Silage growers urged to protect feed supply from Bt-resistant corn rootworm

Some livestock producers in the U.S. corn belt have had to make a difficult and costly decision. They’ve harvested their silage corn for grain to salvage some financial return.

Now a provincial entomologist says Ontario producers may face the same decision if resistance to the Bacillus thuringiensis (Bt) trait in corn continues to grow. Bt-resistant corn rootworm causes lodging and goose-necking in corn plants, making much of the crop impossible to harvest for silage. 

Tracey Baute, field crop entomologist with the Ontario Ministry of Agriculture, Food and Rural Affairs, encouraged Ontario livestock producers to consider alternatives to corn-on-corn and announced the results from the province’s participation in the Corn Rootworm Adult Monitoring Network during the 2022 Forage Focus webinar series last month. 

Why it matters: Resistance to the Bt control gene is now widespread across the U.S. corn belt and increasingly present in Ontario, to the degree that resistant rootworms are migrating to neighbouring farms.

Baute’s forage and grazing colleague, Christine O’Reilly, said some Ontario silage producers have suffered yield losses of 50 per cent or more with confirmed rootworm damage present.

Baute and O’Reilly, along with Corteva Agriscience’s eastern Canada dairy strategic accounts manager Ashley Knapton, presented their concerns in the webinar hosted by the Ontario Forage Council.

Baute stressed that corn rootworm isn’t a new pest and it is native to North America. The introduction of corn with the Bt gene initially allowed producers to put mitigating management strategies to the backs of their minds and grow corn in the same field year after year.

Resistance has changed that.

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Baute said there are four versions of the Bt gene, three of which are closely related, so simply switching from one form of control to another in year-over-year corn isn’t an adequate strategy.

Experience in the U.S. shows that, even with a tight soybean-corn-soybean-corn rotation and a pest that was initially thought to depend solely on corn for larval feeding, at least one species of corn rootworm has adapted to survive.

“They figured out a way to get their eggs into a field that corn is going to come back to the next year,” said Baute. “This is a pest that we need to switch it up and change management tools repeatedly so we don’t have them adapt to what we’re throwing at them.”

The Monitoring Network, which links adult beetle trapping efforts across Canada and the U.S., included 76 Ontario sites in 2022. According to Baute, 57 per cent of those reached the economic threshold of two beetles per trap per day in sites across southwestern, midwestern, central and eastern Ontario.

Sites above the threshold but not reaching nine beetles per day – 28 in all – were in the counties of Elgin, Norfolk, Waterloo, Bruce, Grey, Northumberland, Lennox and Addington, Ottawa, Stormont Dundas and Glengarry, and Prescott and Russell.

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However, 15 fields reached maximums greater than nine beetles per day, with the highest Ontario field reaching 32. These sites, which Baute described as indicating a “high to extreme” risk of the presence of Bt-resistant pests, were in the counties of Durham, Huron, Lambton, Middlesex, Chatham-Kent, Oxford, Perth, Wellington and Hastings.

For all of these counties, “it flags to me that next year, they need a different option for rootworm management and mainly, to rotate.”

She also noted the list includes nine of the top 10 counties for dairy production or feedlots.

“This is actually a direct problem for livestock producers and particularly for dairy production.”

For many livestock producers, rotating to alfalfa is the most obvious alternative. Ideally, costs don’t increase.

“You’re still growing the alfalfa and the corn that you need. You’re just planning better each year about where you’re going to put them,” said O’Reilly.

Alfalfa provides a rotation option

Some producers hesitate to grow alfalfa far away from the barn or silo, added Knapton. But using alfalfa to broaden the crop rotation requires not treating it as a “set and forget kind of crop which, I think, sometimes it can be in our minds.

“It’s so easy. We plant it and then, boom, it’s there whenever we go to cut it. But are we doing everything we can to maximize that production to make sure we’re getting everything we can out of that acre?”

There’s nothing ground-breaking about this, Knapton said. Put enough seed down. Monitor the field to determine the plants and stems per square foot. And “be critical of your aged stands,” even if it looks like there’s still a lot of green.

“Leafhopper can come in and do a tonne of damage before we really recognize it,” she said, encouraging producers to take a good look after a thunderstorm and assess leafhopper activity.

Harvest timing is important. In a dry year, it may be possible to delay harvest to maximize yield and stay within quality requirements.

Apply fertility based on removal rates and pre-crop soil tests, Knapton advised. Alfalfa will probably need phosphorus and potassium. Sulfur and boron deficiencies are also possible. Manure is great but it doesn’t match what’s removed by alfalfa, and ideally should be tested for its nutrient value.

O’Reilly agreed that it’s important to invest in the crop.

“Cheaping out on those inputs might save us something in the short term but, in the long term, that cost per tonne really matters. If we have to drive over that acre every time – three or four times a year – to cut that hay, we might as well make sure we’re doing it for three tonnes per acre rather than one.”

For those unable to add more alfalfa to the rotation, the forage specialist offered other strategies.

A winter cereal followed by a fast-growing summer crop like sorghum or sudangrass could potentially fill a feed bunk. Drawbacks include a small risk of allelopathy with these crops and significantly less energy than corn silage.

Beets, by contrast, are more comparable to corn silage. O’Reilly said they’re not a commonly grown crop in Ontario and do require specialized harvest equipment, but they can be effective in breaking down rootworm resistance and could fit nicely into a dairy or beef ration.

If switching crops isn’t possible, another option is to trade acreage with a cash-cropping neighbour.

O’Reilly noted former Grain Farmers of Ontario chair Mark Brock recently explored this idea through his studies with the Nuffield Scholarship program and encouraged Forage Focus participants to find out more about his findings.

Given that “there really are no other transgenic products coming that can save us from this pest,” Baute stressed that vigilance against Bt resistance remains important.

For those growing Bt corn in counties identified as high-risk or neighbouring counties, “I strongly recommend that you (join the trap monitoring network) next year. The traps are a benefit. Yes, you have to change them once a week for about four weeks – six would be ideal – but it gets you to see what the corn rootworm activity looks like in your field. It gets you more accustomed to ‘is this normal or is this high for this field.’

“While you’re looking, look for those signs of resistance. And what you’re looking for is beetle activity.” If there aren’t already silks or ears on the corn, look for leaf-eating. Look for goose-necking or stunted plants.

“Don’t always assume that it’s because of drought. Stunted plants are often a sign that something is happening at the root level. Dig them up and take a look and see if there’s any root clipping.

“And, of course, if you see lodging after a windstorm, it’s time – in any of these scenarios – for both your seed provider and myself to be notified so we can go in and, if it’s a Bt corn rootworm variety, take and test those beetles and see if you have developed resistance.”