Tough New Weed Problems
Brian Jenks
NDSU, Minot
Many of the weed problems that producers encounter today are not necessarily new weeds. They are often weeds that have been around for years and may have flourished with a change in production practices or environmental conditions. Any change in tillage practices, crop rotation, or herbicides used will reduce or enhance specific weed populations.
Canada thistle
Environmental conditions play a major role in the distribution and competitiveness of weed populations. Weeds such as Canada thistle are known to thrive under more moist soil conditions and is the number one weed problem for many producers in the northern U.S. Data from the Noxious Weed Division of the North Dakota Department of Agriculture indicate that the number of acres infested with Canada thistle tripled from 1993 to 1997. This increase is due in part to above-normal rainfall received in four of the last five years. Research has shown that Canada thistle is more competitive in years of higher growing-season rainfall (2). We have seen Canada thistle populations increasing since 1993 which coincides with above-normal annual rainfall accumulation. We received 26 inches of rainfall in 1998 and we expect high Canada thistle infestations in 1999.
Year Rainfall (inches)
90-year average 16.5
1993 23.2
1994 24.3
1995 17.4
1996 17.7
1997 16.3
1998 26.1
Winter temperatures also influence Canada thistle populations the following growing season. Winter temperatures usually kill Canada thistle roots in the top foot of soil, which delays emergence the following spring. However, the winter temperatures of 1997-98 were very mild and we saw higher than normal populations of Canada thistle emerging very early in the 1998 growing season.
Many producers are moving toward more reduced-tillage practices to conserve moisture and reduce soil erosion. Past research and a recent survey in Manitoba showed that Canada thistle tends to increase under reduced tillage compared to conventional tillage (3, 7).
State noxious weed officials ranked Canada thistle as the number one noxious weed in the northern U.S. based on noxious weed surveys (6). It is a problem in all crops, especially those crops without registered herbicides than can provide at least some suppression. For example, sunflowers, lentils, chickpeas, and crambe have no product labeled that will provide even partial control. Other crops, such as peas, flax, and dry beans have products labeled to provide some suppression, but the cost can be prohibitive. Control options are better in wheat as season-long suppression is possible with some products in wheat. However, low wheat prices and severe disease problems have forced growers to look to alternative crops. Wheat acreage in North Dakota dropped 16% (almost 2 million acres) in 1998 compared to 1997.
Herbicide tolerant crops (HTCs) such as Liberty Link, Roundup Ready, and Imi canola (Smart) are currently available in Canada. As of December 1998, Liberty Link will be available in the U.S. under a Section 18 registration. Roundup Ready and Imi canola may also be approved by EPA in early 1999. These HTCs will give producers a tool to eliminate or suppress weeds that were not controlled with previously registered products.
In addition to providing season-long Canada thistle control, Roundup Ultra will also control or suppress certain perennial weeds (quackgrass, dandelion, and perennial sowthistle) and annual weeds (wild mustard, wild buckwheat, and Russian thistle) that are present in canola. Roundup Ultra and Liberty will also control weeds that are resistant to specific herbicides used in the northern plains such as foxtail species resistant to dinitroaniline herbicides, wild oat that is resistant to ACCase inhibitors, and kochia that is resistant to sulfonylurea herbicides.
Biennial wormwood (5)
Biennial wormwood, is an annual or biennial growing up to 6 ft tall. In the early vegetative stages, it is similar in appearance to common ragweed. Stems are often reddish and greater than ½ inch in diameter. Biennial wormwood is difficult to control because of its extended emergence period, tolerance to many PPI, PRE, and POST herbicides, and detected late after most POST herbicides are applied. It thrives in low areas where soil may remain wet for extended periods of time. It is also a problem in no-till or minimum till fields.
Biennial wormwood is tolerant to most PPI and PRE herbicides used in corn, soybeans, dry beans, peas, lentils, canola, and sunflower. It is susceptible to phenoxy or growth regulator herbicides. Limited research and experience has shown some options for control with products such as Spartan (sulfentrazone) which may be available on a limited basis in 1999 for use in sunflower, Basagran on wormwood less than 3 inches tall, Cobra at 6 fl oz + PO at 1 pt/A on wormwood less than 3 inches tall, possibly Flexstar on small wormwood, Liberty in Liberty resistant crops, and glyphosate on Roundup Ready crops.
A common situation occurs when biennial wormwood survives soil and POST herbicides, then is detected and misidentified later after most other weeds have been killed. Rescue treatments are applied with little effect. Biennial wormwood is easily mistaken for common ragweed. Many herbicides that will control ragweed, such as Basagran, Blazer, and especially FirstRate do not control wormwood. Biennial wormwood appears to be much more tolerant to herbicides as it increases in size greater than 3 inches tall.
Nightshades (5)
Nightshade species are becoming a greater weed problem, especially in the eastern part of North Dakota. The most common species is eastern black nightshade in addition to hairy nightshade and cutleaf nightshade. Eastern black nightshade and hairy nightshade look similar except hairy nightshade leaves and stems are covered with hairs, and berries remain green at maturity. Small eastern black nightshade plants are mostly hairless, but berries turn purple as they ripen.
The nightshades are tolerant to many herbicides, including the sulfonylurea herbicides. Residual herbicides help control continuous nightshade flushes, but with the exception of Authority/Spartan, Balance, Pursuit, and Python, few effective herbicides have residual activity on nightshade. Following the removal of other weeds by herbicides, nightshades have been free to compete with the crop. The increase in nightshade population can be attributed to several sources including selective weed control, contaminated seed or equipment, and high rainfall in recent years resulting in multiple weed flushes.
Herbicide resistant weeds (5)
Continuous use of herbicides with the same mode of action eliminates susceptible species and allows resistant species to dominate. Documented resistant species in North Dakota include the following:
Herbicide resistance is well documented in Canada as well. Resistant weed species include green foxtail, wild oat, chickweed, hemp-nettle, kochia, and wild mustard (4). There is resistance to 9 unique herbicide families (7 herbicide groups). Surveys conducted in 1996 and 1997 in the northern Great Plains of Canada indicated that wild oat and green foxtail resistance to Group 1 herbicides occurred more frequently relative to other herbicide groups (1). Wild oat resistance to Group 1 herbicides was found in greater than 50% of each of the provinces. Five percent of the fields in Saskatchewan were estimated to have green foxtail resistant to Group 1 herbicides. Multiple resistance in wild oat was also documented in these surveys (1).
A survey was conducted in the U.S. in 1994 asking sunflower producers about their worst weed problems (See Table). The survey was repeated in 1997. The survey results indicated that the severity of many weed problems remained the same or declined from 1994 to 1997. However, according to North Dakota sunflower producers, there were four weeds that increased in severity: more than others: Canada thistle, foxtail, kochia, and wild mustard. Herbicide resistance is likely partially responsible for this increase in weed problems with at least foxtail and kochia. We will need to become more familiar with the herbicides, their mode of action, and signs of herbicide resistance to minimize herbicide resistant weed populations.
Literature Cited