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.

Pursuing Carbon Neutrality in Livestock Production

 For livestock facilities improving nitrogen circularity, reducing nutrient loss to air and surface waters, and better utilization of manure have been hallmarks of pursuing sustainability. Limiting the release of greenhouse gases has now been added to that list.

 Achieving carbon-neutral livestock production requires implementing strategies to reduce greenhouse gas emissions and offset any remaining emissions. In carbon footprinting, there are three emissions scopes. Scope 1 Emissions are greenhouse gas (GHG) emissions from operations owned or controlled by the livestock producer. For livestock facilities, Scope 1 emissions are enteric fermentation methane emissions and methane and nitrous oxide emissions from manure storage practices. This category would also include emissions from the combustion of propane used to heat livestock buildings or fuel used to power equipment and tractors that handle mortality compost.

 Scope 2 emissions are indirect emissions resulting from the generation of purchased or acquired electricity, steam, heating, or cooling. Typically, livestock fans would be carbon dioxide generated from producing electricity to run building fans and power other electric motors used in the feed delivery system.

 Scope 3 emissions are indirect emissions in the value chain, including upstream and downstream emissions. These, for example, would include greenhouse gas emissions resulting in feed production, including raising corn and soybean, grinding and delivering it to the production facility, and emissions associated with moving livestock or livestock products (milk, eggs) to the processing facility. It could also include carbon costs associated with converting the animal (or animal product) into a salable product at the grocery store.

 While I attempt to break these into neat categories by scope, it can be challenging as integrated livestock facilities that produce feed on-farm for their livestock could be considered Scope 1 emissions.

 Scope 1

1. Manure Management: Implement efficient management practices to minimize methane and nitrous oxide emissions. Anaerobic digestion systems, which convert manure into biogas for energy production, are a primary example. Other options include aeration, acidification, frequent manure removal and application, and diet modification. Techniques primarily focus on reducing methane emissions.
2. Enteric Fermentation: Methane is produced during digestion and fermentation in an animal's digestive tract. Developing feeding strategies that reduce enteric fermentation is an active line of research for dairy cattle, with products such as Monensin and seaweed being suggested.
3. Feed Efficiency: Enhance feed efficiency by utilizing improved nutrition strategies, including formulating balanced diets and adding feed additives. Improved feed efficiency reduces the amount of feed required to produce a unit of meat, thereby lowering emissions associated with feed production. Improved feed efficiency due to improved grinding and, as a result, digestibility has been a key improvement in environmental performance.
4. Genetic Selection: Select swine breeds or genetic lines with higher feed efficiency and lower emissions. Genetic improvements can help reduce swine production's environmental impact over time and have been key in our environmental efficiency gains. .

 Scope 2

1. Energy Efficiency: Improve energy efficiency in swine production facilities by using energy-efficient equipment and optimizing heating, ventilation, and lighting systems. Optimizing use reduces energy consumption and associated emissions. Examples include switching lighting to LED (an increasingly popular approach in poultry housing). Fan staging to increase fan efficiency, installation of VFDs on fans to improve performance, and closely monitoring minimum ventilation requirements to reduce propane demand for maintaining barn temperature.
2. Renewable Energy: Incorporate renewable energy sources such as solar panels, wind turbines, or biogas systems to generate clean energy for powering swine production facilities. Renewable energy can significantly reduce carbon emissions. Many swine farms have started adding solar panels to help generate clean energy, but so do changes in energy supply technologies throughout the state.

Scope 3

1. Nutrient Management: Optimize nutrient management practices to minimize the release of nitrous oxide, a potent greenhouse gas. Nutrient management involves carefully managing the method and timing of manure or synthetic fertilizers application to crops. Considering nutrient content, timing, and soil conditions. Optimizing rate, timing, method, and soil conditions is a huge topic and will continue to be studied.
2. Select feed ingredients from agricultural production practices that result in lower carbon footprints of the supplied materials. Life cycle analysis is helping differentiate how substituting DDGS may impact feed carbon relative to soybean meal. Similarly, future work will need to identify how recycling manure, use of cover crops, and reduced tillage affect yield and greenhouse gas emissions, and the digestibility and livestock performance interact to help inform ingredient selection and inclusion rates. These factors will need to be balanced with price and availability.

Carbon Offsetting

Compensate for any remaining emissions by investing in carbon offset projects. This can involve supporting activities such as reforestation or using trees and grassland around building sites to increase carbon storage, renewable energy projects that reduce emissions beyond the farm, and implementing or participating in carbon capture and storage projects.

Below, estimates of different emissions for a deep pit swine finishing facility (wean to finish). These estimates are meant to be illustrative, may not include all emissions, and won't represent all farms.

 

1. Manure Management: A deep pit swine storage would emit approximately 192 kg CO_2,e/head-year.
2. Enteric Fermentation: IPCC Tier 1 approach estimates 1.5 kg CH₄/hd-year, or 42 kg CO_2,e/head-year.
3. Barn Heat: Harmon et al. (Sizing minimum ventilation to save heating energy in swine housing) estimated usage of 2 gallons per pig space per year, or 11.4 kg CO_2,e/head-year.
4. Barn Electricity Use: Barn design will impact energy use, but around 25 kWh/pig space per year (see Hanna et al., 2016 – Energy use for field operations, crop drying, and swine housing on University Farms). Iowa is estimated to generate 0.36 kg N₂O/MWh and 430.5 kg CO₂/MWh. These emissions are about 0.5 kg CO_2,e/kWh, or 13.5 kg CO_2,e/head-year.
5. Feed production: According to Benavides et al. (2020), swine feed has about 0.4 kg CO_2,e/kg feed. If we have a feed conversion ratio of 2.5 lb feed/lb live weight gain, then 1 kg CO_2,e/kg of pig sold. Assuming 2.2 turns a year and 280 lb pigs sold, this results in 265 kg CO_2,e/head-year.
6. Feed movement: The average freight truck in the U.S. emits 162 grams of CO₂ per ton-mile. Every pig space needed 0.73 tons of feed/year, and assuming feed was delivered 100 miles gives 12 kg CO_2,e/head-year.
7. Pig movement: Again, assuming 100 miles of pig transport to the processing plant (finish weight of 280 lb) and 100 miles from the nursery facility (15 lb wean pig) gives 5.2 kg CO_2,e/head-year.

 

I had a carbon footprint of 0.5 tons CO_2,e/pig space per year, or about 2 kg CO_2,e/kg live weight produced or 3 CO_2,e/kg of "take home" meat. Two caveats, I didn't include the carbon footprint associated with generating piglets or dealing with mortalities. We can do something easy for mortalities, like assume a 3% death loss and multiply our results by 3%. Assuming culled animals used all the feed and generated all the manure they would have if they were alive, gives a cushion for using energy to handle the carcasses. The approach raises the lifecycle costs to 0.51 tons CO_2,e/pig space per year, and 3.2 CO_2,e/kg of "take home" meat. Unfortunately, I don't have a simple trick to deal with generating piglets. To get this estimate, we need to work on an example sow farm (there is always next time).

 

Figure 1. Percent of swine finishing greenhouse gas emissions from different sources.

 

It's important to note that achieving carbon neutrality in livestock production will require a combination of these strategies, and the feasibility of implementation can vary depending on factors such as location, scale of production, available resources, and livestock species. Regular assessment, adaptation, and improvement are necessary to ensure ongoing progress toward carbon neutrality.

Why is Manure Anaerobic?

Manure is stored with one surface in contact with the atmosphere. That atmosphere contains oxygen. Shouldn’t this oxygen diffuse into the manure and make it aerobic? And the truth is it does, but often the diffusion rate is low compared to the rate at which oxygen in the manure is consumed. As a result, the manure will still be anaerobic. In this month’s article, we’ll look at oxygen diffusion into the manure and discuss the impact manure strength and the amount of organic matter in the manure has on how much of the manure is aerobic.

This analysis is simplified because it doesn’t account for surface disturbance from wind or mixing in the manure that might spread oxygen around. That is to say, it is primarily for illustrative purposes and to get a feel for anaerobic manure storage conditions.

This analysis works down to two parts understanding oxygen movement through the manure and the destruction, or use of that oxygen, to consume organic material. The first part relies on Fick’s law to understand how oxygen moves in manure. It should be noted that this is based on no mixing. Mixing drastically changes oxygen movement as it allows for advection, mixing with the movement of the manure, and diffusion. To get an estimate of just diffusion-driven oxygen movement in manure, though, I did have to make a couple of assumptions. The first is that there is a small layer of manure at the surface with an oxygen content always in equilibrium with the atmosphere. The second is Fick’s Law and the oxygen diffusion rates in water, which describe the movement of oxygen in manure.

The oxygen content of the surface water is estimated using Henry’s Law and assuming Henry’s constant of K_H = 4.34 x 10⁴ atm @ 77°F (25°C). Accepting a partial pressure of 0.21, based on the 21% oxygen content of the atmosphere, gives, 4.84 x 10^-6 moles of oxygen in a mole of water or 4.84 x 10^-6 moles of oxygen in 18 g. And after a little rearranging and a unit conversion, this amounts to, 4.3 mg/L. .

Using this information, Fick’s Law can be used to estimate diffusion. The oxygen diffusion rate in water at 77°F (25°C) is 2.42 x 10^-9 m²/s. The rest of the equation represents the change in concentration over distance. The change in concentration is 4.3 mg/L since we know the concentration is at a maximum at the surface and goes to zero, but we aren’t sure over what distance this will occur yet. To help get an estimate of the distance, we need to think about how quickly the oxygen will be used.

Chemical oxygen demand is often measured on different wastewater materials, including manures, to help understand the strength of the material. The typical COD of deep pit swine manure is around 100,000 mg/L. About 25% of this will often be BOD₅ (the amount of oxygen bacteria will consume in five days as they break down organic matter) or 25,000 mg/L. Assuming that 1/5^th of the oxygen will be broken down daily, 5,000 mg/L of oxygen will be consumed.

The solution becomes iterative as we try to guess the depth that causes all the oxygen to be depleted. To start, assume a 1 m x 1 m x 1 m deep chunk of liquid manure. The diffusion of oxygen can then be calculated  as 0.9 mg/m²-day.

How does this compare with the oxygen demand of the manure? That 1 m x 1 m x 1 m chunk of manure has a 1-day BOD of 5,000,000 mg. Suppose you take the 0.9 mg/m²day/ 5,000,000 mg/m³day= 0.00000018 m. This gives us our first guess of the depth. The smaller depth will cause a much higher diffusion rate and lower oxygen demand. We want to keep iterating this depth process until we find a consistent depth.

If we iterate this solution, we will find that the depth to which oxygen diffuses is 5.6 mm, or

Now iterate the depth you selected in the diffusion equation until it matches the depth of extinction

The diffusion depth of oxygen into manure is 0.42 mm or about 0.02 inches. More than that, that oxygen consumed a tiny amount, 0.00002%, of the oxygen demand.

In lagoon systems, often the manure is diluted to help it flow better, but this dilution also helps the lagoon be more aerobic, at least at the surface. Let’s see what happens if the manure is diluted by a factor of 5. In this case, about 1 mm would have oxygen or 0.04 inches. Even in this case, diffusion alone has a limited impact on making the manure aerobic. To help with this, lagoons often have residual material left in them to help further dilute the manure and to help improve oxygen transfer by providing a larger surface area. In addition, wave action from wind or surface aerators or often necessary to make the lagoon aerobic.

So manure is anaerobic because it has a high capacity to consume oxygen relative to how quickly diffusion allows oxygen to move through the manure.

Anaerobic Digestion – Covered Manure Storages

 Impermeable covers minimize odor and limit ammonia emissions. Covers prevent rainfall from mixing with manure, making manure production volumes more consistent from year to year, reducing the chance of overflow, and providing the potential for methane capture, reducing the farm's environmental footprint. These opportunities indicate that covered manure storage should be favored, yet incorporating impermeable covers has been minimal. Many resources suggest the potential benefits of impermeable covers, but a thorough economic evaluation of benefits is lacking, with the existing tools predating the development of RIN and LCFS Carbon Credit Markets. These markets are game-changing in the potential opportunities they offer. With this in mind, we developed a model to estimate how covers would impact the spreading costs, fertilizer value, biogas production, and carbon credits a farm receives and evaluate implications for Iowa livestock farms.

With that in mind, I've been putting together some estimates of the value covers and potential costs. Many assumptions are involved in these estimates, and hopefully, a future extension publication will walk through some of the calculations. Here we provide a comprehensive economic exploration of the value impermeable covers offer, focused on illustrating both perceived values and those that are the definitive economic drivers in the current marketplace. We looked at three example farms, a 4800-head swine finishing farm starting with deep pit manure storage, a 4800-head swine finishing farm starting with a drain pit and out-of-barn slurry storage, and a 500-head dairy with out-of-barn manure storage. We calculated the cost of modifying the facility to add a cover at each farm and the potential cost savings provided with current economic and carbon credit-based incentives. Evaluations assumed a 5-year or 10-year life with interest set at 8%.

Results are shown in table 1. In general, when all potential values are considered, both the 4800 head swine and the 500 head dairy using lagoon, earthen basin, or out-of-barn manure storage were cost feasible within five years and had net annual incomes of $50,000-$70,000 within five years. Economics didn't appear as favorable at deep pit facilities as construction costs for manure storage modifications increased cost. Combined with a higher carbon intensity score on methane, this produced limited payback opportunities.

Table 1. Estimated cost-benefit ratio for covers at livestock facilities creating CNG.

	5-year life			10-year life
Facility Type	4800 swine	4800 swine	500 head dairy	4800 swine	4800 swine	500 head dairy
	deep pit	lagoon		deep it	lagoon
Methane Value	$44,661	$44,661	$44,030	$44,661	$44,661	$44,030
LCFS Value	$199,352	$420,744	$414,804	$199,352	$420,744	$414,804
RIN Value	$283,311	$283,311	$279,312	$283,311	$283,311	$279,312
Nitrogen Value Savings	$5,635	$4,827	$2,401	$5,635	$4,827	$2,401
Odor Reduction	$6,900	$6,900	$884	$6,900	$6,900	$884
Storage Construction Savings	$0	$1,226	$2,427	$0	$729	$1,444
Manure Application Costs	($2,674)	$398	$2,845	($2,674)	$398	$2,845
Carbon Credits from N2O	($770)	($2,690)	$554	($770)	($2,690)	$554
Pipeline Injection Point	$250,457	$250,457	$250,457	$149,029	$149,029	$149,029
Biogas Cleaning & Compression Equipment	$186,254	$186,254	$183,624	$110,827	$110,827	$109,262
Biogas Cleaning & Compression Maintenance	$74,366	$74,366	$73,316	$74,366	$74,366	$73,316
Biogas Cleaning Operation	$11,018	$11,018	$10,862	$11,018	$11,018	$10,862
Biogas Management Employee	$80,000	$80,000	$80,000	$80,000	$80,000	$80,000
LCFS/RIN Sales Fees (10%)	$48,266	$70,405	$69,412	$48,266	$70,405	$69,412
Manure Storage Construction	$102,391	$0	$0	$60,926	$0	$0
Annualized Cover Installation Cost	$13,948	$13,948	$22,126	$8,300	$8,300	$13,166
Cover Maintenance Cost	$5,569	$5,569	$8,834	$5,569	$5,569	$8,834
Rainfall Removal	$14	$14	$28	$14	$14	$28
Annual Benefit	$536,415	$759,377	$747,257	$536,415	$758,880	$746,274
Annual Expense	$772,283	$692,031	$698,659	$548,315	$509,528	$513,909
Net Benefit	($235,868)	$67,346	$48,598	($11,900)	$249,352	$232,365

 

These results are highly dependent on a farm scale and assumed covered manure storage, not the implementation of an anaerobic digester. Next month we'll look closely at the role farm size plays. In November we'll look at how heated digester systems compare to impermeable covers.

Impact of Manure Timing on Effective Nitrogen Supply

 Nitrogen use efficiency (NUE) is the dry mass productivity per unit N taken up from the soil. The term can be used in many complex ways to illustrate different factors, like the amount of nitrogen supplied from the soil.

I looked at applied nitrogen (from liquid swine manure) as a function of manure application timing and crop yield response. At Nashua, Iowa, we have four years of data looking at manure application timing. Treatments were early fall (October), late fall (November), and spring. We’ve presented this data as the difference in yield. Here, I look at it a different way; how effective was the application in supplying nitrogen?

The process to estimate how effective the nitrogen was at supplying the crop was:

1. Use the difference in yield between the treatments to estimate the percent of maximum yield.

2. Use the Iowa State Maximum Return to Nitrogen nitrogen response curve to estimate effective nitrogen supply.

The results are plotted in figure 1.

So what does this mean? The spring manure achieved 99% of the maximum yield. It has the same yield as the spring applied UAN. The late fall manure yielded around 85% of the maximum yield. The early fall manure yielded 65% of the maximum yield. Reading from the x-axis provides an effective nitrogen supply.

The spring application, by default, supplied approximately 150 lb N/acre (as it is used as the baseline). The late fall application had a yield equivalent to 75 lb N/acre in the spring. The early fall manure had a yield equivalent to if only 25 lb N/acre had been applied. This means that the late fall manure was only taking advantage of about 50% of the nitrogen applied. The early fall manure was only 17% effective. These numbers illustrate the important role application timing plays in using manure

Figure 1. Effective nitrogen application rate from EFM (Early Fall Manure), LFM (Late Fall Manure), and SM (Spring Manure) when all were applied at 150 lb available N/acre at the time of application.