In past columns, I have described several challenges to harvesting crop stovers for a proposed biofuel plant near Spiritwood. These have included the quantity of stover available, amount of time to collect it and ownership issues (tenant versus landowner). While each topic was an initial concern, various solutions were identified to overcome the problems.
The last hurdle our project team has identified may be more of a challenge. How much stover can be removed sustainably without harming the environment?
To obtain more insight, we initially contacted plant and soil science colleagues. However, they quickly directed us to U.S. Department of Agriculture staff within the Natural Resources and Conservation Service (NRCS).
Going into our meeting with NRCS, I naively thought the discussion would focus on wind and water issues, with the result being recommendations for how much straw (length, number of pieces) had to remain on the soil during the winter for protection.
It was the wrong question!
The emerging focus of conservation programs appears to be carbon. More precisely, the goal is to retain about 2,500 to 3,000 pounds per acre of organic matter after a harvest. That amount would be regardless of the crop planted.
A general rule of thumb is that each bushel of wheat produced provides 100 pounds of organic matter. Therefore, with an average state wheat yield of 40 bushels per acre, a total of 4,000 pounds of organic matter per acre is available. If wheat were the only crop, a producer could remove 1,500 pounds of wheat straw per acre, which is the difference between the 4,000 pounds available and the minimum requirement of 2,500 pounds.
However, wheat typically is grown in rotation with other crops. Some of those crops, such as soybeans or sugar beets, don’t leave sufficient material. Consequently, in a sustainable farm plan, the excess 1,500 pounds of wheat straw has to be left to cover the void left from other rotational crops.
Corn and soybean farmers in the Midwest are facing a similar challenge. A 2006 USDA Agricultural Research Service study found that even if corn yields exceeded 200 bushels per acre, less than a ton of corn stover could be removed if the crop is in rotation with soybeans and the land is minimum-tilled. If the producer moldboard plows the field, the scientists recommend that no stover be removed.
If such a policy were to emerge, how should North Dakota producers begin to plan? I see several potential alternatives.
How about leaving the wheat or corn stovers through the winter and then collecting it in the spring? While the spring baling of wheat straw or corn stalks would protect acreage from wind and water erosion during the winter, it does not help with carbon maintenance. The organic material has to be left on the soil to decay. Removing it defeats the goal of maintaining soil health.
Another option or solution is to harvest the stover and then plant a cover crop to provide the required organic material. In North Dakota’s northern climate, plant scientists don’t know how much organic material will be produced before it freezes. There also would be the added expenses of seed and minimum-till planting operations.
Obviously, producers can remove wheat or corn stovers on a periodic basis. To preserve long-term carbon balances and soil productivity, the goal of maintaining 2,500 to 3,000 pounds of organic matter, on average, after each crop should be the goal. With farmers actively rotating some crops in North Dakota, it leaves minimal organic matter at the end of the harvest season, so the opportunities for collecting biomass may be limited.