An international team of researchers representing the Africa Rice Center (AfricaRice), the International Rice Research Institute (IRRI) and Wageningen University, has raised the alarm over the enormous economic impact of parasitic weeds on rice production in Africa, threatening the food security and livelihoods of millions of resource-poor rice farmers and consumers in the region.
Smallholder farmers in the continent are losing every year half a million tons of rice worth about US $200 million because of parasitic weeds. This is roughly equivalent to the annual rice consumption of Liberia, a low-income country, which is highly dependent on rice imports. If the rice lost due to the parasitic weeds had been saved, it would have been enough to feed the total population of Liberia (4.5 million people) for a whole year.
Parasitic weeds are among the most destructive and problematic weeds to control. “When these plants invade food crops, they turn into ferocious weeds,” said Dr Jonne Rodenburg, Agronomist at AfricaRice. The most important parasitic weed species in rice are Striga asiatica, S. aspera, S. hermonthica and Rhamphicarpa fistulosa. They are all endemic to Africa and can also parasitize other cereal crops like maize, sorghum and millet.
The team of researchers reveal that these parasitic weeds, which survive by siphoning off water and nutrients from host crops, have invaded 1.34 million hectares of rainfed rice in Africa, affecting an estimated 950,000 rural households. They are increasingly becoming severe due to an intensification of agricultural production and climate changes.
The areas affected by parasitic weeds are home to some of the world’s poorest farmers. Studies by AfricaRice and partners have shown that parasitic weeds seem to predominantly affect women farmers in Africa as they are often forced to grow rice on the most marginal and parasitic weed-infested plots.
Parasitic weeds threaten rice production in at least 28 countries in Africa that have rainfed rice systems. The most affected countries are Burkina Faso, Cameroon, Côte d’Ivoire, Guinea, Madagascar, Mali, Nigeria, Sierra Leone Tanzania and Uganda.
The researchers warn that these parasites are spreading fast in the rainfed rice area and if nothing is done to stop them in their tracks, the damage will increase by about US $30 million a year.
These findings were revealed in a recent article by Rodenburg, Demont, Zwart and Bastiaans, entitled “Parasitic weed incidence and related economic losses in rice in Africa,” published in Agriculture, Ecosystems and Environment 235 (306-317). It is published as open access (http://www.sciencedirect.com/science/article/pii/S016788091630528X).
Rice is the second most important source of calories in Africa. It is also critical for smallholder incomes. Demand for rice is growing at a rate of more than 6% per year – faster than for any other food staple in sub-Saharan Africa (SSA), because of changes in consumer preferences and urbanization. Rice production is increasing across SSA, but the continent still imports some 40% of its rice.
Until now, there has been little information on the regional spread and economic importance of parasitic weeds in rice in Africa. “We have presented in this article best-bet estimates on the distribution as well as the agronomic and economic impact of parasitic weeds in rice in Africa,” explained Dr Rodenburg. “In fact, this is the first multi-species, multi-country impact assessment of parasitic weeds in Africa.”
The article focuses on the four most important parasitic weeds in rice. Striga species – known under the common name “witchweed” – occur in at least 31 countries with rain-fed upland rice systems. Rhamphicarpa fistulosa – known under the common name “rice vampireweed” – threatens rice production in at least 28 countries with rainfed lowland rice systems.
Dr Sander Zwart, AfricaRice Remote sensing and Geographic information systems specialist, explained that for this study, a map of rainfed rice production areas, compiled from different databases, was overlapped with parasitic weed observation data retrieved from public herbaria to visualize regional distribution of these four important parasitic weeds.
From this overlap, probabilities of actual infestation were estimated. These estimates together with secondary data on parasite-inflicted crop losses and efficacy of weed control were combined into a stochastic impact assessment model.
The knowledge acquired on the distribution as well as the agronomic and economic impact of parasitic weeds in rice in Africa underlines the importance of finding effective measures to control these pests through research.
AfricaRice and its partners have been investigating and developing efficient parasitic weed management strategies that are affordable and feasible for resource-poor rice farmers. “A range of high-yielding, short-cycle, farmer-preferred rice varieties have been identified with resistance or tolerance to different species and ecotypes of Striga, as well as varieties with good defense against R. fistulosa,” said Dr Rodenburg.
He explained that such varieties can be combined with different agronomic measures, such as late sowing (against R. fistulosa) or early sowing (against Striga), and the use of organic soil fertility amendments. Growing a leguminous cover crop such as Stylosanthes guianensisand following a zero-tillage approach also contribute to effective control of Striga, as demonstrated by agronomic experiments conducted by AfricaRice and its partners.
To study institutional and socio-economic constraints underlying the challenge posed by the parasitic weeds, and to raise awareness and improve communication on efficient management strategies, AfricaRice and its partners have brought together stakeholders, including national research institutes, extension services, crop protection services and private sector representatives in workshops in East and West Africa.
At a time where there is a decline in public sector investments in agricultural research, efficient targeting of resources is becoming increasingly important. “The results of our studies emphasize the importance of targeted investments in further research, the development and dissemination of control technologies and capacity building of farmers, extension agents and other stakeholders, to reverse the observed trend of increasing parasitic weeds in rice,” stated Dr Rodenburg.
Herbicide-resistant weeds didn’t fall from the sky or rise from fields in a mutant mutiny, but they are here nonetheless. With new herbicide technologies going mainstream this season, growers must continue dogged resistant-weed management programs to preserve viable chemistries for as long as possible.
Herbicide-resistant weeds didn’t fall from the sky or rise from fields in a mutant mutiny, but they are here nonetheless. With new herbicide technologies going mainstream this season, growers must continue dogged resistant-weed management programs to preserve viable chemistries for as long as possible.
“In general, herbicide-resistant weeds become a problem over time when they are selected to survive by the overuse of a single herbicide or single mode of action. In all weed populations, there are very low levels or frequencies of herbicide-resistant plants in comparison to susceptible plants,” said Eric Prostko, University of Georgia Extension weed specialist during an American Society of Agronomy webinar “Growing for Tomorrow: How Weed Resistance Management Can Lead to Sustainability”Feb. 1 sponsored by BASF.
The U.S. leads the world with 156 unique cases of herbicide-resistant weeds. “If you grew up in the U.S. like me, you are likely always proud to see American athletes win Olympic gold medals. The more the better, right? Unfortunately, the U.S. is also the gold medal winner for herbicide resistant weeds,” Prostko said.
Australia currently comes in second place with 84 unique cases of herbicide-resistant weeds, and Canada takes third with 64 cases.
Worldwide there are 478 unique cases of herbicide-resistance weeds. The most frequent modes of action that weeds have developed resistance to are the ALS inhibitors (SUs and IMIs), PS II inhibitors (triazine and ureas) and the ACCase inhibitors (dim and fop grass herbicides), he said.
The over-use of glyphosate on glyphosate-tolerant crops has led to the rapid development of glyphosate-resistant weeds over the last two decades, he said. Today, there are 36 weed species worldwide with resistance to glyphosate with 16 of those species in the U.S.
PPO-resistance is now a growing concern, too. “The evolution of PPO resistance is scary because many growers have been relying on herbicides with this mode of action to help manage herbicide-resistant pigweed. Currently, three weed species have evolved PPO-resistance in the U.S., including tall waterhemp, Palmer amaranth and giant ragweed. PPO-resistant Palmer amaranth is under investigation in Alabama, Mississippi, North Carolina and South Carolina,” he said.
But resistant weed problems do not necessarily result in yield loss. In Georgia where farmers have dealt with glyphosate-resistant pigweed for more than a decade, cotton and peanut yields have continued to increase. But it has come at great cost.
The cost to fight resistant weeds with herbicides in Georgia cotton, for example, has increased since 2004 from $28 per acre to $68 per acre plus a 10 percent to 20 percent increase in cost of mechanical cultivation and an increase in the need for hand weeding going from just under $3 per acre to almost $24 per acre today.
So what can growers do or do better, especially in handling herbicide-resistant Palmer amaranth?
1 – Start weed-free at planting using a combination of tillage, cover crops and herbicides. Although the benefits of reduced tillage systems are many, they have also helped contribute to some of our resistant weed problems. “The deeper and longer Palmer amaranth seeds are buried, the less seed germination will occur. Burying pigweed seed with a moldboard plow every three years or so can be beneficial, particularly in problematic fields. In some cases, deep tillage may not be a practical option,” he said.
2 – For growers who cannot or will not use deep tillage, well-managed cereal cover crops can be used to help reduced the emergence of some weeds. Since Palmer amaranth seed requires light for germination, a heavy rye biomass, for example, can prevent light from reaching the soil surface which ultimately influences germination and emergence. Getting an adequate crop stand in extreme cover crops can often be challenging and requires diligence and practice.
3 – Another tactic that exploit’s the influence of light on weed seed germination and emergence is narrow-row planting. Studies in many crops show narrower rows typically result in better overall weed control.
4 – Use herbicides with multiple effective modes of action. Most, if not all, herbicide labels today have their modes of action listed in plain view. “You no longer have to be a weed scientist to identify different modes of action,” he said.
5 – A strict crop rotation “can be extremely beneficial for the management of herbicide-resistant weeds because multiple herbicide modes of action can be used over time. In the case of a typical cotton-and-peanut rotation in the Southeast, eight different herbicide modes of action can be used over a two year period,” he said.
6 – Don’t cut rates to save money or for any other reason. “The use of reduced herbicide rates has been proven to be one of the factors that can increase the rate of herbicide resistance development. … The bottom line: only full-labeled rates should be used for weed control.”
7 – Reduce the seed bank in field. Many growers know it but it’s worth saying again: pigweed is a ‘seedy’ plant, producing 500,000 to 1 million seeds per plant.
Cotton and soybean seed traits tolerant to new formulations of dicamba and 2,4-D, which are on track to be widely available and legal to spray over the top of crops this season, are expected to be planted in some regions. Both herbicides are in the same auxin herbicide family. “First and foremost, these new auxin technologies are not a miracle cure for all your current weed problems. You will still need to start clean, use residuals and make timely postemergence applications,” he said.
“Although much is being said about the new auxin technologies these days, I am not hearing very much about the fact that auxin resistance in weeds has already occurred,” Prostko said. “These herbicides are not new. Currently, eight weed species in the U.S have already developed auxin-resistance. Auxin technology stewardship will be even more important as we head into the future.”
Chad Asmus, technical marketing manager for BASF, spoke during the webinar and echoed Prostko’s concerns and recommendations for growers to reduce the risk of herbicide resistance developing in their fields, highlighting the company’s newly formulated dicamba herbicide Engenia, which can be sprayed over the top of dicamba-tolerant cotton and soybean.
The new herbicide has a BASF patented molecule called BAPMA, which, Asmus said, is the lowest volatility salt of dicamba with the highest loading and lowest use rate at 12.8 fluid ounces per acre; and it’s rain fast in 4 hours.
The Engenia label comes with many requirements, including buffer areas. Asmus stressed that growers must follow the label requirements thoroughly. Here are a few of the general federal requirements for Engenia:
Use the TTI11004 spray nozzle.
Boom Height: ≤ 24” above the target.
Application Volume: ≥ 10 GPA.
Ground Speed: ≤ 15 MPH.
Wind Speed and Direction: 0 to 15 MPH
For wind speeds ≤ 3 MPH confirm there is no field level temperature inversion.
Do not spray with wind speeds >10 MPH blowing toward neighboring sensitive non-specialty crops.
Do not spray with any wind blowing towards neighboring specialty crops.
Asmus said growers need to also pay special heed to any state-specific label requirements that might apply to Engenia this year. An updated list of EPA-approved tank-mix products for Engenia can be found at http://www.EngeniaTankMix.com.
Dr. Wondi Mersie of Virginia State University, PI of the IPM Innovation Lab’s “Biological control of the invasive weed Parthenium hysterophorus in East Africa” project.
Feed the Future IPM Innovation Lab and Invasive Weed Species
The IPM Innovation Lab presented two papers at the “International Conference on Biodiversity, Climate Change Assessment and Impacts on Livelihood” in Kathmandu, Nepal. Dr. R. Muniappan, Director of the IPM Innovation Lab at Virginia Tech presented on the ecological distribution of the four alien invasive weed species, Ageratina adenophora, Chromolaena odorata, Lantana camara, and Parthenium hysterophorus. He pointed out the adoption of Ageratina adenophora to humid and temperate conditions, Chromolaena odorata to humid and tropical areas, and Parthenium hysterophorus to arid and tropical situations. However, several taxa of Lantana camara found in the tropics have adopted all the three conditions mentioned above.
Dr. Wondi Mersie of Virginia State University, PI of the IPM Innovation Lab’s “Biological control of the invasive weed Parthenium hysterophorus in East Africa” project, presented a paper on the adverse effect of Parthenium hysterophorus on biodiversity of above ground vegetation and the seed soil bank of rangelands in Ethiopia.
An arundo gall wasp depositing eggs into the main stem of a giant reed. Photo by John Goolsby.
The release of tiny insects to combat an invasive weed is paying off, according to a recent study by the U.S. Department of Agriculture.Scientists from the USDA’s Agricultural Research Service released arundo gall wasps (Tetramesa romana) and arundo scale insects (Rhizaspidiotus donacis) several years ago as part of a biocontrol program to kill a weed called “giant reed” (Arundo donax) along the Rio Grande in Texas. The weed, also known as “carrizo cane” and “Spanish reed,” clogs streams and irrigation channels, weakens river banks, stifles native vegetation, affects flood control, reduces wildlife habitat, and impedes law enforcement activities along the international border.
Recent research conducted by entomologist John Goolsby demonstrates that these insects have helped control giant reed over more than 550 river miles. Measurements taken in 2014 documented a 22-percent decrease in plant biomass along the Rio Grande since the insects’ release in 2009. Measurements in 2016 show a further decrease of 28 percent and significant recovery of native riparian vegetation.
Giant reed grows between three and seven inches a day and reaches heights of 30 feet along the Rio Grande. The weed increases the population of cattle fever ticks by creating an ideal habitat for them, which makes it difficult for USDA inspectors to detect tick-infested cattle and deer. As the riverbank transitions back to native vegetation, the plant community supports greater abundance and diversity of tick-feeding ants and beetles that act as biological control agents.
To accelerate weed removal, scientists have combined “topping” — mechanically cutting cane — with insect releases. Topping suppresses growth for more than a year and makes plants more susceptible to insect attacks. Combining topping and insect releases gives a high, long-term suppression of cane and allows native trees to grow and start shading giant reed.
“We’ve thinned the cane out significantly,” Goolsby said. “The biggest decline in plants correlates with the greatest number of our biocontrol agents—the wasp and scale.”
Sustainable alternative to methyl bromide for tomato production
Anaerobic soil disinfestation determined effective for controlling weeds, nematodes in Florida fresh-market tomato
August 25, 2016
American Society for Horticultural Science
Field studies in two Florida locations evaluated and compared anaerobic soil disinfestations (ASD) and chemical soil fumigation (CSF) performance on weed and nematodes control, and on fruit yield and quality of fresh-market tomato. Results indicated that ASD (applied using a mixture of composted poultry litter and molasses as carbon source) may be a potentially sustainable alternative to conventional CSF for controlling plant-parasitic nematodes and weeds without causing negative effects on fruit yield and quality.
Tomatoes were grown in field studies to compare anaerobic soil disinfestations (ASD) and chemical soil fumigation (CSF). ASD applied using a mixture of composted poultry litter and molasses as carbon source was shown to be an promising alternative to conventional CSF.
Credit: Photo courtesy of Francesco Di Gioia
Following the phase out of methyl bromide, scientists continue to explore effective, viable, and more sustainable options for vegetable crop production. Among nonchemical alternatives, anaerobic soil disinfestation (ASD) is considered to be one of the most promising methods. ASD has been determined to be effective with a range of crops and environments against several soilborne fungal and bacterial plant diseases, plant-parasitic nematodes, and weeds.
A study in the June 2016 issue of HortScience focused on the effects of ASD in an open-field, fresh-market tomato production system. Field studies were conducted to evaluate and compare ASD with chemical soil fumigation (CSF) treatments for controlling weeds and nematodes, as well as for influence on tomato fruit yield and quality. In experiments conducted in southwestern (Immokalee) and northern Florida (Citra), conventional CSF was compared with two ASD treatments, which consisted of amending the soil with 22 Mg·ha-1 of composted poultry litter and two rates of molasses (13.9 and 27.7 m3·ha-1) as a carbon source.
Analyses showed that the application of ASD did not negatively affect commercial tomato fruit quality, and that quality and the mineral content of fruit produced with ASD was comparable or higher than that of fruit produced in CSF plots.
In both locations, the application of ASD provided a level of root-knot nematode control equivalent to, or more effective, than the CSF. Additional results showed that, in Immokalee, the CSF provided the most significant weed control, “but ASD treatments also suppressed weeds enough to prevent an impact on yield,” the authors said. In Citra, all treatments, including the CSF, provided poor weed control relative to the Immokalee site.
“Overall, the results of the two locations demonstrate that the ASD technique may be a valid and sustainable alternative to the conventional CSF, and could be transferred at commercial level,” the authors said. “Molasses rates showed similar performance in terms of root-knot nematode and weed control, yield, and fruit quality; therefore, the lower molasses rate could be suggested to reduce the cost of the ASD treatment.”
On-going research is focused on substitutions for composted broiler litter and minimizing nutrient inputs in an ASD system.
American Society for Horticultural Science. “Sustainable alternative to methyl bromide for tomato production: Anaerobic soil disinfestation determined effective for controlling weeds, nematodes in Florida fresh-market tomato.” ScienceDaily. ScienceDaily, 25 August 2016. <www.sciencedaily.com/releases/2016/08/160825113223.htm>.
Surprised at how a field full of herbicide-resistant weeds results? It had its origins years earlier. Birds do it, bees do it, even educated fleas do it, as the old Cole Porter song goes.
So why can’t waterhemp (or weeds) do it?
That last line probably isn’t funny if you’re battling herbicide-resistant weeds. Still, it says much about the way herbicide-resistant waterhemp infests your fields in the first place.
That field full of weeds – waterhemp, Palmer amaranth, marestail, or something else – started some time before.
It’s akin to when a point guard catches a cute courtside TV reporter’s eye. Waterhemp is dioecious, with male and female plants. Sparks quickly fly.
“Since waterhemp is dioecious, pollen is already moving around,” says Pat Tranel, University of Illinois (U of I) weed scientist. “Pollen can be viable up to 120 hours, and it can move ½ mile from the pollen source.”
Initially, resistance is rare
Every herbicide selects for its own failure, even a brand-spanking-new one. In every weed species, rare genetic biotypes exist that resist a herbicide. This varies among herbicide sites of action, says Ian Heap, director of the International Survey of Herbicide-Resistant Weeds.
Rare herbicide-resistant biotypes exploit management uniformity, much as a star center who shreds double-teaming defenders. Every herbicide application heightens the odds that resisters survive.
Really quick in the case of some weeds like Palmer amaranth. It’s double trouble if the weeds are herbicide-resistant biotypes. By producing up to 1.8 million seeds per plant, glyphosate Palmer amaranth infested 20% of the field areas in less than two years in a 2008 University of Arkansas analysis.
You may be doing a laundry list of best-management practices to forestall resistance. Rotating herbicides’ sites of action. Lacing your row-crop rotation with small grains. Regularly scouting fields and rounding oddball weeds.
Unfortunately, all can be for naught due to an often overlooked resistant-weed spreader: waterfowl.
Kevin Bradley, University of Missouri Extension weed specialist, reports an MU trial found that weed seeds can remain intact after passing through a mallard duck’s digestive system.
Think you’re cruising in a corn planter that goes 10 mph? It has nothing on mallard ducks, which can fly up to 48 mph for up to 38 hours. Potentially, they can move weeds like Palmer amaranth, waterhemp, common lambsquarters, giant foxtail, and smartweed nearly 1,740 miles in 1½ days.
Glyphosate seems synonymous with herbicide-resistant weeds. In most cases, though, weeds resist multiple sites of action. Here are herbicide action sites that waterhemp resists and initial U.S. confirmation year.
ALS Inhibitors – 1993
Triazines – 1994
PPO inhibitors – 2001
Glycines (Glyphosate) – 2005
Synthetic auxins (2,4-D) – 2009
HPPD inhibitors – 2009
“Waterhemp won’t stop at six herbicide sites of action,” says U of I’s Tranel. “It will develop resistance to anything being developed, whether it’s the seventh, eighth, or ninth.”
It’s important to note that no waterhemp biotype exists in all six herbicide sites of action. Still, multiple site resistance abounds. In 2009, for example, weed scientists confirmed an Illinois waterhemp biotype that resisted four sites of action.
Nightmare crosses between pigweeds occur, but so far at a low rate.
Glyphosate-tolerant waterhemp and Palmer amaranth are bad now, but imagine how fearsome they could be if this weed equivalent of pro basketball’s Stephen Curry and LeBron James teamed up together! Could they cross-pollinate to create some sort of freak frankenweed?
Now that would be bad – and it is currently happening with pigweed species like waterhemp, Palmer amaranth, and spiny amaranth.
“There is some hybridization occurring,” says Kevin Bradley, University of Missouri Extension weed specialist. “There are some weeds I walk by and say, ‘Wait a minute. That doesn’t look quite right.’ The leaves will look like waterhemp, but the head looks like Palmer.”
2012. That’s the year Bill Molin, USDA-ARS plant physiologist in Stoneville, Mississippi, began tracking spiny amaranth and Palmer amaranth hybrid. Resistant to both glyphosate and ALS herbicides, the hybrid is the result of a cross between Palmer amaranth growing in a cotton field and spiny amaranth in a neighboring pasture.
Palmer and waterhemp are doing it, too
Researchers at the University of Illinois and Colorado State University have also discovered amaranthus hybrids of Palmer amaranth and waterhemp.
“The hybrids often look similar to the normal variation of the species,” says Pat Tranel, University of Illinois weed scientist. In many cases, hybridization between species can only be confirmed by genetic analysis, says University of Missouri’s Bradley.
The Good News
Imagine if that imaginary basketball player brimmed with hybrid vigor by crossing Curry with James. Fortunately for you, this hybrid vigor so far doesn’t extend to amaranthus species. Palmer amaranth and waterhemp shoot bricks when it comes to hybridizing. In 2011, field studies by Colorado State University weed scientists showed that glyphosate-resistant Palmer amaranth transferred glyphosate resistance to:
Spiny amaranth 0.4% of the time.
Waterhemp 0.2% of the time.
Smooth pigweed 0.01% of the time.
Remain watchful, though.
“I think the danger in Missouri is with pastures between crop fields,” says University of Missouri’s Bradley. “There, Palmer amaranth that transfers resistance to spiny pigweed in the middle of a pasture is something of which we are aware.”
It’s akin to the anxiety a coach feels when his star forward, at midshot, flashes an on-court smile at a cheerleader. “Wherever these two plants are close together, there is a chance to hybridize,” Molin says.
How to control?
Double- and triple-team it just as you would a 40-point-a-night forward. “Broaden the spectrum of herbicides used,” says Molin. It’s important that this spectrum include herbicides with multiple effective modes of action, he says.
Look beyond the field, too. Control weeds in fencerows, ditches, and other places these weeds can grow, he says.
Even more good news
Low hybridization of pigweed species also means low rates of hybridized seed being shed.
Still, bear in mind that the average waterhemp plant sheds 250,000 seeds. If waterhemp hybridizes just 0.2% of the time, the plant still sheds 500 seeds that can germinate and emerge the next year.
Precision weed control
No new tools are on the horizon, but there’s still technology to help you manage weed issues.
Herbicide-resistant weeds aren’t man-made, but that’s not to say man isn’t to blame. “Farmers have been selecting for herbicide-resistant weeds,” says Lisa Behnken, Extension educator for the University of Minnesota. “They do that by being very predictable.”
Herbicide-resistant weeds occur naturally in the weed population. Farmers became more predictable, and the repeated use of the same chemical tools caused resistant weeds to become a larger part of the population. “Waterhemp became resistant to ALS herbicides before glyphosate technology was even around,” says Behnken.
Missing free throws frustrate the crowd on game day. The same goes for weed specialists when applicators continue to use flat-fan spray nozzles. “They haven’t adopted the air-induction nozzles,” says Fritz Breitenbach, Extension specialist for the University of Minnesota. “We’ve done enough work to show that they’re superior.” It’s a simple correction that could change the outcome of the game.
In a University of Minnesota study, Behnken found that only 9% of farmers were mapping weeds. “We’d like to see that change,” says Behnken. As you are going across the fields during harvest, map the weeds. That allows you to spend money wisely on your herbicide program, and you can focus on the spots where you have more difficult problems.
You only have one first chance to prevent a weed before it gets established, says Russ Higgins, University of Illinois Extension educator. Once it’s established, it becomes widespread across the field, and then it becomes costly and difficult to control.
Make Multiyear Plans
Last-minute-play calls are no longer acceptable management plans. Weed control needs to be a system approach, which includes multiple practices, says Behnken. “The majority of farmers are lucky if they make a one-year plan.”
Instead, she recommends farmers create multiyear plans with backup plans. In those plans, Behnken recommends tactics that have both chemical and nonchemical tools. Also, don’t include the same herbicide or mode of action every year.
Few new weed-control tools exist, says Breitenbach. “They have to rediscover old tools,” he says. Breitenbach recommends that you go back to incorporating cultural practices for more comprehensive weed control, such as:
Reevaluating planting dates
Ever run into the former high school wallflower at a class reunion who now mimics a flashy NBA star? In the weed world, that’s waterhemp.
Waterhemp used to just line banks near streams and rivers. If this small-seed broadleaf did meander into your crop fields, tillage would bury it. No more. These days, less tillage ensures this shallow soil dweller has ample germination and emergence.
This emergence – which has started as early as late March in Illinois – can continue all through July, says Pat Tranel, University of Illinois weed scientist.
Late-season flushes separate waterhemp from other weeds. A 1996 Iowa State University study showed that most wooly cupgrass emerged May 10-20. Peak waterhemp emergence didn’t start until June 24.
Use full rates of preemergence residual herbicides as close to planting as possible.
Follow with overlapping contact and residual postemergence herbicides.
Apply multiple effective sites of action to help manage multiple-resistant weeds.
The seeds one waterhemp plant can make with no competition, such as in a prevented-planting case.
It pays to control weeds early. That’s because 1% is the soybean yield loss that results when control occurs on 6-inch waterhemp with infestations of fewer than 10 plants per square foot.
Waterhemp is the league scoring champ when it comes to corn and soybeans. It infests row crops all the way from Louisiana to North Dakota. It’s particularly thick in Iowa and Missouri, infesting 90% or more of soybean acres.
That’s some wingspan. The seed head can reach up to 3 feet long!
Palmer amaranth is an aggressive, invasive weed native to the desert regions of the southwest U.S. Facing a drought? That won’t slow Palmer. Given it originates from the desert, it’s drought-resistant.
Palmer is akin to a towering center who grows every game. It grows nearly 2 inches per day. It’s also as prolific a scorer as a 3-point shooting guard.
The cheapest way to manage a weed is to prevent it from getting established in a field, says Iowa State University’s Bob Hartzler. He encourages farmers to hand-weed, especially the female plants, before it is established.
Why is Palmer a challenge? It’s already knocked out two main defenses. Most populations resist glyphosate and ALS-inhibitor herbicides. If you let Palmer amaranth play out all season, yield loss could be up to 91% in corn and 79% in soybeans.
It’s easy to confuse Palmer and waterhemp seedlings, says Hartzler. Waterhemp has a shorter petiole and longer, narrower leaves than Palmer. The best trait to differentiate seedlings is the presence of petioles longer than the leaf blade on Palmer. Once they have flowered, the sharp bracts on female Palmer are a dead giveaway.
Diversity of weed management must expand beyond herbicides, says Mike Owen, Iowa State University Extension weed scientist. Repeated use of the same chemical tools selects for resistance. Instead, try a combination of cultural practices and chemical tools for control.
Harvest infested fields last. Otherwise, your combine will spread Palmer seeds. Flag areas where you spot Palmer amaranth, says Russ Higgins, University of Illinois Extension educator. When it’s in the introduction stage, there’s still an opportunity to eradicate it before it becomes a widespread threat, he says.
Palmer infests row crops from Louisiana to Ohio to North Dakota. In a 2015 survey, it was on 60,000 acres in Kentucky, according to University of Kentucky’s J.D. Green. Nearly one third of Illinois counties have documented cases of Palmer.
Lack of crop diversity and no-till spur this winter annual’s spread.
Marestail is number one, but not in a good way. It was the first glyphosate-resistant weed confirmed in the U.S. when it surfaced in Delaware in 2000. Since then, it’s moved at full speed into other areas like the Midwest.
Marestail rosettes often appear in the fall in the northern U.S. A fall application of 2,4-D or dicamba plus chlorimurin or metribuzin products offers the best residual control, says the University of Missouri’s Kevin Bradley.
Preemergence herbicides are a must! Larger weeds are tough to kill. Limited postemerge corn options exist, and there are even fewer in soybeans, say University of Nebraska weed scientists.
Spring and summer flushes of marestail south of I-70 in the Corn Belt are becoming more common. This likely is signaling a change in plant biology, due to herbicide selection and changes in crop rotation.
Why not rye?
A cereal rye cover crop planted after corn, soybean, or wheat harvest can nix fall marestail emergence and aid control. “A glyphosate and Fierce preemerge provides nearly 98% control of marestail at planting,” Bradley says. “Take out the cereal rye, and the same treatment provides just 40% control.”
Glyphosate-resistant marestail migrated to the mid-South after its 2000 Delaware discovery. It now infests 90%of Kentucky soybean acres. It’s since moved to the Midwest and now plagues an estimated 30% of Iowa’s soybean acres.
This old fencerow and ditch dweller now infests crops
Its 17-foot maximum height would make it any team’s starting center. Typically, though, it’s 1 to 5 feet taller than the field’s crop. Giant ragweed biotypes exist that have resisted ALS inhibitors (Pursuit) in seven states and EPSP synthase inhibitors (glyphosate) in 12 states. There are confirmed giant ragweed biotypes resisting both herbicide action sites in Ohio, Minnesota, and Missouri.
A single plant can produce an estimated 1 billion pollen grains during its lifetime. Its pollen triggers more than allergies; it creates cross-plant pollination potential. This creates genetic diversity that creates more herbicide-resistance potential.
Why is it a problem?
Giant ragweed used to dwell in undisturbed areas like fencerows and drainage ditches. Since the late 1980s, it’s moved into cropland. It’s now prevalent in cropland due to:
Crop rotation. It surfaces more when corn is rotated with soybeans vs. continuous soybeans.
Tillage. Buried giant ragweed seed escapes surface weed predators like insects and mice.
Stem-boring insects. They enable large giant ragweed plants to survive glyphosate applications.
The most effective herbicide programs combine preemergence and postemergence herbicide treatments with two or more herbicide sites of action. Scouting after the first post trip can pinpoint escapes.
Giant ragweed infests more acres in the eastern Corn Belt. In Kentucky, for example, giant ragweed infests 40% of soybean and corn acres, Still, infestations occur farther west, with infestations estimated at around 10% in Iowa, mainly in north-central regions of that state.