Unneeded/Excessive Pesticide Use? Back »

Written by Bob Fanning, former SDSU Extension Plant Pathology Field Specialist.


Weed samples often make their way into the Regional Extension Centers for identification and control recommendations. Recently, a local producer brought a “weed” into the Regional Extension Center that was present in his pasture, and more obvious in his neighbors’. The producer was primarily interested in identifying the plant in order to determine which herbicide would be most effective in controlling it. The “weed” turned out to be a native wildflower known as penstemon or big beardtoungue.

This scenario is very typical throughout our rangelands, where we’ve trained ourselves to assume that everything not grass is a weed that should be controlled through chemical application. In the case of this penstemon, although it has little value to cattle, it is a natural part of the grassland ecosystem and has little effect on the overall production of the pasture. In fact, some penstemon species decrease with poor grazing management, and so their presence may be one indicator in assessing a grazing plan.

In this particular case, SDSU plant taxonomist, Gary Larson, offered words of wisdom and caution in relation to the value of these native wildflowers to our agricultural landscape. In his words, “Native forbs like this one are being extinguished through wholesale herbicide application to grasslands. Native bees important for pollinating native and cultivated fruits, etc., are being decimated as natural nectar sources needed through the growing season are lost. I'm seeing far fewer bees and less fruit set in pollinator-dependent fruits in corn/soybean/pasture country here in the eastern part of the state. We have a pollinator crisis that is intensifying. I'm not a tree hugging true environmentalist, just an observant realist. How do we debunk the notion that anything not grass is a weed?”

Our state range specialists offer similar cautions in relation to overall pasture management and health. Native broadleaf flowering plants have a place in the pasture system. Even if cattle do not directly consume the plants, they offer system functions that grasses do not. For instance, the native legumes such as leadplant and scurfpea fix nitrogen in the pasture. Producers should carefully evaluate the cost-benefit ratio of expending dollars on broad scale herbicide applications that may provide little return in the way of beef production. Further, most herbicides are non-selective, and non-target impacts to other plants is a real concern. The key is identifying and researching the plant and then determining best management practices. Sometimes that can be simply enjoying the flower’s presence in the pasture. 

This “theme” carries over to other areas. Entomologists promote that there are other ways to control insects than just insecticides. For example, wheat producers are likely hearing of aphids in their fields. There are ladybird beetles and other predatory insects there too, and if at high enough populations, can keep aphid numbers below the thresholds. Applying insecticides when insect pest thresholds haven’t been reached may not be economical, and the predators will also be “controlled”. Insecticides are also not the only solution for alfalfa weevils. Granted, the weather doesn’t always cooperate to allow early cutting, and even so, the weevils sometimes survive to feed on the regrowth and justify an insecticide application. Alfalfa weevils do have natural enemies and insecticides should be used with care to minimize the effect on these beneficials. There are situations where including an insecticide along with another pesticide application is being practiced, even though there are only a few undesirable insects present. Often, producers may find themselves coming back for another application because the beneficial insects being taken out with the first application.

A similar phenomenon occurs with fungicides. In addition to killing harmful fungi, fungicides also kill good fungi. These good fungi help to control aphids, grasshoppers, and other insects as well as plant pathogens such as bacteria. Extensive fungicide use has also shown to be detrimental to microbial activity in the soil.

In rangelands, the best example can be found in the success of biological control methods for leafy spurge. Producers across the Midwest have realized the benefits of allowing a tiny beetle to manage their leafy spurge problem and in many cases have opted out of the annual expenses associated with broad scale chemical control.

Integrated Pest Management, or IPM, practices have been encouraged for several years. IPM principles stress crop scouting, following economic thresholds and considering alternative control methods. It’s important to recognize that healthy grasslands contain more plants than just grass, not all insects are pests, and not all fungi are bad.

In this case, the producer did the right thing. Before spraying an unknown plant he sought advice, assessed the cost/benefits, and realized his assumptions needed to be assessed before implementing a management plan.

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