Nitrogen Circularity in Swine Finishing

 

The circularity of nitrogen in swine finishing production refers to the management and utilization of nitrogen within the swine production system to minimize waste. Nitrogen is an essential nutrient for pigs, but its improper management can lead to environmental issues such as water pollution, ammonia loss, and greenhouse gas emissions. The concept of circularity refers to the percent or fraction of nitrogen that is kept within the cycle and can be used again.

Nitrogen circularity in swine finishing encompasses at least four areas.

 

1. Feed Management: Balancing the feed composition with the pig's nutritional requirements minimizes nitrogen in manure. Nitrogen is lost during manure storage and recycling for crop production. As such, maximizing nitrogen retention in the pig is critical for improved circularity. However, the choice of managing manure as a waste or resource is often dictated by the ability of farmers to use the manure more cost-effectively than commercial fertilizer could be applied. If manure is treated as a waste, typically, circularity is reduced. Finally, diet ingredient selection impacts system efficiency by its relationship to crop production and crop selection and the nitrogen use efficacy in the cropping system. 
2. Manure Management: Efficient handling and treatment of swine manure can minimize nitrogen losses and environmental impacts. During manure storage, nitrogen is volatilized to the atmosphere and lost. Technologies such as anaerobic digestion, impermeable covers, and acidification can help reduce this loss. Retaining nitrogen for recycling for crop production is critical for circularity.
3. Nutrient Management: Implementing nutrient management plans on swine farms can guide the appropriate application of manure to cropland, ensuring optimal nitrogen utilization. By matching the nutrient content of manure with the crop's nutrient requirements, over-application can be avoided, reducing the risk of nitrogen loss and increasing circularity. Manure application method and timing strongly impact the ability to recover applied nitrogen.
4. Crop Rotation and Cover Crops: Integrating crop rotation and cover crops into the farming system can improve nitrogen cycling. Selection of rotations can reduce nitrogen need for subsequent crops and increase yield relative to limited rotations. Nitrogen-fixing cover crops, such as legumes, can capture atmospheric nitrogen and make it available to subsequent crops. Cover crops have been shown to reduce nitrogen leaching, potentially keeping more nitrogen in the profile for use by subsequent crops.

 Nitrogen circularity is a complex issue that requires a holistic approach. Regulations, incentives, and education can play a vital role in supporting and encouraging the adoption of sustainable nitrogen management practices in the swine finishing industry, but a critical first step is understanding how different systems compare and how other options impact nitrogen circularity. Nitrogen circularity, and our understanding of it, is an ever-evolving topic that we can continue to better understand to make informed decisions that help keep our cropland in Iowa the most productive in the world at providing the food, fuel, and fiber we need.

 To illustrate these concepts, we will look at four systems focused on two different diets (Corn-DDGS and Corn-Soybean Meal), two different manure systems (deep pit and lagoon), and one manure application rate (yield goal rate with any purchased nitrogen fertilizer applied at the ISU maximum return to nitrogen rate) and cropping system focused on an optimized cropping system to supply soybean needed for the diet; manure is then applied to equivalent acres of corn to the amount of soybean that was raised with any manure nitrogen remaining above this level applied to corn raised in a continuous corn rotation.

 Schematics are shown in the four figures below. While the presentation is linear, showing a flow from left to right, the final crops on the right only need processing to become the feed ingredients on the left and begin the cycle again.

 

Figure 1. Corn-soybean meal diet with a deep pit manure storage and manure applied using Iowa yield goal and commercial fertilizer applied at MRTN.

Figure 2. Corn-DDGS diet with a deep pit manure storage and manure applied using Iowa yield goal and commercial fertilizer applied at MRTN.

Figure 3. Corn-soybean meal diet with a lagoon manure storage and manure applied using Iowa yield goal and commercial fertilizer applied at MRTN.

Figure 4. Corn-DDGS diet with a lagoon manure storage and manure applied using Iowa yield goal and commercial fertilizer applied at MRTN.

 

Each figure shows a nitrogen flow through the swine finishing cycle, including an estimate for nitrogen fixation by the soybean. There are many ways of interpreting the cycle, but I like to start with efficiency as an engineer. Efficiency is defined as wanted output/inputs. In this case, the output would be the nitrogen in the pig. Nitrogen inputs include the amino acid and synthetic fertilizer nitrogen purchased to support crop production. Soybean complicates this slightly –the nitrogen they provide is an input, but some of the nitrogen comes from biological fixation and the rest from the soil. As I can't find a reference for how to include them, I will estimate biological fixation and use that fraction as an input.

Another metric that is receiving more discussion is circularity. One means of defining circularity is the percent of required nitrogen inputs obtained by recycling. This would be the amount of nitrogen land applied with manures compared to the total nitrogen needed for fertilizer (both manure and synthetic fertilizer) and the amino acids in the feed. This might be termed "self-reliance" circularity as it is the fraction of circularity an integrated farm could control.

An alternative circularity metric is the "recycling rate." The recycling rate is the percentage of recycled products available for recycling. In our examples, this would be the land-applied manure compared to the as excreted manure plus the nitrogen in the pig. At first glance, including the nitrogen in the pig sounds funny as it is the product, and we want it to be consumed. However, nitrogen is available for recycling at the slaughter and rendering plant and in the human wastewater treatment system, and including this term recognizes that fact and shows how urban locations and areas of consumption must find ways to recover this nutrient to create circular systems.

 

 Table 1. System comparisons on different performance metrics including efficiency and two circularity metrics.

System	System Efficiency	Self-Reliance Circularity	Recycling Rate Circularity
Corn-Soy Deep Pit	67	71	52
Corn-Soy Lagoon	51	46	32
Corn-DDGS Deep Pit	43	42	54
Corn-DDGS Lagoon	35	27	34

 

The recycling rate tells how well we return potential residues into places of value. Nutrients lost to volatilization during manure collection and storage are challenges that must be addressed. While deep pit storages are an improvement over lagoons, innovation is needed to help improve recycling rates further. Nutrients exported with the pig may or may not be recycled depending on what happens at the slaughter facility, the rendering plant, and the human wastewater treatment plant. Within this analysis, I excluded them as they are beyond my scope; however, for a better idea of how the agricultural system is performing, this needs to be included and must be addressed, as these nutrient exports are on the same order of magnitude as nitrogen losses during lagoon storage.

Self-reliance circularity indicates what percent of the nitrogen needs are supplied by items an integrated crop-livestock farm (or system) could control. An alternative way of viewing this number is 100 minus the self-reliance circularity is our reliance on nitrogen sources outside the farm's control. An integrated cropping and swine finishing farm with a deep pit manure storage is currently proving around 71% of the nitrogen it needs to feed a pig and fertilize its crops. An interesting take-home here is that using soybean to biologically fix some of the nitrogen for the diet improves self-reliance circularity significantly as it reduces nitrogen demand to the following crop and provides nitrogen to the manure that didn't originate from synthetic fertilizer or recycling materials. This metric could also be improved by reducing ammonia losses during manure collection and storage, improving rate selection for manure as a fertilizer (probably mostly through improved application timing), and innovating ways to recycle nitrogen that ends up at the slaughter, rendering, and human wastewater treatment facilities.

Finally, we have the system efficiency metric. This metric again favored soybean-based diets, though in this case, this was at least in part due to diet formulations that facilitated greater nitrogen retention in the pig and less excretion and at least in part from reduced nitrogen inputs needed to achieve corn production. This metric could also be improved by reducing ammonia volatilization during collection and storage, as demonstrated by the differences between the lagoon and deep pit systems.

A few caveats – this is by no means an end-all-be-all analysis. Many assumptions were made to facilitate the analysis; for example, I compared nutrient removal with corn grain to the nitrogen fertilizer application rate to facilitate calculations rather than trying to estimate N losses with nitrate leaching, surface water runoff, or denitrification losses. In doing so, I inherently assumed nitrogen additions through deposition didn't matter and that the soil nitrogen level was in equilibrium and unchanging. Similarly, I looked at nitrogen soybeans fixed from the atmosphere and didn't show nitrogen losses during their production. Water measurements in tiled fields show nitrogen leaching in these production systems. Similarly, co-products are produced from both diets – either soybean oil or ethanol. While neither contains nitrogen and thus doesn't impact my analysis with these parameters, recognizing that the system is more complicated and contributing other goods is important in using these metrics.

All this is a fancy way of saying things we already know. We, agricultural and agricultural researchers, need (1) to innovate new solutions that help reduce ammonia losses during manure collection and storage, (2) improve nitrogen rate selection with fertilizers but especially manures (again probably through innovations that facilitate improvements in manure application timing), (3) be mindful of how crop rotation effects alter nutrient needs and alternative in-field management practices (like cover crops) could alter losses and our ability to recycle nutrients, and (4) that on-farm management only goes so far and that recycling of consumed nutrients from urban areas where livestock products are consumed has almost as large of impact on nitrogen recycling metrics as the ammonia volatilization losses.