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An annual weeds job is to produce seed so that the species will continue on next year, everything else is secondary. Unlike summer annual weeds such as foxtail or velvet leaf which typically germinate and produce seed within a summer growing season, winter annuals actually germinate in the fall and begin growing before winter. During winter these weeds go into dormancy, but at the first signs of spring, winter annuals come out of dormancy, bolt and produce seed before corn and beans are usually planted.
What Are Some Winter Annual Weeds?
Some commonly seen and problem winter annual weeds include henbit, horseweed (mares tail), pennycress, shepherds purse, curly dock, perennial dandelion and the mustard family.
Are We Seeing More Winter Annual Weeds in No-Till?
As the use of no-till farming increases, winter annual populations and problems in the fields seem to be increasing. Many producers, especially in dry-land situations, have found that one of the benefits of no-till is that it helps conserve moisture but, no-till may not interfere with the life cycle of winter annual weeds like tillage does. Consequently, many no-till users feel they are having more winter annual weed problems in their fields than they had when they used more tillage. Another speculation on why winter annuals are popping up more in no-till fields is the increased use of Roundup-Ready soybeans. Since Roundup has no residual there is no long term control of weeds. When conventional soybeans were the norm, traditional herbicides provided residual control that kept many of the winter annuals from germinating or growing in the fall.
Can Winter Annual Weeds Do Much Damage?
Research at the University of Nebraska shows that winter annuals can use as much as three inches of soil moisture in 30 days. In another study, Agriculture and Agri-Food Canada, concluded that 8% to 11% of the soils moisture could be saved by controlling winter annuals early. “Winter Annual Weed Pest Alert” Remember, patches of winter annuals are a preferred area for cutworms to lay their eggs in.
Controlling Winter Annuals?
The biggest issue for control is timing. Herbicides used to control winter annuals work best when applied before the weeds have bolted. Typically, this means checking fields early and spraying as soon as temperatures warm up enough for fast plant growth to begin. Studies done by the University of Nebraska regarding spring applied products and the percentage of winter annual weed control experienced are (1) 2,4-D with 65% control, (2) 2,4-D plus Banvel with 83% control, (3) Atrazine COC with 100% control and (4) Roundup with 93% control. The University of Missouri did a fall application study for henbit and documented the following results (1) Canopy with 100% control, (2) Canopy XL with 98% control and (3) Sencore with 94% control. In most cases, a herbicides effectiveness or control can also be influenced by the air temperature at which the herbicide was applied. In many situations, cooler application temperatures than the manufacturer recommends may render a herbicide less affective.
CREDITS: University of Nebraska
University of Missouri
Kansas State University
In recent years more and more farmers have become aware of weeds that have become resistance to popular herbicide programs. As agricultural production continues to intensify farmers have increased their use of herbicides to manage weeds. In addition, farmers are relying more on continuous use of herbicides with similar modes of action (MOA) or even the same herbicide. Farmers have been selecting and developing weed resistance since the late 50’s when the first weeds were identified resistant to 2,4-D and atrazine. The number of weeds resistant to herbicides has grown to over 250 species worldwide. The number of species resistant to a given herbicide family range from over 70 species resistant to the triazines (i.e.: atrazine) and ALS/AHAS (i.e.: Glean, Spirit, Pursuit) herbicides, to just a handful for glyphosates, and only a couple of weeds for the chloroacetamide (i.e.: Dual, Harness) herbicides. With the exception of a few new herbicide derivatives, only the newest herbicide family, the HPPD mode of action group, has not yet had any weeds identified as resistant. The only herbicides presently labeled in this class are isoxaflutole and mesotrione (Callisto). There has been only two new herbicide MOA’s in the past 15 years, including glufosinate in 1994 and the HPPD’s since 2000. It is unlikely that there will be any new herbicide MOA’s launched within the next 8-10 years. The lack of new MOA’s will put increased pressure on farmers to better manage the products we have today to prevent further losses in weed control options. Weed resistance is defined as the inherited ability of a weed to survive a rate of herbicide, which would usually give effective control. There are differences in opinions on exactly what “use rate” defines resistance. Most researchers follow the WSSA’s guidelines that once a weed is no longer controlled at a rate 6 times greater than before, it is considered resistant. For example, if 1 lb per acre provided effective control of a given weed, then if a portion of that weed’s population could no longer be controlled at 6 lbs per acre, then the non controlled weeds would be considered resistant to that herbicide MOA. One of the common misconceptions is that weeds “mutate” to become resistant to herbicide treatments. Weeds actually do not mutate very easily but primarily rely on their diverse genetic codes to select for resistance to a specific herbicide family. There are two typical ways that weed resistance develops within a population. The first is through a simple selection process. In some weed species there exist a small number of weeds that have the inherent ability to bypass a given herbicide’s mode of action. If a farmer makes a treatment and kills all but these “resistant” weeds, they remain to produce seed or pollinate with others. If the farmer continues to use the same single herbicide program for consecutive years, eventually these escaped weeds will build up a larger and larger portion of the field population – or natural selection. The second method of developing weed resistance is through cross breeding similar to what seed companies use to develop traits. This occurs when plants within a weed population have different levels of “tolerance” to an herbicide MOA. When farmers use low or reduced rates of the herbicide, these tolerant plants can survive (escape) and cross breed to potentially develop a stronger trait in the next generation. If this pattern continues, this cross breeding will develop a stronger trait of resistance in each subsequent generation, ultimately developing a population of resistant weeds. This is how Shattercane developed resistance to reduced rates of the ALS herbicides. It is important to note that not all weeds have the ability to develop resistance, and not every herbicide MOA will develop resistant weeds equally. Herbicides with very specific sites of activity, like the ALS/AHAS herbicides, tend to have a greater likelihood of resistance selection. The more specific the herbicide site of activity, the higher the probability that a weed can bypass that herbicide’s mode of action. By contrast, herbicides with a very complex MOA such as the chloroacetamides (i.e. Dual) have only two species that have selected for resistance in over 25 years of use. Farmers that are at the greatest risks for developing resistance are those that:
Weed resistance is a growing treat to American agriculture. As fewer new herbicides are developed, there is an increased need to manage the tools that we have. The increased adoption of GMO crops puts increased weed resistance pressures on a select group of herbicides, significantly reducing their long term input value. By adopting the simple herbicide resistance strategies above, farmers can help prolong the use of these valuable tools. A good resistant strategy can incorporate several herbicide groups including GMO technology into a long term sustainable system. Use each herbicide where it provides the greatest value while using sequential or tank mixes to improve control. Incorporating a resistance strategy can lead to improved performance and value, otherwise “If you abuse it, you will lose it!”