Cover crops and the scrutiny on crop nutrients

Far from old-fashioned, cover crops are one tool to keep nutrients in the field.

Focus on crop nutrient use efficiency by the government and the agricultural industry is on the rise. In fact, nutrient use efficiency has earned its own acronym—NUE. With a new focus on NUE from the feds, it is important for producers to understand how to make sure applied nutrients end up where they’re supposed to—in the plant, not lost to watersheds. Nutrient programs like the 4 Rs (right rate, right timing, right source and right place) are backed by The Fertilizer Institute, The International Plant Nutrition Institute, The International Fertilizer Industry Association and the Canadian Fertilizer Institute. The focus on efficiency is here to stay.

Cover crops are an old method for increasing crop NUE, and they have been a hot topic for the past few years. USDA-NRCS and local conservation departments are encouraging cover crops. Cover crop meetings across the country feature farmer cover-crop success stories. Typically, the benefits of using cover crops are described in terms of soil health and quality. Improving soil health and soil quality should lead to a better overall crop-growth environment that increases efficient use of applied and available plant nutrients. These are good talking points, but understanding how cover crop systems can be used in a crop rotation to get these soil-improving benefits can be a challenge.

The first thing to do when planning a cover crop system is to set goals. Is reducing soil erosion your main goal? Or is weed control or nitrogen fixation by legumes? What about reducing compaction? A couple of years ago, the Conservation Technology Information Center surveyed 700 growers to see what goals farmers assigned to their cover crops. Farmers where also asked about the benefits they had experienced from cover crops as well as their biggest cover crop challenges.

Most respondents wanted to reduce soil compaction and erosion. Following that, in order, goals were: nitrogen scavenging, weed control, yield increases and nitrogen fixation.
According to farmers who answered the survey, the biggest challenges to using cover crops were establishment, time, species selection and seed costs. These surveys help whittle down what most people desire from using cover crops and the perceived challenges involved. Visit www.sare.org for more information on this survey and a full analysis.
It is important to use local extension and agronomists to help you implement a cover crop system. Also, talking to other producers with experience in cover crops can be beneficial. Attending cover crop conferences and trainings will help, but may also be an overwhelming informational experience. Be sure to focus on what your personal goals are. Talk to as many experts as you can.

Cover crop considerations:
1. Do not plant corn after a cereal rye cover crop. This is an issue that has been debated. Some growers have attested to growing corn after cereal rye. They say that as long as the cereal rye is killed at least two weeks prior to corn planting, they have seen no reduction in yield. However, others have had experience with cereal rye causing an allelopathic effect on corn that caused yield reduction.

My recommendation is to avoid cereal rye before corn. Use a different cover crop to avoid the risk of a yield hit. Additionally, if cereal rye is used before going into a non-corn crop, make sure the cereal rye is killed off at least two weeks prior to planting the crop.

2. Do not use annual ryegrass/marshall ryegrass/Italian ryegrass as a cover crop in a cropping system. Annual ryegrass is very good at reducing soil compaction and erosion; however, it is becoming resistant to glyphosate and can be tough to kill. While this may be an excellent choice for use in pastures, it is one to avoid in corn/soybean systems.

3. Inoculate legumes with the correct inoculum. Various legumes can be used as cover crops (crimson clover, berseem clover, alsike clover, red clover, hairy vetch, sunn hemp, etc.). Each legume needs a specific strain of rhizobia bacteria. Make sure to inoculate with the correct strain for the legume being used.

4. Make sure your soil test NPK levels are adequate to support growth of the cover crop and your regular crop. If soil test levels are not where they should be, the cover crop will not grow nor do the job you want it to. In turn, you may reduce nutrient availability for your cash crop by allowing the cover crop to take up nutrients that would have otherwise been used by your regular crop. In some cases, nutrient scavenging may be the goal, but you should still make sure you know your soil fertility to pick the best cover crop. 

5. In general, you will need 40 to 60 days of growth before a killing frost to maximize cover crop biomass production. Planting a cover crop too late may result in a poor or non established cover crop. Planting too early in some years may result in too much growth and nutrient/water uptake. Know proper planting timing for your cover crop mix and realize that not every year is going to be the same. A safe bet with cover crops is to plant species that will winterkill. If the winterkill mix is well-established and good growth is achieved for a couple of months, you will most likely add organic matter to the soil and provide cover to help reduce erosion. Additionally, you may see benefits from weed suppression and possibly improved soil health in general.

The last thing to think about when implementing cover crops is to remember that it is probably not going to be a one-year fix. Increasing soil organic matter and improving soil structure take many years. Some benefits like fixing nitrogen and scavenging nutrients may be achieved in the short term, but for overall soil health and quality improvement, long-term strategies for using cover crops and cash crop rotation will be required. The best thing to do is keep learning and keep experimenting with different mixes to see what works best on your farm.