The small, black Palmer amaranth seed has many ways of reaching new farms. It can be introduced by poorly cleaned equipment, slip through seed screening systems or even arise from natural selection if farmers over-rely on key herbicides.
But falling from the rear end of a duck – complete with its own “starter rate” of organic fertilizer – well, that just seems downright unfair.
Nonetheless, the spread of weed seed by waterfowl could explain how invasive weeds – including Palmer pigweed – often pop up in puzzling places. That’s according to a soon-to-be-released University of Missouri-Columbia study.
Researchers have at least suspected that waterfowl were distributing weed seeds, although it was unclear how much or how far they might move seed from south to north. The idea for the Missouri studies took shape late in the last decade.
“Around 2009-10, we were starting to see Palmer in the bottoms of both the Missouri River and Mississippi River,” said Kevin Bradley, an Extension weed scientist at the University of Missouri and author of the study.
This didn’t make much sense to Bradley or other scientists because Palmer deposited by a moving river would have come from the north. That was the common logic. But Palmer was developing in areas where it had never been found upstream, so these populations were inconsistent with what was known about distribution at the time.
The pigweed seemed to be moving from the south to the north by some other means.
Producers who farm in the river bottoms told Bradley they had often seen waterfowl in fields prior to discovery of Palmer on that land. Naturally, they wondered if the ducks and geese might be depositing seed as they made their way to breeding grounds to the north.
Bradley asked graduate student Jaime Farmer, now an agronomist with Pioneer, to help explore that theory. The connection between waterfowl and weeds wasn’t new to Farmer, who was brought up on a farm. He recalled his grandfather hand pulling shattercane from fields and thinking they had them cleaned up.
“Then snow geese came in over the winter,” Farmer remembers, “and a bunch of shattercane would emerge the next year where they had been feeding.”
To pinpoint what was happening, Bradley and Farmer constructed two studies. Roughly speaking, the two projects looked at:
- What went into the birds.
- What came out of the birds – and when.
Bradley and Farmer took a comprehensive approach and tried to build as big a sampling as possible.
What Goes In…
In the first study, they collected waterfowl in Missouri. Their goal was straightforward – determine which weed seeds were being consumed by ducks and snow geese and then transported through Missouri by the waterfowl. Plus, determine if seed were still able to germinate.
During the two-year project:
- Researchers dissected 526 hunter-donated ducks and geese from all parts of Missouri.
- They removed seed from the ducks’ digestive systems and then planted the seed.
- A little over 35,000 plants from dozens of weed species emerged, including barnyardgrass, Palmer amaranth and waterhemp, a pigweed cousin of Palmer amaranth and the Midwest’s predominate pigweed species.
- Waterfowl species included mallards, Northern pintail, green-winged teal, American coot, lesser scaup, snow geese and others.
In the first year of the study, researchers found an average of 18 pigweed seed in the digestive system of each duck. Factoring in estimates of a U.S. duck population of 49 million, that’s nearly a billion pigweed seeds potentially flying around the United States looking for a place to plop – nothing to quack about.
What Comes Out…
The researchers also designed a feeding study to determine the viability of weed seeds after passage through a duck’s digestive system. Live ducks were fed known quantities of 13 different types of agronomically important weed seed, and “we collected what came out of the back end,” Bradley said.
From these droppings, viable seed from 11 of the weed species were recovered.
Not surprisingly, pigweed showed the highest percentage of viable seed. These small, hard seed have a reputation for remaining viable in soil for a couple of years, so a quick pass through a duck’s digestive tract probably would be like a stroll in the park for that pigweed seed.
“We were still getting viable pigweed seed 48 hours after ingestion,” Bradley said. “We didn’t carry the study past 48 hours.” Click here for waterfowl study details.
Based on type of seed, a duck’s flying speed of 48 miles per hour and other variables, the scientists concluded that ducks can jet small weed seeds, such as those from Palmer amaranth and waterhemp, as far as 1,000 miles.
Larger-seeded weed seeds, like those from giant ragweed, can short-hop on “Duck Airlines” almost 200 miles in any direction after ingestion.
Northbound Versus Southbound Ducks
In the waterfowl collection study, hunter-harvested ducks were collected in-season during their trek from breeding grounds in the north to wintering grounds in the south. As might be expected, the contents of the southbound ducks’ digestive tracts yielded a large percentage of waterhemp seed. Again, waterhemp is the most pervasive pigweed-type weed in the Midwest.
Bradley wanted to conduct studies on northbound ducks to see if they contained a higher percentage of Palmer amaranth, the number one weed nemesis of the South, but he encountered a maze of permits required for taking ducks out of season, and decided against it.
Bottom Line: Be Vigilant
Based on the study’s findings, Bradley’s advice for farmers is to add another level of vigilance to their scouting.
“If you have seen geese or ducks in your fields, watch those areas more closely the following year,” Bradley said. “If you catch weeds early, rogue them out. Everybody has been educating Midwest farmers about Palmer amaranth and what it looks like. Don’t let it go to seed. If it’s just one plant, you have to get it out of there.”
“We’ve told farmers that if they have fields with ponds in them, fields that flood or bottom ground where birds are coming off rivers, refuge or roosting areas where they can feed, that is a situation where they might introduce weed seed,” Farmer added.
Bradley said future waterfowl/weed seed studies might consider the nutrient effects of duck droppings and how this might affect the emergence and growth of weed seed contained within them.
“It makes sense,” Brandley noted. “We’ve seen this happen in cattle, where weed seed is deposited in the perfect fertilizer and gets off to a good start, although we’re aren’t so sure with ducks.”
The previous study phase did not track individual birds, which brought up the question of how often the waterfowl might have visited agricultural fields. “In the future, we’re going to have to get a better idea of where the birds are going, and where they’re dropping the seed,” Farmer said.
Nevertheless, it’s easy for Farmer to visualize the duck/weed seed connection, especially since he lives a mere three miles from the Missouri River on the south side.
“Many of our fields drain into these little ponds,” he explained. “The ducks defecate in them. The seed can float beyond the banks into an area where the farmer may not spray or manage. The weed population starts there, then it moves into fields and becomes an even bigger issue.”