Ducks are a hunter’s sweetest dream, but 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 the usual suspects: used equipment purchases, custom harvesters, hay, contaminated pollinator seed, cottonseed and gin trash. But new research by University of Missouri Extension weed scientists shows growers can tick off all those boxes of defense and still get nailed by a feathered threat from above. Slicing sky at 60 mph, a flock of mallards with a tailwind often travels hundreds of miles in a handful of days, discharging waste with a rapidity rarely matched in the animal kingdom. They may eat supper in Arkansas, and defecate all the way to Iowa. Waterfowl droppings, strafe bombed throughout the Mississippi Flyway, can be laced with resistant weed seed. Simply, Palmer creep can quickly morph into a successful Hail Mary.
Prior to 2008, Palmer presence in Missouri was confined to the Bootheel region but began to march up the Mississippi River along bottom lands, presumably transported during flooding. Just eight years later in 2016, Palmer is confirmed in 31 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 that Palmer’s debut on their fields matched with a heavy presence of geese in prior months. “I never discount anything a farmer tells me,” Bradley says. “I listened and began working with wildlife personnel to develop a project to find out if waterfowl could be a significant cause of pigweed spread.” Bradley 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. Farmer’s weed-and-feed study offered an unprecedented peek into the potential for waterfowl to move agronomically important weeds across large distances. Starting in October 2014, Farmer reached out to 20 friends in his ducking 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 ducks on ice, and called Farmer. When freezers filled, he picked up the birds and tagged them in a log kept for the MDC according to county, date of harvest, and hunter’s name. With 237 ducks from 13 species acquired, the next phase of research was ready. “We couldn’t have done this without the hunters,” says Farmer, now a Pioneer field agronomist. “The University of Missouri wouldn’t sign off on a Benelli for the research, but I did get to participate in the harvest.” After removing seed from the esophagus, gizzard and intestine, Farmer rinsed it through a sieve and planted each specimen in a greenhouse for a three-month window to test germination. From the 237 ducks, 14, 395 weed species emerged (30% were either waterhemp or Palmer). In 2015, Farmer repeated the process, obtaining 125 duck specimens and observing germination of 20,412 plants (2.3% were either waterhemp or Palmer). Farmer also extracted seed from 111 snow geese specimens and observed emergence of 86 weed species (9% pigweed).
Following collection of the hunter-harvested waterfowl data, Farmer next tested post-consumption seed viability. How much seed remains viable in duck droppings? At a MDC breeding house south of Columbia in the fall of 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. “There was no variation in meal size because we knew the exact numbers of weed seed and weight,” Farmer says. “Then we monitored the ducks for 48 hours.” 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 until the flight’s end. “Birds have stopovers and don’t fly in a straight line, but the mechanical potential is there,” Farmer notes. “Frankly, the numbers are scary and show how weeds can make tremendous jumps.” An average of 11,400 total seeds (all 13 weed species) were fed to the ducks in eight feeding trial runs, and an average of 2,500 seeds were recovered in viable condition after each trial. Specifically for Palmer: 3,183 seeds fed per trial with 815 (25.6%) were recovered viable. Specifically for waterhemp: 4,205 seeds fed per trial with 963 (22.9%) were 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 (either Palmer or waterhemp) in each duck harvested during 2014-15. The USFWS estimated a population of 48.4 million breeding ducks in 2016. Taken strictly by the numbers, ducks have the potential to transport 871 million pigweed seeds during any given segment of migration, according to Bradley.
In October 2016, on the heels of Bradley’s research, MU's IPM announced the first finding of multi-resistant Palmer in Missouri, located in a river bottom north of St. Louis. According to MU's IPM, the confirmed case marks the first occurrence of Palmer in the state with resistance to group 14 herbicides. The continued spread of resistant pigweed comes in tandem with a jump in herbicide and management costs. For producers already paying $45 to $50 in weed prevention, the prospect of increased financial pressure looms large. “This has been eye-opening and wakes people up,” Bradley says. “Waterhemp has exploded up north and many of my colleagues partially attribute the spread to waterfowl transport,” he adds. Christy Sprague, an Extension weed scientist with Michigan State University, has long-suspected resistant weed spread by waterfowl, based on waterhemp pop-ups across Michigan. “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 doesn’t paint a picture of total gloom. "Over 1,000 miles from the source is a very realistic estimate of how far a mallard might transport waterhemp or Palmer,” he says. “Farmers are scared of Palmer and that works toward better prevention.” Bradley’s research shows proximity is a hollow weed defense when ducks are overhead, and location is no antidote to resistant pigweed. Look to the skies. “For any Midwest state, this means of pigweed spread is very possible,” he says. “Stay tuned.”