Features Agronomy Genetics/Traits
Reliance on stacked-trait corn hybrids

Ontario corn growers have been relying more and more on stacked-trait hybrids, hybrids that have more than one genetically modified (GM) trait, since the first ones became available about a decade ago. And the popularity of stacked-trait hybrids will likely continue as more traits and new combinations of traits are added to the stacks.

According to Statistics Canada, GM corn in Ontario has increased from 27 percent of the total area seeded to corn in 2000 to an estimated 69 percent in 2010. Statistics Canada does not track the amount of stacked trait-corn grown, but Mervyn Erb, an independent crop advisor in the Brucefield, Ontario, area, says, “Most everything now is stacked because almost every Bt hybrid has either a Roundup or Liberty tolerance trait.”

Mike Nailor of Monsanto estimates that 60 to 70 percent of the GM corn acres in Ontario are planted to stacked-trait hybrids. That could be in the right ballpark; for instance, in the Ontario Corn Committee’s 2009 Hybrid Corn Performance Trials, 76 percent of the hybrids were stacked-trait.

Pat Lynch, an independent agronomist, lists some key factors that have driven adoption of GM corn, including the stacked-trait hybrids. “Probably number one is that’s the direction corn companies are going, the way their breeding programs are. Number two, there is an economic response to the growers. Number three, the growers are asking for them; that may be because of promotion, because of economics, or because ‘all my neighbours are doing it, so I should too.’ ”

Dr. Steven King of Pioneer Hi-Bred notes, “As long as the stacked hybrids offer something of value to our customers, they will continue to purchase those options. Right now the adoption rates are really high; in fact, there’s not much more room to grow stacked hybrids because, to ensure stewardship of these technologies, a certain proportion of the land has to be devoted to refuge acres, which in Canada is typically 80 percent Bt and 20 percent refuge.”

Of course, the idea behind a refuge is to plant corn that does not have a Bt insecticidal toxin, with the goal of slowing down development of Bt-resistant insects by ensuring some of the target insect’s population is not exposed to the toxin. Although preventing resistance is very important, Lynch concedes planting a 20 percent refuge for every Bt corn field is “very, very impractical” for growers and is difficult to enforce, which leads to less than perfect compliance.

So corn companies are developing options to make it easier for growers to meet refuge requirements. For instance, Pioneer has developed the “refuge-in-a-bag” concept where the refuge seed is mixed with the Bt corn seed at the correct percentage to meet regulatory requirements.

“In May in the US, the EPA (Environmental Protection Agency) granted approval for the first generation “refuge-in-a-bag” hybrid. The Canadian approval is not through yet. It’s up to the Canadian government to decide whether that’s a concept they feel is worthy in Canada,” says King.

In the US, Pioneer’s refuge-in-a-bag has a reduced refuge requirement. For instance, its Optimum AcreMax 1 technology contains 90 percent of a herbicide-tolerant hybrid with Bt traits for corn borer and corn rootworm, and 10 percent of a hybrid that lacks the rootworm trait. The 20 percent refuge for corn borer has to be planted separately. Pioneer has submitted regulatory approval requests for Optimum AcreMax 2, a second-generation integrated refuge for above- and below-ground insect control using multiple modes of action.

King adds, “If the integrated refuge concept is approved by the Canadian regulators, it will offer a very simple, convenient refuge option for the grower. Right now they have to keep track of the different acres that they are planting to the different types of technologies,” and also the acres they are devoting to the 20 percent refuge. With a refuge-in-a-bag or integrated refuge concept, you build that all in. And the other thing we’re doing with that concept is really increasing productivity because of the reduction of the components unprotected against either the European corn borer or corn rootworm. By doing that you increase the yield per acre compared to what you would have had with a 20 percent block refuge, for example.”

Monsanto is addressing the refuge issue with multiple modes of action to control insect pests. “One of our newer products, called Genuity SmartStax, has multiple modes of action to protect against insects, both above and below ground,” says Nailor. “That means more consistent control versus just having one mode of action. It also means the trait is going to last longer. Because it’s more durable and has multiple modes of action, the government has seen fit to reduce the degree of refuge from 20 percent to five percent. So farmers can now protect 95 percent of their acres against these yield-robbing insects.”

Implications for corn production in Ontario
“The biggest impact of stacked-trait corn (and global warming, if there is such a thing) is higher yields. When you look across North America, corn yields have been going up all the time. It’s unprecedented. Even when you’ve got stress conditions, the corn just keeps on pumping,” says Erb.

He adds, “Everybody kind of grumbles about paying $230 a bag for the seed, but when your corn averages 190 bushels on all your acres, that’s a lot of happy people.”
Lynch agrees on the yield benefits. “When we look back at the corn yield in Ontario, at different times we reached yield plateaus. When we went to hybrid corn, we went to a new yield plateau. When we went to some of the new herbicides, we went to a new yield plateau. I strongly believe we are heading for another plateau. In 1991, I was with a grower who harvested 200 bushels to the acre on a small part of his farm. I said then ‘I’ll never see 200 bushels to the acre again.’ The last two years we’ve had many fields that yielded 200 bushels to the acre, and even parts of fields with 300 bushels to the acre. We are getting this with the genetically modified stacked hybrids.”

Erb and Lynch note that some stacked hybrids have other advantages such as improved standability. Lynch adds, “Another advantage is a healthier corn product. European corn borer is a significant vector in spreading of Fusarium, and Fusarium produces vomitoxins that can be very unhealthy both to people and livestock.”

Lynch sees one downside of the growing reliance on stacked-trait hybrids. “The biggest disadvantage will be acceptance by the end-market users, whether these are European people, Pacific Rim people. There is a very strong anti-GMO movement in the world.”

Nailor thinks anti-GM markets will continue to be an issue, although he sees some signs of changing attitudes in the European Union. “There have been some recent announcements that they are going to accept some of our more advanced traits, where before they hadn’t.”

Looking ahead
The corn companies are working on adding new traits to the stacks that meet the needs of growers and corn markets.

Lynch says one of the next traits to be added will be tolerance to 2,4-D. “Right now we have trouble with glyphosate-resistant weeds. Many of those weeds are susceptible to 2,4-D. One company is now breeding corn hybrids that are tolerant to or resistant to 2,4-D. Corn is fairly tolerant to 2,4-D when it’s small. But if we knew we had glyphosate-resistant weeds like giant ragweed and we could spray corn with 2,4-D when it’s a little bit bigger, that would be phenomenal.”

Erb sees merit in adding 2,4-D-tolerance to a stack, but he thinks some other additions may have drawbacks. “Every company that’s got a herbicide wants to get tolerance to it in the stacks. We’re going to start adding tolerance to grass herbicides, like Assure and Venture, to the stacks. They are going to advertise it as resistance management for herbicides; if you buy a Roundup- or Liberty-tolerant stacked corn that’s also stacked with Assure or Venture tolerance, then you can rotate your chemistry. But how do you kill the volunteer corn plants in your soybean field or edible bean field when that corn is resistant to Assure or Venture?”

Nailor says Monsanto is working on traits to drive yields higher: “We have traits coming along to protect against drought and traits that are going to offer higher yielding hybrids and varieties. This new Genuity SmartStax is going to become the platform onto which these new technologies are launched.”

Further down the road, Lynch envisions the possibility of designer corn hybrids. “Supposing you want a high-amylase hybrid (amylase is an enzyme that turns the corn’s starch into sugar for ethanol production). The corn companies can probably give that to you with stacked genes. But unless there’s a big enough market for that, you’re probably not going to get it.”

Erb notes another likely limitation on the number of trait options available. “The corn seed companies just can’t warehouse all these different stacks and all these different parts of stacks. You can’t have enough seed production and you can’t store it all. They’re going to have to thin out the lines.”

Nailor says, “I think there are always going to be choices available for farmers; the challenge is to find the right balance of trait offerings in a variety of different genetic backgrounds. It’s the combination of great genetics protected by advanced technology that helps farmers to yield more, and each farmer has different needs depending on their soil type, crop rotation and insect pressure. Focusing efforts on delivering a simplified line-up of advanced traits provides the opportunity for companies to deliver those traits in more diverse genetics to better suit the unique needs of each field.”

King explains another limitation on the size of the stacks. “Conceptually, adding more traits has appeal because you are able to offer many protection genes, or any genes of value, for that matter, in the same hybrid. Technically though, it has some difficulties because the more genes we introduce, the harder it is on a plant because a certain amount of the plant’s resources have to be devoted to making these new proteins (genes carry instructions for making proteins, like the insecticidal proteins that are the Bt toxins). So, what we in the seed industry have to ensure is that the options we are developing and testing still have their productivity at the highest levels.” n