Wheat harvest is ramping up and as of this past Thursday, storms have avoided most of the immediate area. With temperatures hovering around the 100 degree mark and good southerly winds, the green in fields is rapidly disappearing. If the weather holds, harvest could be pretty well wrapped up by the end of this week if combines are available. Let’s finish up the discussion of cover crops. Part I described what they are while Part II discussed the possible benefits of cover crops. Now, what are the potential problems and pitfalls? These potential problems aren’t intended to dissuade producers from using cover crops but to help them enter the process with their eyes open.
• First certain cover crops have a potentially negative effect of the next crop. Corn following a freshly killed rye crop for example has been demonstrated to significantly inhibit germination and growth.
• The cost of certain cover crop seed, especially those fixing nitrogen like Austrian winter pea and cowpea, are extremely expensive. They can fix and make available substantial amounts of nitrogen for “free” but that leads to the next point.
• For the plants to fix nitrogen or help cycle nutrients, they need to be able to grow as long as possible prior to the next crop. This is especially true of winter cover crops that provide most of their benefits growing from late winter into spring. If the winter/spring period is dry this can deplete the soil of necessary moisture for crop establishment. If the period is extremely wet you often find phytotoxic effects on the crop, especially under no-tillage. A long-term study in at the K-State Agronomy Field south of Pleasant View in Reno County showed tremendous benefits to winter cover crops with adequate moisture and quite negative effects under dry conditions. The negative effects were lessened the earlier the cover crop was terminated under dry conditions.
• Leguminous cover crops require a certain soil chemical and physical range of soil conditions to fix nitrogen. If the soil is too acid or poorly drained, the cost won’t justify the benefit.
• The cover crop must be adequately terminated or it can end up acting as a weed. Some cover crops are much easier to terminate than others. The problem can be more difficult when tillage is eliminated.
• Cooler soil temperatures with a heavy surface cover can inhibit crop growth and development while exacerbating potential phytotoxic effects of the cover crop.
• While they can aid in weed, disease, and insect control, they can also harbor these pests. This is especially true when the cover crop is similar to the succeeding crop.
• Finally, cover crops to be require the input of time, labor, and money to be effective and require an increased level of management to succeed.
Again, this isn’t to say not to use them as there are potentially great benefits to cover crops but to be aware of the pros and cons. Producers, especially those on sandy soils can see huge benefits (weather permitting) to their operations through increased organic matter, increased soil water holding capacity and surface cover to prevent erosion. Those with more productive soils can also benefit greatly.