Control weeds without herbicides? Biological alternative is part of a ‘new frontier’
Published: December 7, 2016 on Kansas Agland Website
I hate this time of year. Every November and December I find myself sitting at the kitchen counter figuring up all the untold thousands of dollars I’m going to spend on herbicides.
Isn’t there another way, an easy way out? Why do we have to spend so much money on herbicides?
The recent glory days in farming are gone. We were making so much money not all that long ago that we actually looked forward to expenses. Just think of all the money we saved on income taxes!
Yes, those days are dead and gone. We are now back to normal, or worse. We are now seeing expenses for what they have always been – a living nightmare. So is there another way to control weeds, without expensive herbicides or, at least, using less?
In fact, there is. There’s an entire frontier out there just waiting for us. The early American pioneers saw the same thing and termed it “everything west of the Mississippi.” This is a pretty big deal.
We farmers have exactly the same kind of potential today, only it’s not with geography but biology – the biological control of weeds.
Years ago we started growing triticale on our farm here in Lane County. Right off the bat we noticed there were no weeds in the fields after harvest. Same thing the next year and the next year. Then it finally dawned on me: There’s something going on out there that is really different. It’s not like our wheat fields, which turn green after harvest and we have to go spray them. (And that takes money.)
But even worse today, we’ve got a little problem with herbicide resistance. Many weed species including kochia have developed resistance to multiple classes of herbicides. Some years ago, KSU weed scientist Phil Stahlman put in some post-harvest weed plots on our farm and concluded that the only way we could kill the weeds in our wheat stubble was with our undercutter and mechanical tillage.
But with triticale and rye, we didn’t have a weed problem after harvest, or not nearly to the degree we did with wheat. Initially I thought it was because triticale is so much more competitive in plant and root growth rates. In addition, it’s taller, leafier and produces two to four times more stubble. A later explanation was the concept of allelopathy – the plant itself producing chemicals that are secreted into the ground, which inhibits growth and development of weeds. Or it could be a combination of physical suppression and competition combined with allelopathy. The bottom line is that I can save one or two expensive herbicide applications.
While this is common with triticale and rye, you occasionally see it in wheat. One of the most spectacular cases I’ve ever seen occurred a number of years ago when I planted three different wheat varieties on the same field. Everything was the same except for variety, planting date, seeding rate, fertility rate and date of harvest. Then at some point after harvest, as I was driving around looking at fields to see which ones I needed to spray for control of post-harvest weeds, I came to this field. The first third was as green as Ireland with weeds. But the next third was nothing but beautiful, clean stubble – no weeds whatsoever. I was absolutely shocked. Then I kept driving until I hit the last third of the field and it was again as green as Ireland – right to the line.
Later I was talking with Gail Wicks at the University of Nebraska’s North Platte Experiment Station, and he mentioned that they used to rate wheat varieties on this characteristic just like we do today, with a whole range of things like how resistant varieties are to wheat streak mosaic or leaf and stripe rust or how drought-hardy the varieties are.
He also told me about another spectacular case where a farmer in western Nebraska was out spraying post-harvest weeds. As he was going around and around the field, he came to an area where the weeds just stopped. There were no more weeds whatsoever. The rest of the field was nothing but beautiful clean stubble. It was like he had hit a force field. As he sat there stunned, trying to figure out what had happened, it finally dawned on him: “Oh, that’s where I ran out of wheat and had to switch varieties.”
This past year, we had Colorado State variety Byrd planted on a field beside CSU’s Antero. Same thing. We had hardly any weeds where we had the Byrd and clearly had enough on the Antero to warrant a herbicide application. This year we had Byrd planted beside KSU’s Joe. Lots of kochia in the Joe. None in the Byrd. We also had TAM 112 planted beside TAM 204. Lots of kochia in the 204. None in the 112.
So what’s it worth to not have to spray a field?
From a farmer’s point of view, it would be really nice to know which wheat varieties were very competitive or had the ability to control weeds through allelopathy. Further, I think it would be even better if wheat breeders were selecting varieties with these abilities.
While discussions are in their early stages, researchers around the world have also become aware of the new frontier. Australian researchers have found that certain wheat varieties like Tasman are strongly allelopathic and can seriously inhibit ryegrass root growth. They also say if we had more such varieties, we’d be able to cut back on herbicides, the extensive use of which has resulted in rapid development of herbicide-resistant weeds. Also, cutting back on herbicides would have a very desirable environmental impact.
University of Nebraska researchers say that probably the main reason we haven’t crossed the Mississippi River yet to develop biological controls is that we didn’t need another control strategy since herbicides were working well and they provided economical weed control. Ironically, one of the reasons we now need to cross the Mississippi is again because of herbicides – not because they’re working so well but because they’re not working. Or because of their expense.
The Nebraska researchers, along with Australians, say that because of herbicides, we started selecting for wheat varieties that were less and less competitive. That means we have become more and more vulnerable to weeds and the likelihood of needing to spray has increased. In short, modern wheat varieties are much less competitive with weeds than older types.
Obviously, some of the most competitive wheat varieties have been taller, like Turkey Red, which did a good job of shading the ground. Canadian and British agronomists agree that the most competitive plant characteristic is plant height, which helps limit light penetration to the soil surface. But over time, taller varieties have been selected because they were lower-yielding and lodged. Still, some shorter wheats have also been competitive because they had a high tillering ability.
Another way of restoring competitive ability is to select wheat varieties for early vigor and leaf size. Swedish agronomists say that besides selecting for fast shoot growth rates, it’s very important to have rapid initial root growth rates in order to out-compete weeds for nutrients, water and space. This early vigor may replace the need for taller varieties. The Swedes say the best breeding program is to select for a number of competitive traits as well as allelopathy.
At the vegetative stage, crop-seedling allelopathy could be used to suppress weeds. At the post-harvest stage, crop-residue allelopathy could be used for weed suppression. The Australians say, however, that biological methods are the least exploited area in weed management, and that is in large part because of the limited knowledge about the genetic control of crop allelopathy.
The Australians also point out that while crops have been made resistant to insects, pathogens and herbicides through genetic engineering, biotechnology has not produced crops that control weeds with allelochemicals, again because of limited knowledge in the area. One of the most exciting of these areas involves inserting genes to produce allelochemicals that are not found in the crop.
For instance, for years people noticed that there were no weeds living under eucalyptus trees in California, and it was just assumed this was because tree roots were drying out the soil and weeds couldn’t germinate. Only later was it found that the tree was secreting a toxic material that kept weeds from germinating. How exciting would it be if that gene were inserted into wheat or corn. Those crops could now control their own weed problems and herbicides would no longer be needed.
Back here in Kansas, K-State wheat breeder Allan Fritz has been working with the Australian variety Tasman, which has strong allelopathy. Using it in crossing programs, he has developed decent allelopathy, but crosses have poor agronomics. However, he is now crossing them with some elite materials.
K-State agronomist John Holman, at the Garden City Experiment Station, has been doing research to see if certain cover crops grown during fallow could reduce kochia density and biomass. While he looked at a number of cover crops, he found that winter triticale alone or in mixtures reduced kochia density by 78 to 94 percent and reduced kochia biomass by 98 percent compared with kochia in chemical fallow. He concluded that fall-sown cover crops that produced more biomass were most effective at suppressing kochia. However, his research does not point out whether the kochia suppression is due to allelopathy or plant competition/suppression. Nonetheless, his work supports the role that biological control can play and that it can be used in an integrated weed control program.
Vance and Louise Ehmke grow certified seed wheat in Lane County.
Read the article on Kansas Agland Website