A manure injector in a field

Putting a value to manure as a fertilizer is complex because of varying nutrient levels in the soil and in the manure. (Photo: Joseph L. Murphy/Iowa Soybean Association)

Manure shows value as a commercial fertilizer replacement

May 21, 2020 | Bethany Baratta

When managed properly, replacing commercial fertilizer with swine manure has economic benefits to farmers and benefits to plant and soil health.

“It’s really a wonderful fertilizer replacement,” said Dan Andersen, associate professor of agricultural and biosystems engineering at Iowa State University (ISU).

Manure from swine finishing barns in Iowa has an average nutrient value of $30 per 1,000 gallons considering potassium, phosphorous and nitrogen.

Andersen says the average swine manure in Iowa contains:

10 to 15 pounds of phosphorous per 1,000 gallons of manure

20 to 30 pounds of potassium per 1,000 gallons of manure

40 to 50 pounds of nitrogen per 1,000 gallons of manure

Putting an exact farm-level value to any kind of manure is complex, said Heath Ellison, ISA senior conservation agronomist. That’s because manure varies from farm to farm depending on the species from which the manure is obtained, the nutrient levels of the manure based on animal diets and age, how well the manure is agitated or mixed before application, amount of bedding material in the manure, and other factors.

Once the manure is applied, nutrient availability is affected by application method, soil organic matter and biology, soil moisture conditions and soil temperatures. Manure testing and utilizing appropriate application recommendations helps, but manure application into a dynamic soil and hydrology system is nuanced and complex.

Manure management

Instead of treating manure as a waste product, farmers should think of manure as a valuable source of nitrogen, phosphorus, potassium and micronutrients.

Iowa soybean, corn and pig producer John Weber has seen improvement in manure management within the past few decades.

“Farmers are realizing the agronomic value and spending a lot of money to get it properly applied,” said Weber, who farms near Dysart, Iowa. “And rightly so, it’s a valuable input.”

Before applying manure on the field, both manure and soil tests are necessary to determine the nutrient contents and needs.

Iowa pig, soybean and corn farmer Dave Struthers grid samples his fields to determine which areas of the fields need the nutrients that his swine manure provides. Struthers grows 1,100 acres of row crops and markets around 5,500 hogs per year in a feeder pig- to-finish swine operation.

About 30% of the manure produced on his farm operation comes from a deep-pitted confinement barn. The remaining 70% comes from pigs raised in deep-bedded hoop barns. Manure from the hoop barns also contained corn stalks or bean stubble, which is used as bedding for pigs growing in those barns.

Because of the varied composition of the manure from Struthers’ farm, various manure application methods are practiced. Liquid manure is broadcast on the fields and then incorporated in the field during the fall. The solid manure is applied through a vertical beater manure spreader.

Other available methods for applying manure include using a tank wagon, a tank wagon with a supply semi, and an umbilical system. Each have their own benefits, and the economics of using each method depend on the location of the field, freight charges and application fees if hiring a commercial applicator.

Money saved through nitrogen management

On top of cost savings farmers can reap from substituting manure application for nitrogen fertilizer application, Iowa farmers could save millions of dollars by reducing their nitrogen applications to a rate suggested under the Iowa Nutrient Reduction Strategy, a recent study found.

The Iowa Soybean Association (ISA) completed the study with 20 Iowa farmers with support from the Walton Family Foundation, Environmental Defense Fund and Iowa-based Regional Strategic, LTD.

Completed during the 2018 cropping season, the study found that the overall nitrogen- per-acre target among the 20 participants for corn following soybeans averaged 173.65 pounds of nitrogen per acre. This is 40.65 pounds per acre more than the Maximum Return to Nitrogen Rate (MRTN) referenced in the Iowa Nutrient Reduction Strategy and 33.65 pounds per acre over the current updated online MRTN calculator.

Based on the estimates in the Iowa Nutrient Reduction Strategy, there are 17.1 million acres of crops in a corn-soybean rotation and 3.9 million acres in a continuous corn rotation in Iowa. If half of the corn-soybean rotation is planted to corn in any year, reducing nitrogen application to the MRTN would save Iowa farmers between $59 million and $98.5 million based on assumptions about the nitrogen price, form, rate and acres applied.

Nitrogen is just one consideration of manure usage. Farmers using manure to displace commercial nitrogen use must look at it as part of a system, said Peter Kyveryga, ISA director of analytics notes that the MRTN is a benchmark only for commercial nitrogen. Other strategies should be used to evaluate the use of manure as a source of nitrogen and other inputs.

“The key is that farmers who use manure should also use better adaptive management strategies (soil test, remote sensing, predictive models) rather than MRTN,” he said.

Factors influencing profitability

Struthers says manure is valuable to his farm, but he struggles to accurately put a value to the product because of several variables.

“We know the costs of the commercial fertilizers, but we haven’t really been able to put a value on the increased organic matter, water-holding capacity and what we’re getting for macronutrients and organic carbon in the manure that we don’t get in commercial fertilizer,” Struthers said.

The Iowa Soybean Association (ISA) put manure through 89 trials* to determine if swine manure could completely replace commercial nitrogen needs. Peter Kyveryga, ISA director of analytics, tested yield differences between adding manure and adding manure with an additional 50 pounds of commercial nitrogen.

The objective of this study was to determine the yield benefits of adding 50 lb./acre of commercial nitrogen (N) to the farmer's normal application of fall swine manure on corn. Farmers applied 50 lb. of commercial N, usually in the spring or sidedress, in addition to their normally applied fall swine manure.

In trials comparing the use of commercial nitrogen, it looks like commercial N behaves logically; you apply more, and the yield response levels off quickly and you don’t need to apply extra,” Kyveryga said. “In the manure trials, it’s not as straightforward because there are other factors to consider like the rate at which manure is applied, the nitrogen content, which could vary from load to load, and finally manure nitrogen losses over the winter and during wet spring.”

In corn after corn, the average normal N rate from manure was 190 lb./acre, (considering the range of 140-288 lb./acre), and the average manure plus 50 application rate was 240 lb./acre. In corn after soybean, the average normal N rate from manure was 170 lb./acre, (considering the range of 100-245 lb./acre), and the average manure plus 50 application rate was 220 lb./acre. (Note: Farmers supplied manure N rates estimated primarily from manure analyses, but occasionally obtained from book values.)

Between 2006 and 2014, 15 of the 24 trials in corn after corn and 35 of the 65 trials in corn after soybean had a significant effect on yield response from the additional 50 lb./acre N.

In corn after corn, the average yield response (difference between manure and manure plus 50 treatments) to manure plus 50 was 11.5 bushels/acre, with a 90% probability of being between 7.6 to 15.4 bu/acre.

In corn following soybean, the average yield response to manure plus 50 was 8.3 bu/acre with a 90% probability of being between 6.5 to 10.1 bu/acre.

Kyveryga says the decision to apply an additional 50 pounds of N/acre at the current economic situation depends on both rainfall and existing nitrogen content in the manure.

Evaluating various cropping systems

The ISA Research Center for Farming Innovation understands the challenges of managing crop inputs, especially one as dynamic as manure, in today’s cropping systems. The Center is currently pursuing opportunities to work with farmers to conduct in-field research evaluating manure and other nutrient inputs in conjunction with the use of conservation cropping practices such as cover crops and reduced tillage. This research will help Iowa farmers better understand how these cropping systems can work together to improve agronomy, water quality, and overall farm profitability.

Additional studies are being conducted at Iowa State University to understand how the timing of manure application impacts crop yield. There are also studies being conducted at ISU to determine how timing of manure application in a cover crop system impacts yield.

Andersen said cover crops could be a great mitigation strategy on farms where manure is applied earlier than preferred (early-mid October).

“In those cases where we’ve drilled in cover crops and injected manure, we’ve seen cover crops hold more nitrogen in the soil and make nitrogen more crop-available in the season, so crop yields are actually improving compared to using manure without cover crops at the same time.”

To evaluate the value of manure nutrients and the costs associated with manure application and inputs, use the decision tool provided by ISU Farm Management Specialist Kelvin Leibold.

*Use the Economic Analysis button in the ISOFAST application to adjust for current market price and your application costs.