. Each plot was 3mx6m. All herbicide applications took place with a push type bicycle sprayer. Water volumes applied were 14 US gal/A and the boom height was approximately 20 inches above canopy.
The Ashern and Springstein sites received the same treatments. The Ashern field was located on the SW 9-26-8W. The Ashern site was sprayed October 21, 2011 with an air temperature of 12 degrees and 22km/H winds. The Springstein field was located on the SW 34-9-1W. Spraying took place November 3 2011. Air temperature was +3 degrees Celsius and wind speed was 19 km/H. See table 1 below for a list of the treatments that were used.
Table 1. Description of treatments used at the Ashern and Springstein sites.

The Steep Rock field was located on the SE 14-29-9W. Spraying took place October 21, 2011, at 12:30; the air temperature was 10 degrees Celsius, with winds at 19 km/H. A list of the treatments used can be seen in table 2 below.
Table 2. Description of the treatments used at the Steep Rock site.

The following spring, 2012, crop tolerance and weed control ratings took place. Three ratings took place in May, June, and July. Assure II was sprayed on the fields to control the grassy weeds. No other herbicides were applied on any of the crops throughout the season. The crops were sprayed for insects and diseases, based upon the growers’ normal production practices.
Finally plots were to be harvested with a Wintersteiger plot combine. The seed would then be cleaned and weighed to obtain yield data.
There were some unfortunate alterations to the original project plans. First, in Ashern, the site that was selected was an area in the field which had very high levels of Canada thistle. In addition, the alfalfa population was very low, resulting in the weeds out-competing the crop. The decision was made to terminate the plot after tolerance ratings had taken place. Flixweed was spread throughout the plot area, which resulted in the addition of flixweed control ratings at this site. Then when control and tolerance ratings began at the Springstein site, no effects from any of the treatments could be observed. It was decided that there were other factors that caused this, and no more data was collected from this site. Finally, at Steep Rock, the field that was selected had Canada thistle issues in 2011, but in 2012 there was no Canada thistle in the field. Crop tolerance ratings and yield ratings took place, however trampling of the plot made yield data unreliable. Night flowering catchfly was spread throughout the plot, and therefore night flowering catchfly control ratings were recorded.

Figure 1 shows the effects of several fall applied herbicides that were used in the alfalfa seed crop at the Ashern site. PrePass and Ally show descent control in early spring. However, near the end of June the Canada thistle is re-established and all treatments show poor control.
Figure 1. Control of Canada Thistle in alfalfa seed crops in Ashern with fall applied herbicides
The abundance of flixweed at the Ashern site allowed for flixweed tolerance ratings to be taken as shown in figure 2. Prepass, Benchmark, Velpar, and Ally all had good flixweed weed control. The other products were still considerably better than the untreated check.
Figure 2. Control of flixweed in alfalfa seed crops with fall applied herbicide in Ashern as of June 11,

In Steep Rock, the Embutox and Express Pro tank mix, along with the Telar, Authority, and Pursuit tank mix had the least alfalfa tolerance mid may at 68%, and 73% respectively. In August the Telar, Authority, and Pursuit Tank mix was the only treatment that resulted in crop tolerance below 80%.
Figure 3. Tolerance of alfalfa seed crops in the 2012 growing season to herbicides applied in the fall of 2011 in Steep Rock.

High levels of night flowering catchfly at Steep Rock resulted in the collection of weed control data at this site. Embutox and Telar had the best control of Night Flowering Catchfly with 90% Control. The Telar, Authority, and Pursuit tank mix, as well as the Blazer and Telar tank followed with %70 control, and %68 control respectively. The results can be seen in figure 4 below.
Figure 4. Control of night flowering catchfly in alfalfa seed crops on Sept 5, 2012 with herbicides applied in the fall of 2011

Discussion and Conclusion
At the Ashern site, both the Prepass and the Ally suppressed the Canada thistle in early spring. For the most part, the other products gave very little control. By the end of the season the Canada thistle had re-established itself and there was very little control from any of the products tested. It is important to remember that the only herbicide used in spring was Assure II. Had a spring herbicide been used, we could have seen Canada thistle control last for more of the season. Some growers have reported good Canada thistle control with Velpar. Since this study showed very little Canada thistle control with Velpar, it suggests that Velpar activity is strongly affected by the soil properties of the field. However, the Velpar was available in the soil to some degree, as it showed very good control of flixweed. Velpar, along with Ally, Benchmark, and Prepass all gave 90% or greater control of the flixweed.
None of the treatments in Springstein had any effect on the weeds or on the alfalfa crop. There are a few possible reasons that this may have occurred. First the crop was sprayed in late fall on November 3, 2011. It is possible that at this point in the season the crop and the weeds had become nearly 100% dormant, which means that there may have been no foliar uptake from the herbicides which rely on foliar uptake. However, many of the herbicides applied have a soil uptake component. Since the fall of 2011 and the spring of 2012 were very dry it is possible that these chemicals were still on the crop trash in the spring. There was a controlled crop burn done on the field for disease control that spring. Any chemical that was still in the trash would have been destroyed.
In the Steep Rock field, Canada thistle levels were very high in 2011, which resulted in the grower deciding to cut the crop for hay. This proved to work very well as there was very little Canada thistle in the 2012 crop. As mentioned earlier the Embutox and Express Pro tank mix, along with the Telar, Authority, and Pursuit tank mix had the least alfalfa tolerance mid May at 68%, and 73% respectively. In August the Telar, Authority, and Pursuit Tank mix was the only treatment that resulted in crop tolerance below 80%. Based on this data, growers should be very cautious with these tank mixes. While crop tolerance is quite high for many of the other tank mixes, growers still need to be careful as there is no yield data to support these results. Night flowering catchfly is a winter annual, or annual weed that is often an issue in alfalfa seed crops. Some of these tank mixes did a very good job controlling the weed. As mentioned, the Embutox and Telar had the best control of Night Flowering Catchfly with 90% control. The Telar, Authority, and Pursuit tank mix, as well as the Blazer and Telar tank mix followed with 70% control, and 68% control respectively.
This study shows that more work is needed to find a consistent and effective way to control Canada thistle in alfalfa seed crops. MFSA will continue to investigate this in 2012, using a combination of fall and spring applied herbicides.
A special thanks for this project to all MFSA staff involved in the project, the chemical companies who supplied product, Paterson Grain in Arborg, and the growers that allowed MFSA to use their land.

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