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Ducks Trigger Weed Dynasty

Look to the sky—research finds resistant pigweed spread by waterfowl

Ducks are a hunter’s sweetest dream, but they can be a farmer’s nightmare akin to flying time bombs expelling rapid-fire payloads of resistant-weed seed. Wonder where the next pigweed outbreak will come from? Listen for quacks and honks, and look up.

Palmer amaranth typically hitches a ride north on used equipment, custom harvesters, hay, contaminated pollinator seed, cottonseed and gin trash. But new research by University of Missouri (MU) Extension weed scientists shows farmers can tick off all those boxes of defense and still get nailed by a feathered threat from above.

Prior to 2008, Palmer presence in Missouri was confined to the Bootheel, but it began marching up the Mississippi River along bottomlands, presumably transported during flooding. Just eight years later, Palmer is confirmed in 32 counties. But to MU Extension weed scientist Kevin Bradley, the northern spread of Palmer due to flooding didn’t ring true when matched against the river’s southern flow. Several Missouri producers told Bradley Palmer’s debut on their fields matched with a heavy presence of geese in prior months. 

Bradley began working with wildlife personnel to develop a project to find out if waterfowl could be a significant cause of pigweed spread. He designed an experiment carried out by MU graduate student Jaime Farmer in two phases. First up, waterfowl collection to discover what weed species were consumed and transported in Missouri. Second, a follow-up feeding study to determine seed viability. 

Starting in October 2014, Farmer reached out to 20 friends in his duck hunting network and asked for help to obtain specimens for testing, with everything permitted by the Missouri Department of Conservation (MDC) and the U.S. Fish and Wildlife Service (USFWS). After hunting trips scattered across the state, the group of licensed hunters pulled the meat, put the ducks on ice and called Farmer. When freezers filled, he picked up the birds and tagged them in a log kept for MDC according to county, date of harvest and hunter’s name. With 237 ducks from 13 species, the next phase of research was ready.

After removing seed from the esophagus, gizzard and intestine, Farmer rinsed it through a sieve and planted each specimen in a greenhouse for three months to test germination. From the 237 ducks, 14,395 weed species emerged (30% were waterhemp or Palmer). In 2015, Farmer repeated the process, obtaining 125 duck specimens and observing germination of 20,412 plants (2.3% of which were waterhemp or Palmer). Farmer also extracted seed from 111 snow geese specimens and observed emergence of 86 weed species (9% were waterhemp or Palmer).

Following collection of the hunter-harvested waterfowl data, Farmer next tested post-consumption seed viability. How much seed remains viable in duck droppings? At an MDC breeding house in fall 2015, he fed 13 varieties of weed seed to captive ducks via controlled tube feeding to ensure a precise quantity of seed. During feeding, the ducks were placed in metabolic chambers with freedom of movement, but segregated to ensure the integrity of fecal sampling. 

Every four hours, Farmer and his research team removed collection tubs and checked viability through chemical analysis. Of the 13 weed species, 11 (lambsquarters, waterhemp, smartweed, Palmer, common ragweed and more) were passed within four hours and remained viable. The majority of seed passed within 12 hours. However, harder and smaller seeds last longer in the digestive tract: The tiny Palmer seeds often were passed in viable condition at 40 hours and beyond.

How does 40-plus hours of passage equate with flight? A flight speed of 48 mph maintained for 38 hours could equate to 1,824 miles, plenty of distance to take a duck from any Southern state far across the Canadian border, theoretically dropping pigweed seed at flight’s end. “The numbers are scary and show how weeds can make tremendous jumps,” Farmer notes.

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An average of 11,400 total seeds (all 13 weed species) were fed to the ducks in eight feeding trials, and an average of 2,500 seeds were recovered in viable condition after each trial. For Palmer: 3,183 seeds were fed per trial with 815 (25.6%) recovered viable. For waterhemp: 4,205 seeds were fed per trial with 963 (22.9%) recovered viable.

Farmer believes the study proves ducks can disperse Palmer and waterhemp seed over vast distances. “You can farm in isolation, but still have weed resistance suddenly pop up in your fields, delivered by wildlife, not equipment or hay,” he says. “Everyone needs a multi-pronged weed management program already in place.”

Significantly, Farmer found an average of 18 pigweed seeds (Palmer or waterhemp) in each duck harvested in 2014 and 2015. USFWS estimated a population of 48.4 million breeding ducks in 2016. By the numbers, ducks have the potential to transport 871 million pigweed seeds during any given segment of migration, Bradley adds.

In October 2016, on the heels of Bradley’s research, the first finding of multi-resistant Palmer was confirmed in Missouri. The case in a river bottom north of St. Louis marks the first occurrence of Palmer in the state with resistance to group 14 herbicides.

The spread of resistant pigweed comes in tandem with a jump in herbicide and management costs. For farmers paying $45 to $50 per acre in weed prevention, the prospect of increased financial pressure looms large. 

“Waterhemp has exploded up north and many of my colleagues partially attribute the spread to waterfowl transport,” Bradley says.

Christy Sprague, a weed scientist with Michigan State University Extension, has long-suspected resistant weed spread by waterfowl, based on waterhemp pop-ups across the state. “Equipment and animal feed such as cottonseed and gin trash have moved waterhemp and Palmer around our area, but there is certainly the 
possibility that waterfowl are also a contributing factor,” she says

Despite Palmer’s spread, Bradley says it isn’t all bad. “Farmers are scared of Palmer and that works toward better prevention,” he notes.

Bradley’s research shows proximity is a hollow weed defense when ducks are overhead. Look to the skies. “For any Midwest state, this means of pigweed spread is possible,” he says.

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