The Roundup -

By Tim Fine 

Fall Herbicide Application


Narrowleaf hawksbeard. (Source

In order for an industry to survive and be sustainable, it must be able to adapt and change. Agriculture, our nation's number one industry is no different. Changes in farming practices, crop varieties, management practices, and much more are constantly being researched and recommended. One such practice that has become fairly commonplace in agriculture production is the advent of minimum tillage or no-tillage systems for crop production.

I will not go into too much detail in regards to the hows and whys of no-till production but the science behind it is that when you reduce the number of times the soil is disturbed you also reduce the amount of disturbance to the beneficial organisms that are working under the soil which allows for a potentially more fertile and productive soil, saves on the fuel bill, and reduces the detrimental effects that compacted soils play in reducing crop yields.

Many agricultural producers in our region (and a vast majority across the nation) have adapted their systems to no till for these very beneficial reasons, however, just like most changes in any industry there are some trade-offs to changing practices and it is not always a win-win situation.

One of the issues that has recently reared its ugly head is that we are starting to see new weed species invading cropping areas and even some species that are not new are becoming more of a problem. While there are several factors that contribute to this, not making a fall tillage pass through a field allows some weed species to thrive. So I thought that I would list just a few of the weeds that are becoming an issue and discuss some management strategies.

Horseweed or marestail (Conyza Canadensis)- If you have attended the ARS Field Day, EARC Field Day, or the Pulse Day held in Richland County, Montana, you probably have heard me talk about this weed. Horseweed is a winter or summer annual and has directly benefitted from no-till production. We do have glyphosate resistant horseweed in Richland County but that does not mean that every plant in the county is resistant to herbicides containing glyphosate. Typically, horseweed is in the basal or rosette stage in the fall so an application of glyphosate (if the plant is not resistant), dicamba, or 2,4-D ester may go a long way in preventing further infestations.

Narrowleaf hawksbeard (Crepis tectorum)- Narrowleaf hawksbeard is still a relatively new plant that is just beginning to rear its ugly head in Richland County. Prior to June, I was fairly certain that we did not have this weed in our county but at one of the field days a producer pulled a weed and brought it to me. As he was walking towards me, I realized that, unfortunately what he had pulled was most definitely narrowleaf hawksbeard. This weed is another one that acts as either a summer or winter annual and will sometimes be in the rosette stage in fall. Just like with horseweed, a fall application of 2,4-D or glyphosate could be beneficial to prevent the spread. On a side note, this weed has really taken off in counties to the north of us. So much so that if you are driving through Sheridan, Daniels, Valley, and parts of Roosevelt counties in mid to late June you will notice yellow ditches, CRP fields and hay fields primarily because of this yellow-flowered weed.

Canada thistle (Cirsium arvense)-As opposed to the two weeds listed above, Canada thistle is not a relatively new weed to deal with and it is not an annual weed. Regardless of management strategies, control of Canada thistle normally takes repeated attempts to get satisfactory control. Fall applied herbicides probably give the best opportunity for control of this weed. A fall herbicide application to Canada thistle moves through the plant's system as sugars are being sent from the leaves to the roots. According to the North Dakota Weed Control Guide ( "products containing glyphosate or clopyralid fall-applied to Canada thistle in the rosette stage provides greater control then when applied to bolting or flowering stems.

What a field would look like at the end of the season if control measures are not taken against horseweed. (Source

There are plenty of other weeds that I could have suggested and I am sure that as the years progress and things change there will be plenty more weeds that come down the pike. If there is a take home message it is that if these weeds that are in the rosette stage in the fall are controlled then, you can save yourself some headaches the following crop season.

I should point out that there are other products available that will provide control of these weeds. The ones listed are the most common but by no means are the products listed above an exhaustive list. Careful consideration should be made when selecting herbicides as some will have re-plant restrictions. Information in this article is provided for educational purposes only. Reference to commercial products or trade names does not imply an endorsement of them by MSU Extension. Common chemical and trade names are used in this article for clarity for the reader. Inclusion of a common chemical or trade name does not imply endorsement of that particular product or brand of herbicide and exclusion does not imply non-approval. This article is not intended to replace the product label.


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