Balancing Act: Carbon Flows in a Continuous Corn System with Stover Harvesting for Livestock Bedding and Manure Reapplication

In the vast landscapes of modern agriculture, the intricate dance of carbon within ecosystems plays a pivotal role in sustaining soil health and fertility. One such intricate choreography unfolds in continuous corn systems where the harvest of corn stover for livestock bedding intersects with the subsequent application of stover-laden manure. Let us explore this delicate balance of carbon flows and its implications for soil health.

Harvest: The Carbon Exodus

Continuous corn systems rely on the consistent cultivation of corn year after year. In this cycle, corn stover, comprising stalks, leaves, and cobs, is a crucial component. However, the decision to harvest corn stover for livestock bedding initiates a subtle but impactful carbon exodus from the agricultural ecosystem. When corn stover is removed from the field, it takes a significant portion of organic carbon. This organic matter, a key constituent of healthy soils, contributes to soil structure, water retention, and fertility. The concern arises when this organic carbon, essential for microbial activity and nutrient cycling, is carted away, potentially leaving the soil in diminished fertility.

To help put some numbers to this, let us assume 200 bushels of corn, a harvest index of 0.5, and a shoot-to-root ratio of 0.21.

200 bu/acre x 56 lb/bu x 0.845 lb dry matter/lb grain = 9464 lb dry grain/acre

Using the harvest index, we can get stover produced.

0.5 = 9464 lb grain/(9464 lb grain + lb stover) gives 9464 lb stover/acre

Finally, the root-to-shoot ratio can be used to calculate how much root biomass is generated.

0.21 = root/9464 lb stover gives root = 1987 lb root/acre

The Livestock Connection: Stover as Bedding Material

The journey of corn stover continues beyond the fields. Harvested stover finds its way to livestock operations, where it takes on a new role as bedding material. Livestock farmers recognize the value of corn stover for providing comfortable and absorbent bedding for their animals.

A starting estimate for beef cattle bedding use in a bed pack barn is around 5 pounds per head per day or about 1800 lb per year. We want to harvest about 1 ton per head.

The Return: Manure Application

As livestock bedding breaks down, it decomposes, releasing nutrients and organic matter into the manure. When this nutrient-rich manure is reapplied to the fields, it returns a significant amount of organic carbon. This return of organic matter is vital to soil health and carbon sequestration.

A typical bed pack manure is around 20 lb N per ton (Assume 50% 1st year available, 5% volatilization losses), and each space generates 6 tons of manure annually. To make this easier, I will assume a manure application rate of 6 tons/acre at 70% moisture or 1.8 tons dry matter.

The carbon flow in this system resembles a cyclic rhythm, echoing the sustainable principles of nutrient recycling. The manure application not only replenishes the carbon lost during stover harvesting but also contributes to the overall organic matter content in the soil.

The Balancing Act: Where does this leave us?

So now, the fun part is making assumptions to track organic matter. There are many complicated models to help track soil organic carbon, but in the interest of time and ease, I will make assumptions to simplify what is happening. I am assuming 10% of above-ground biomass is transformed into soil organic matter and that, similarly, 10% of applied manure becomes soil organic matter. For roots, I am going to be more aggressive and say 30% of that material becomes soil organic matter. Some fraction of the soil organic matter also needs to mineralize per year, and unfortunately, I do not have a good rule of thumb on what this is – so I estimated it for a continuous corn system by assuming equilibrium at a known soil organic matter content, in this case, 5% soil organic matter which suggested annual organic matter mineralization of 1.5%. This organic matter mineralization was then used in the stover harvest-manure application case to estimate the soil organic matter content under this operation. Carbon flows estimated are shown in Table 1.

Table 1. Carbon flows in a continuous corn system and in a continuous corn system with stover harvest and manure application. 

	Continuous Corn	Continuous Corn Stover Harvest & Manure Application
Grain Yield	200	200
Corn Mass	9,464	9,464
Harvest Index	0.5	0.5
Corn Stover Dry Mass	9,464	9,464
Root to Shoot	0.21	0.21
Root Mass	1,987	1,987
Manure Applied	0	3,600
Stover Harvest	0	2,000
Respired Corn Stover	8,518	6,718
Respired Corn Root	1,391	1,391
Manure Respired	0	3,240
Corn Stover to SOM	946	746
Corn Root to SOM	596	596
Manure to SOM	0	360
SOM Added	1,543	1,703
		0
Original Soil Organic Matter	5.0	5.5
Organic Matter in Acre	100,000	110,400
Percent OM Mineralized per Year	1.5	1.5
Organic Matter Mineralized	1543	1703

A few thoughts, this exercise is meant to be illustrative, and not determining of an exact field as the overall process was vastly simplified. However, with that said, our results indicated that higher organic matter would be expected in a field where stover is harvested for bedding and then that material applied as a manure. This should be expected, because on net, the more organic matter is added because of the cattle excrement and is removed from the field.

Of course, there are many complicating factors. Examples include, how does the use of manure impact tillage choices at the farm? If the farm was no-till and then choose to use manure, perhaps a tillage pass is added. This tillage pass would almost certainly increase the rate of soil organic matter mineralization. Does more organic matter in the soil increase the percent of organic matter mineralized per year? I think it almost certainly has to, but I didn’t model it as such here because I don’t have data to suggest a new rate of mineralization. With that said, even with these caveats I think the balance and process are illustrative as they help us understand some of the approaches we can use to get a feel for how different practices might impact our soil.

Conclusion: A Symphony of Sustainability

In the realm of continuous corn systems where corn stover is harvested for livestock bedding and later reapplied with manure, the carbon flows represent a dynamic interplay between extraction and replenishment. Striking a balance between the needs of crop production and livestock management is essential for ensuring the long-term sustainability of agricultural ecosystems. In many respects what I showed here is simple, I matched the harvested area with an area that is going to receive manure. There could be places where this isn’t the case. By understanding and respecting the nuances of carbon flows, we can foster agricultural systems that not only meet the demands of the present but also preserve the vitality of the land for generations to come.