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A connection between waterfowl and the spread of resistant weeds?

Missouri study sheds light on the role of waterfowl in spread of resistant weeds

It’s that time of year when the migration of waterfowl has begun and hunters unlock the gun cabinet and pull on camo. It turns out those wonderful ducks may be carrying more than the odd band and a desire for warmer climes.

Is the rapid spread of herbicide-resistant pigweeds in the Mid-South at least partially attributable to the flyway? A recent study out of Missouri says it is quite likely.

In 2014, Jaime Farmer was a weed science program graduate student under Dr. Kevin Bradley at the University of Missouri. That fall, Farmer -- now a Pioneer agronomist for western Missouri -- began collecting hunter-harvested ducks. In 2015, Farmer and colleagues did a feeding study and repeated the collection of ducks and snow geese.

Farmer, who spoke with Delta Farm Press in early December, explains what the study showed.

What precipitated the start of the study?

“A couple of things came together. First, my predecessors in the program were avid waterfowl hunters. Dr. Bradley had been tracking the movement of Palmer amaranth throughout Missouri for a few years before I got there.

“One of the things they noticed was that a lot of the areas in counties they were finding the weed were in river bottoms. Western Missouri has two main river systems flowing through. Until then, most of the Palmer amaranth had been contained in the Bootheel.

“The first thought was ‘maybe it’s flooding that dispersing the seed in these bottoms.’ But what was interesting was the movement of the Palmer was from south to north – opposite of the water flow direction. That’s when they came up with the possibility that waterfowl may be responsible for spreading the seed. Farmers pointed out that a lot of ducks and geese had been in fields over the winter and then they found pigweeds there the next growing season.

“When I got to the program, he asked if I could figure out a way to answer whether waterfowl was the answer. We began looking at literature, decided to find out if waterfowl were even eating Palmer amaranth and waterhemp seed and, if so, what potential does that seed have to make it through the bird and still be viable.”

What did you do with the duck and geese collections?

“The process involved working with the Missouri Department of Conservation. I also worked with the University of Missouri Wildlife Biology Department. They showed me how to dissect and remove the digestive tracks, segment it out and taught me how each section works.

“The easiest way to identify a plant species is to let it grow out. So, we asked legal hunters for the carcasses after they’d breasted the birds. We had a network all over the state when hunting season began in 2014. They followed the protocol required by the state and when their freezer was full, I’d go collect the birds. They were tagged with when they were killed, where, the species, etc.

“Once we had enough birds, we’d go in the lab and check the esophagus, the gizzard and the intestine. So, we had samples from three sections of the birds. We took the collections to the greenhouse and planted them. Once the seeds germinate, it’s a lot easier to identify the plants.

“It worked out really well. The first season, we collected 237 ducks and had 14,395 plants emerge in the greenhouse.”

On snow geese…

“In Missouri, we’ve had 60-day waterfowl seasons for the last two years. The northern zone season starts earliest and that means there’s an open season somewhere basically from the end of October through January. 

“Then, the conservation order for limitless snow geese begins in February. Usually the push of migration depends on weather. The biggest push for the project years was in March. So, we did the same process with snow geese as we did with ducks.

“After the first year, it became very apparent how different ducks and snow geese were feeding. With snow geese, we found a lot of waste corn and we did find some Palmer amaranth and waterhemp seed. With ducks, we’d found that almost 30 percent of the 14,000-plus plants we grew out were pigweeds.”

On the second phase of the study…

“So, we knew what the birds were eating but wanted to find out how much seed actually makes it through and remains viable. Some older feeding studies had looked at retention rates and feeding efficiencies. We modeled our experiments on those.

“We got some live adult mallards, an even mix of males and females. We took 13 agronomically important weed seeds we’d found during the first year of field collections. Those seeds were all different sizes. The smallest were waterhemp and Palmer amaranth to the largest with sunflower, giant ragweed and others.

“We fed one gram meals that contained seeds that had all been counted. Then, we waited and collected what they passed every four hours for 48 hours. Since we knew what they’d been fed we didn’t need to grow the seed out. We used a chemical process to stain the seed embryos to let us know if the seed was still viable.

“From that study – and we did seven replications – we got viability and retention rates, recovery rates. That gave us a better idea how long a bird could carry each type of seed.”

On the numbers…

“All in all, it was about 18 months’ worth of work. For both seasons, we had 362 ducks collected in the field. From those, 34,807 plants emerged with 30 percent being pigweed. For snow geese, there were 163 collected and 213 plants emerged – the majority of that being waste corn.

“After the first season, to get over 14,000 plants to emerge in the greenhouse was a surprise. I just didn’t expect that many to come up. And there were over 50 plant species in the mix so the variety the ducks were eating was eye-opening, as well. Ducks eat whatever is available, no discretion.

“We were confident we’d find agronomically important weed seed in the birds. The amount, though, the sheer numbers along with how long the seed can remain viable in the birds was a shock.”

On farmers’ observations…

“Initially, we thought, ‘maybe birds are transporting weed seed by walking through mud and flying off to another location.’ For the most part, I found hardly anything on their feet or in their feathers.

“It was interesting when I was explaining the study to my grandfather and some of his friends up in northeast Missouri. They laughed and said ‘yeah, we’ve known ducks transport seed for years.’

“Turns out, they used to have hand-eradicate shatter cane, the biggest problem for farmers way back. It would take a few years but they’d clean a field up pretty well. They said, ‘Then snow geese would come in during the winter and feed. We knew the next growing season we’d have to deal with shatter cane again.’ They were always under the assumption the geese were bringing the seed in.”

The takeaway…

“You may have no herbicide-resistant Palmer amaranth or waterhemp. You may have the cleanest of fields. You may clean your equipment and take precautions to keep these weeds off your ground. That’s great, keep doing those things.

“But understand, unfortunately, that doesn’t mean these resistant weeds can’t come in with waterfowl. Don’t be complacent. Make sure you maintain a good weed management program on your property. Resistant weeds can become established incredibly fast.

“Farmers in Missouri are scared of resistant Palmer amaranth -- waterhemp is bad enough. They see how things have progressed in the Mid-South and Southeast and don’t want to deal with those problems.”   

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