written by Dr. Jeff Stachler, Weed Scientist, UMN and NDSU
Scouting fields for weeds throughout the growing season is extremely important to maintaining herbicide effectiveness and planning for future weed control decisions. Scout fields now and at harvest to determine the effectiveness of this season's weed control practices. If weeds are present now, determine why they are present. If weeds are present due to herbicide resistance, then weed control and cropping practices must be different next season and beyond.
Determining if resistance is present in a field can be difficult at times, especially with low-level resistance such as resistance to glyphosate and ACCase-inhibiting herbicides.
If a single weed species is left behind after a herbicide application and all other susceptible species are controlled, then herbicide-resistant biotypes may be present. If low-level herbicide resistance is present in a field then normal-appearing plants will be next to dead plants and a continuum of responses between normal and dead will likely be present.
For additional assistance in determining if low-level or glyphosate-resistant biotypes are present in a field, view a video entitled "Scouting for Glyphosate Resistance". The video can be found at the following web site and will be most helpful if the last herbicide application occurred within the last six weeks:
In addition to scouting for resistant plants, determine the presence of perennial weed patches to begin planning future management strategies. Pay close attention to the presence of dandelions.
One effective tool to managing perennial weeds is to apply glyphosate as a pre-harvest application. Consult the North Dakota Weed Control Guide (http://www.ag.ndsu.edu/weeds/weed-control-guides/nd-weed-control-guide-1) for specific recommendations for timing and rates. Fall herbicide applications are an excellent strategy for controlling winter annual, biennial, and perennial (if frost has occurred only those perennial species surviving the frost will be controlled) weeds.
Currently Available Weed Control Strategies:
If weeds are present in a field at this time of the season, few options are available. For Roundup Ready sugarbeet growers, glyphosate can be applied at 0.75 pounds acid equivalent/acre (lb ae/A) up to 30 days prior to harvest as long as the maximum seasonal use-rate has not been exceeded. Another option for sugarbeet growers having large numbers of weeds in a field is to mow the weeds off to just above the sugarbeet canopy. This strategy will only be effective for weeds above the canopy and will be most effective when the first seeds begin to develop.
There are no herbicide options available at this time of the season for all other crops, besides pre-harvest applications. The best way to remove weeds at this time of the season is to remove them by hand. Most weeds are producing viable seeds at this time of the season therefore removal of the pulled plants from the field is necessary to prevent distribution of the seeds to the seedbank. Hand-removal or rogueing of resistant biotypes is important to maintaining herbicide effectiveness.
For sugarbeet growers, if weeds are present in a field, plan to harvest those fields as soon as possible. The process of topping sugarbeet usually destroys the majority of plants. The sooner plant destruction occurs the fewer viable weed seeds are produced. Thoroughly clean the topper after use in a field having weeds to ensure weed seeds are not being spread to another field.
For corn and soybean producers, harvest the weediest fields last, to save time cleaning out the combine. Combines will easily move weed seeds from one field to another. Always clean out a combine when moving from a field having weeds to a field not having weeds. If cleaning harvesting equipment is too time-consuming the only other option is to record the harvesting sequence of fields and which fields had weeds present. Then next season increase the weed management strategies for those fields harvested immediately after a field having weeds and those fields having the weeds.
If a single waterhemp plant in a one acre area survives a herbicide application and produces 100,000 seeds (this is a conservative number since a single waterhemp plant can produce over one million seeds), what may happen in the future? If just 25% of those seeds emerge next season and if only 10% of those plants are resistant, then 2,500 waterhemp plants per will be present at the end of the next growing season in that one acre area.
If soybeans were planted in that one acre and 180,000 soybean plants are present at harvest, then only 1.4% of all plants at harvest will be waterhemp. If those 2,500 plants each produce 100,000 seeds and 25% of those seeds emerge the following season and only 10% of those plants are resistant, how many waterhemp plants will be remaining just two years later? There will be 6,250,000 waterhemp plants at harvest in that one acre! This helps to explain why people have said there were no weeds or few weeds present last year, but now there is a problem. This is why it is so critical to obtain 99 to 100% weed control after an herbicide application.