Biomass and Cover Crops

Biomass refers to what’s left over in the fields once your cover crop has been terminated. Biomass is a crucial element in soil chemistry, as well as in the overall health of your land.

There are two basic elements to the biomass equation: the root mass that is produced during the life cycle of the cover crop; and the residue left on the surface of the ground after the cover crop has been terminated in the spring. The surface residue is useful to the new cash crop, whether corn or soybeans, because in decay, it gives back nitrogen to the juvenile cash crop growing there. Use of annual ryegrass as a cover crop can effectively cut the amount of nitrogen you apply to corn in early summer because of the nitrogen it gives back to the soil.

But it is the subsurface biomass of cover crops that are most important to soil health and, ultimately, the productivity of the field. The slow decay of the root structure continues to feed the microorganisms in the soil, and also helps to create a soil structure that let’s corn roots grow deep and water to soak in, rather than running off.

Over a period of years, the University of Nebraska did field studies of different cover crop species in different soil types, to measure the amount of biomass produced. In all cases, it was annual ryegrass that produced the most biomass of all cover crops. They refer to annual ryegrass as “rye” but that is distinctly different than “cereal rye.”

The following two paragraphs from a recent Successful Farming article talk about the value of annual ryegrass compared to other cover crop species.

In eastern and northeast Nebraska, the pre-harvest planting achieved the biomass threshold, producing on average 1,900 to 2,500 lb/A of biomass by late April to early May, whereas the rye planted post-harvest produced approximately half of that amount. Pre-harvest planted cover crops had lower emergence than post-harvest planted cover crops, but had more time to grow and tiller, compensating for low populations. In south-central Nebraska, both planting times reached the threshold, but the post-harvest planting produced more biomass. This site receives less rainfall in the fall, restricting the emergence of pre-harvest cover crops.

Overall, the mix biomass was lower than the rye biomass. The brassicas in the mix winterkilled, and hairy vetch and winter pea produced very little growth. Thus, the mix can be thought of as rye planted at 30 lb/A. Despite this low seeding rate, the mix produced more than the threshold biomass at the south-central sites in both planting times, and at the northeast site in the pre-harvest planting.

The article also distinguished between planting the cover crop before the harvest (broadcast from a high-clearance rig or aerially) or after harvest (seed drill). Generally, the drilled seed has a better record of emergence but, because of being planted later, it still doesn’t produce the biomass of the earlier broadcast seed.