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Using Polycultures to Improve Productivity and Pest Control

By John Tooker
 
Pest management is a challenge faced by every grower. To combat pest populations, growers rely on a variety of tactics. Organic growers tend to rely on cultural and biological tactics whereas conventional growers have a wider range of solutions to manage pests, but many tend to rely too heavily on chemical controls, many of which can negatively influence human or environmental health while being costly. As of a few years ago, conservative Penn State estimates of popular preventative pest management costs are approximately $12 per acre of herbicides, $15 for fungicides, and $10 for insecticides. Because of these costs, there is an opportunity to improve profitability of farming by developing cropping systems that rely less on inputs and more on ecological interactions.
 
An experiment comparing cultivar mixtures and moncultures of wheat. The different shades of green reveal that the plots are planted with different varieties or mixtures (Image by John Tooker, Penn State).
 
One way my research group and our collaborators are trying to take advantage of ecological interactions is by growing polycultures, or more than one crop species in a field at once. This is an old idea. In fact, as you may recall, native Americans relied on a production system that combined corn, beans, and squash—the ‘Three Sisters.’ The traditional view has been that this combination of three crop species yielded better than their respective monocultures because the beans fix nitrogen, the corn provided a pole for the beans to climb, while the broad leaves of squash helped suppress weeds. Recently, research has shown that this combination of crops had more than just this simple trio of benefits. The three plants species have different root structures, allowing the crops to partition soil resources and capture more water and nutrients in an area of soil. From an insect pest management perspective, the diversity of crop species likely promoted higher populations of beneficial insects, including pollinators, which improved crop productivity by pollinating the beans and squash, and natural enemies, which helped control pests.
 
Some of our current research at Penn State seeks to gain similar multifunctional benefits by mixing compatible crop species or crop cultivars. In one effort, we are focusing on silage production. Many dairy farmers that produce silage grow corn and often soybeans, but most keep them in separate fields. In recent years we have encountered a growing number of farmers, particularly in central PA, who are experimenting with growing corn and soy together in the same field with the idea that the crops will benefit each other—the soy providing nitrogen and some protein with the corn providing most of the biomass. Building on this idea, we conducted a series of experiments mixing corn & soy, corn & sorghum, and corn, sorghum, soy, and sunflowers.
 
A polyculture of corn, soybean, sorghum, and sunflower that we are studying for its benefits for silage production (Image by John Tooker, Penn State).
 
We found that the mixtures can produce biomass similar to what monocultures of corn can offer, but some of the mixtures provide benefits. For example, the mixtures tend to have higher nutrient content or those with sorghum tend to have higher crude fiber, both of which are likely to be helpful for animal nutrition. From an insect pest management perspective, we find that mixtures do not suffer much from pests because they seem to foster good natural-enemy communities that can help to protect the crops from insect damage. While there appears to be benefits to growing mixtures, there are also challenges. The different sized seeds can be challenging to plant in a single pass, so two passes might be more effective. We also had some challenges with harvesting, mostly in plots with sorghum that lodged. Clearly, this system is a work in progress, but we are continuing to refine it and we can provide more detail in the future. If you have experience growing mixtures and would like to share, we would love to hear from you—please get in touch with me.
 
The other polyculture project that we are pursuing is similar, but the mixtures occur with cultivars within a species, so there is no challenge in establishing or harvesting the crop. We have worked most with cultivar mixtures of wheat, mixing 3 to 6 cultivars of the same maturity. Our work and similar research elsewhere is revealing that cultivar mixtures of small grains tends to yield 5-10% more than monocultures because the cultivars vary in their productivity based on annual conditions, and the performance of one cultivar can compensate for the performance of others that are not growing as well. From an insect pest management perspective, we have found in some experiments that cultivar mixtures have fewer insect pests, but few of our fields have developed challenging pest populations, so we have not had a good test of their ability to protect against insect pests.
 
We have been finding that the mixtures can produce biomass similar to what monocultures of corn can offer, but some of the mixtures provide benefits. For example, the mixtures tend to have higher nutrient content or those with sorghum tend to have higher crude fiber, both of which are likely to be helpful for animal nutrition. From an insect pest management perspective, we are finding that mixtures do not suffer much from pests in part because they seem to foster good natural-enemy communities that can help protect the crops from insect damage. While there appear to be benefits to growing mixtures, there are also challenges. The different sized seeds can be challenging to plant in a single pass, so two passes might be most efficient. We have also had some challenges with harvesting, mostly in plots with sorghum that has lodged. Clearly, this system is a work in progress, but we are continuing to work on it and can provide more detail in the near future. If you have experience growing mixtures and have some experiences you would like to share, we would love to hear from you—please get in touch with me.
 
The other polyculture project that we are pursuing is similar, but the mixtures occur with cultivars within a species, so there is no challenge to establishing or harvesting the crop. We have worked most with cultivar mixtures wheat, mixing together 3 to 6 cultivars of the same maturity. Our work and similar research elsewhere is revealing that cultivar mixtures of small grains tend to yield 5-10% more than monocultures because the cultivar vary in their productivity based on annual conditions and the performance of one cultivar can compensate for the performance of others that are not growing as well. From an insect pest management perspective, we have found in some experiments that cultivar mixtures can have fewer insect pests, but few of our fields have developed challenging pest populations, so we have not had a good test of their ability to protect again insect pests.
 
For pathogens, we found that cultivar mixtures can protect against leaf diseases because different varieties have different susceptibilities to rusts and blights, but the mixtures have not protected as well against head diseases, perhaps because there is not as much variability in susceptibility to head scab. Unfortunately, the mixtures pose a challenge when it comes to protecting against head diseases because the individual varieties within a mixture tend to have different flowering times (even if they are in the same maturity class) and, as a result, a wider flowering window, which in a wet year can be difficult to protect with one application of fungicide. A solution to this challenge is to find wheat varieties that flower at the same time, but screening wheat varieties is time consuming and the market for wheat varieties is pretty dynamic, so varieties available one year may not be available in the next, meaning that wheat screening would have to be somewhat continuous.
 
We’ve encountered a very limited number of companies offering premixed bags of cultivar mixtures, but if you do find them, those varieties are likely pretty compatible. Because premixes are rare, folks interested in cultivar mixtures may have to buy multiple varieties, mix them in their drill and experiment on their own. Our work is demonstrating some benefits but managing head diseases remains a challenge. If you have any interest in or experience with cultivar mixtures, I welcome you to get in touch.
Source : psu.edu