Cover crops can help fight pests
Low crop prices have farmers discussing the potential cost savings of buying conventional non-GMO corn seed. Some estimates show by switching from GMO to non-GMO seed, farmers might save close to $80 per acre on corn. Over several years, yield comparisons show the cheaper seed maintains similar yields as the GMO seed. However, making the switch means rethinking how you are going to control insect pests and weed pressures.
Cover crops can be a key part of an integrated pest management plan to help control pests. New research studies are supporting what longtime cover crop farmers have been observing on their farms. That is, the cost of producing corn and soybeans can be brought down by using cover crops long term. Still, the costs of establishing and managing cover crops are frequently reported as the main barrier to using them on a lot of acres.
With these two things in mind, farmers are asking the question: Could switching to conventional corn hybrids and soybean varieties pay for the cover crops and provide the same insect pest and weed control?
Since cover crops are still fairly new to Iowa and the Corn Belt, few studies about their benefits to crop protection have been conducted here. As described in “Managing Cover Crops Profitably,” cover crop research results from the warmer eastern and southern areas of the U.S. show that cover crops can reduce weed, insect and disease pressure for the succeeding cash crop. Cover crops also can help farmers cut back on nitrogen while maintaining cash crop yields.
But in the eastern and southern regions of the U.S., cover crops are able to grow for much longer periods of time than in the Corn Belt, where growing only corn and soybeans takes up nearly all of the growing season. Since farmers are adding cover crops to their farms because of the many water quality and soil health benefits, could cover crops also help reduce farmers’ costs of growing corn and soybeans? What does research tell us about the potential crop protection benefits of cover crops?
Consider corn rootworm, specifically western corn rootworm, which is a pest of economic significance to farmers. This pest can severely damage corn roots. Currently, farmers have several methods to choose from for controlling pests like western corn rootworm. These include planting insect-resistant corn hybrids, applying granular soil insecticides, using insecticide seed treatment and applying insecticide treatments to kill mature insects. Crop rotation, for example, rotating corn with soybeans, is another method of controlling western corn rootworm.
Corn rootworm control
Corn rootworms are a pest that has quickly become resistant to chemical management, and in Iowa in 2011, the first cases of rootworms developing resistance to genetically modified corn were confirmed. Recent research in South Dakota suggests that even in colder climates, cover crops can impact crop pests of corn. In that study, Jonathan Lundgren and colleagues in 2010 found that western corn rootworm populations and their natural enemies were affected by a winter cover crop.
At Brookings, S.D., a cover crop of slender wheatgrass was broadcast-seeded at 30 pounds per acre in September of 2006 and 2007. Half of the experiment was planted to cover crops, and half of the plots were not. The next spring glyphosate was used to kill the cover crop near the time of corn planting. In both years the same glyphosate-tolerant corn hybrid was planted across the entire study area.
To make sure western corn rootworm pests were present, researchers artificially infested all of the plots with rootworm eggs. Then they watched. Western corn rootworm instars, or “grubs,” were counted and their size was measured. Also, natural enemies including beetles, spiders, ants and ladybugs were counted.
Lundgren and his team found that the number of rootworms at the largest rootworm instar growth stage (third instar), were much less following a cover crop than when following bare soil. Also, corn root damage was much lower following a cover crop than when following bare soil.
Finally, the natural enemy populations collected from the soil surface were in greater abundance during June and July following the cover crop. Lundgren found a strong negative correlation between the number of natural enemies (such as grubs) and the number of third instar corn rootworms. The number of rootworms sized small (first instar) and medium (second instar) were similar between the two treatments, but by the time the grubs grew bigger, these rootworms were eaten.
Corn rootworm management is not cheap, and the costs of management can increase if the pests become widespread.These crop pests have displayed a high rate of becoming resistant to either chemical management or genetically engineered corn.
In 2011, Aaron Gassmann and other researchers from Iowa State University documented the first western corn rootworms resistant to corn traits that are specifically targeted to control these pests. They found that the longer these genetically modified corn hybrids are planted in consecutive years in a field, the greater the potential for this pest to develop resistance. This suggests that cover crops should be part of a corn rootworm resistance management plan, but more importantly, cover crops can save farmers costs on protecting their corn crop from this pest.
Other important corn pests
What about other economically important corn pests? Can they be controlled by crop rotation? Another study, this one conducted in 1984 in Ontario, asked a similar question to the one posed in the study by Lundgren in South Dakota. Researchers in the Canadian study measured another pest that has formerly caused great economic devastation — European corn borer, or ECB (Ostrinia nubilalis).
In this study, 9 pounds of red clover per acre were interseeded into standing corn at 10 and 25 days following corn planting and was compared to a control where no clover was interseeded with corn. Corn in this study was not genetically engineered. At corn harvest, 40 cornstalks from each plot were cut at the ground level and rated for ECB damage. Damage from ECB was less in the treatments that were interseeded with red clover 10 days after corn planting. In the red clover treatments, ECB damage was 5.6% in 1984 and 2.6% in 1985, compared to 11.5% and 7% damaged in stalks where corn was the sole crop.
European corn borer has not been an economically devastating pest for corn farmers for more than a decade now, due to the common use of Bt corn. However, corn rootworm remains a significant pest. Rootworm damage can reduce yield significantly and be costly to manage.
Overall, costs of growing a cover crop could be offset by combining it with a conventional, non-GMO hybrid. These two studies suggest that cover crops can provide alternative pest management options to genetically engineered traits.
Carlson is Midwest cover crop research coordinator for Practical Farmers of Iowa.
This article published in the February, 2015 edition of WALLACES FARMER.
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