Questions dairy farmers have about cover crops and manure
This last year I gave a number of presentations throughout Minnesota about integrating cover crops and a livestock system with manure. Following are some of the questions asked about incorporating cover crops into a livestock farm’s system: Which cover crop species should I plant? When do I plant cover crops? What is the most cost-effective/efficient method to plant cover crops? What effect do cover crops have on next crop’s yield? Do I need additional or less nitrogen for the next crop? What is the forage quality of cover crops? What is the herbicide/cover crops interaction? What is the financial return of planting cover crops? These are all good questions; research is ongoing in many of these areas. Like many other issues in agriculture, many of the answers to these questions are: “It depends”.
What is interesting about these questions is that they are not about the reasons to plant cover crops, which include wind and water erosion control, nitrogen sequestration (holding nitrogen for later release), increasing organic matter, weed control, and improving soil health. So does this mean everyone already understands the benefits of cover crops and they just need to figure out how to manage them? It does seem that most farmers “get it” that cover crops can minimize soil and wind erosion.
What species we plant depends on the time of year, issues we are attempting to correct, the next crop we will plant, and if we want to use it as forage or pasture. Many people do not consider the nitrogen sequestration value of some cover crops. Everyone knows how difficult nitrogen is to manage for corn production; nitrate leaching from spring rains is one of the challenges. It would be a big deal if a cover crop could lessen nitrate leaching issues.
If you are considering planting a cover crop after soybean or corn silage harvest your choices are limited. Warm season species may germinate but they will not do much in the cool fall weather. Turnips and radishes are cool season species, they will germinate and grow in late fall, but they will not have enough time to produce the large tubers you expect from them. For cover crops planted after corn silage or soybeans, you are mostly limited to cereal grains. Annual cereals, like oats, barley, and spring wheat, will germinate and grow until frost kills them. A benefit of annual cereals is that they will usually be terminated by the frost; the negative of annual cereals is that they usually will be terminated by the frost. What this says is that it depends on your needs of the cover crop. Annual cereals will have excellent erosion control in the fall and fair-to-little erosion control in the spring, but you usually do not have to terminate it with herbicides.
If nitrogen sequestration and spring erosion control is high on your list, then a winter cereal would be a good choice. Winter cereal rye is suggested if you have little experience with winter cereals or are planning on terminating the cover crop in the spring. Winter rye is cheaper and hardier than winter wheat or winter triticale. Since most dairy farmers inject liquid manure in the fall, we set up a trial where we injected liquid manure late fall (ideally after soil temperatures are below 50° F) into winter cereal rye that had been planted after corn silage or soybean harvest.
In our 2016 trial, we measured 24” soil nitrate levels when the rye was terminated in the spring, during the last two weeks of April. The strips under the winter cereal rye had lower levels of soil nitrate than the bare ground strips by an average of over 60 pounds per acre. What happened to the nitrogen? It showed up in the winter cereal rye above ground biomass. Nitrates in the soil are subject to leaching from spring rains. If this nitrogen can be held by the rye in the spring, preventing its leaching, and then is made available to the corn later in the summer, it would be a win. Can this be accomplished? In the 2016 trials, we averaged the same corn yield with and without the rye. Some yield was lost when the rye was over 10 inches tall at termination; therefore, there was a small, but statistically insignificant, yield gain when the rye was less than 10 inches at termination.
So, can we plant cereal winter rye after corn silage or soybean harvest and inject liquid manure into it later in the fall? Yes, as long as the injector system is not too aggressive, covering up the rye. Sequestering manure nitrogen could be one of the greatest benefits of cover crops on a dairy farm.