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Why Mix Varieties of Wheat in a Field?

You’ve probably heard of the value of diversification in your financial portfolio. Having more than one type of financial investment reduces risk of losing all your money. My research involves lowering risks for farmers who grow wheat in a sustainable practice of crop mixtures.

A typical wheat field is one variety, a monoculture production. This tends to be common because it is the most efficient method. However, this type of specialization often requires additional inputs, fertilizers, pesticides, and other chemicals, to maintain the high yields that is expected of modern agriculture. This, in turn, can cause a cycle of reliance on these chemical inputs.

Various head diseases are shown among the different varieties of wheat in the study. Credit: Julie Baniszewski

Consider a wheat field with a severe pest infestation, such as aphids or cereal leaf beetle. Once these pests arrive in susceptible wheat field, chemicals may be required to control the pest before it damages the crop. There is risk to hurting beneficial insects like ladybird beetles that might be able to help control the pests naturally. Even ground beetles could be injured. The farmer must control the aphids, as they can kill a lot of the wheat plants – decreasing the food supply and harming the farmer’s profits. But using pesticides also starts to create a loop of continually needing more pesticide application as “good insects” are controlled along with the pests.

My work involves an alternative solution that reduces risks for farmers and keeps growing a healthy food supply. Our research group is working to create crop mixture systems. Crop mixtures can be intra-specific, containing the same species, or inter-specific, containing different species.

Aphids, a common pest in wheat, were lightly glued onto cardstock and monitored for 24 hours to quantify predation by native insects. The card on the right shows that four tiny aphids remain; the card the left shows the aphids were consumed by a ladybird beetle. Credit: Julie Baniszewski

In many cases, interspecific mixtures, also called polycultures, have a different set of challenges in modern agriculture. It’s difficult to use specialized machinery for multiple crops.

Intraspecific mixtures can be much easier because farmers can manage them as they would a typical monoculture. In the context of a wheat crop, a mixture would consist of two to four varieties of wheat. Each variety would have a different set of traits. One may be high yielding, another resistant to insect pests and yet another resistant to specific diseases. By combining these different sets of traits, the crop is more consistent year to year. The field is more diverse genetically and better able to reduce or withstand pest abundances and diseases – just like diversifying a stock portfolio.

A wheat field in one year of the researchers’ experiments. Here, different varieties are noticeable because of physical differences – namely color or hue and if the head has awns on it or not. Credit: Julie Baniszewski

With these wheat mixtures in mind, we created a three-year field experiment to determine if mixtures could:

  • suppress diseases,
  • increase predation of insect pests and
  • increase yield and economic value.

Each year, we planted a wheat mixture comprised of four varieties. We compared the mixture results to stands with only the single varieties planted separately. We sprayed only half of each plot with chemical pesticides. This was to measure if the mixtures provided an environmental benefit similar to chemical inputs. We quantified insect predation using aphids, diseases, yield, and economic return over variable costs for each of our plots.

Shown here is the difference of a fungicide application within one variety. The left side of the photo shows the control side of the plot that had no chemical fungicide applied. The slightly darker coloring of the wheat is largely due to disease. The right side of the photo shows the fungicide-sprayed side of the plot. The lighter coloring is healthier wheat, which often also has a few additional days to mature, thereby increasing yield. Credit: Julie Baniszewski

The yield results were about the same as similar studies, so no surprises there. We found that fields sprayed with fungicides did have higher yields – but only enough to cover the expenses related to fungicide application.

Some good results: the mixed variety plots had inhibited spread of foliar disease in the field comparable to the fungicide application. Although the wheat mixture was not the highest yielding, nor did it provide the greatest return over variable costs, it did inhibit disease to an extent consistent with a fungicide application – about 20-25%.

The value of crop mixtures is both a portfolio diversifier and an insurance policy. Although a higher-yielding, single variety wheat field may do well consistently for several years, having the same genetics in the same field has vulnerabilities. In one of those years, there could be a severe pest or disease outbreak, or a severe drought or climate event that that one variety is vulnerable to.

By using crop mixtures, we increase the genetic diversity and safeguard against a complete crop failure that could occur with that single variety. The genetic diversity from the other combining varieties in mixtures may have disease or pest resistance and may thrive under otherwise stressful climatic conditions. That is, diverse crop mixtures, like financial portfolios, are more stable than any single variety.

By Julie Baniszewski, Pennsylvania State University, American Society of Agronomy and Crop Science Society of America. Our members are researchers and trained, certified, professionals in the areas of growing our world’s food supply while protecting our environment. We work at universities, government research facilities, and private businesses across the United States and the world.

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