(This article was written and reviewed by manure science experts with Iowa State University Extension and Outreach.)
The Iowa Nutrient Reduction Strategy was launched in 2013 to address water quality issues related to nutrient runoff in the state's rivers and streams, particularly nitrogen and phosphorus. The strategy was a collaborative effort between the Iowa Department of Agriculture and Land Stewardship, the Iowa Department of Natural Resources, and Iowa State University Extension and Outreach.
As we acknowledge the 10th anniversary of the strategy, it is important to recognize the progress made in reducing nutrient pollution and the successful implementation of manure management practices. But just as important, it is a chance to reflect on areas we still need to improve.
In this article, we examine key achievements and areas where improvement is still needed.
The nutrient reduction strategy has helped raise awareness about the impacts of nutrient runoff on water quality and the importance of implementing conservation practices. Farmers today are more engaged in the conversation, with knowledge of nitrogen and phosphorus movement in agricultural ecosystems. Farmers are studying best practices and considering how to add them to their operation, and how they can fit. At the 2021 manure applicator training, ISU Extension and Outreach surveyed participants and 85% responded that they had learned more about the nitrogen cycle and the importance of manure application timing. Similarly, 35% of commercial manure applicators at the training stated that the nutrient reduction strategy has changed their clients’ expectations about manure application.
Cover crop use on acres receiving manure has followed the state's trend for cover crop use. Most liquid manure is injected, and at the onset of the INRS, there was a fear that this injection would hurt or destroy the cover crop stands and make it not worth establishing cover crops. Iowa State research has demonstrated cover crops are generally more effective for reducing nitrogen loss to tile water in fields that receive manure, than commercial fertilizer, due to the timing challenges of utilizing manure and the higher utilization of fall application compared to other fertilizers.
Moreover, as many fields with a long history of manure have higher soil phosphorus concentrations, reducing erosion in these areas is more critical than in many other fields. Additionally, USDA’s Agricultural Research Service, Iowa State, and the Natural Resources Conservation Service, have all demonstrated that low-disturbance manure injection systems do little harm to cover crops. Survey response data from the 2020 Manure Applicator Certification program suggested that 29% of confinement site applicators had tried cover crops, but more than half of those who had tried cover crops used them on less than 80 acres.
Manure acres receiving cover crops at a rate similar to other fields suggests farmers using manure have overcome the extra concerns from using manure. Research indicates that cover crops reduce nitrate loss by 30-50%. Cover crops reduce phosphorus loss by 30%, with the number probably higher on fields with a manure history and a higher level of soil phosphorus.
Increased N:P ratio in swine manure
Although manure is a complete fertilizer containing all the nutrients needed to support crop production, it often isn't balanced to crop nutrient needs. In the 1990s and 2000s, a great focus on manure research was on phosphorus separation from livestock manures. Most manures have low nitrogen to phosphorus ratios. An increased N:P ratio in manure can positively affect water quality, depending on the specific circumstances and management practices. If being applied at a nitrogen-limited rate, it helps limit phosphorus buildup in the soil and reduce phosphorus runoff. From 2000 to 2020, the N:P ratio of swine manure went from 1.5:1 to 3:1 on average, making it approximately balanced for corn-soybean rotations in Iowa. This change occurred due to modified feeding practices, including increased use of phytase and greater use of DDGS in swine rations, but also offered opportunities for water quality improvement. The Iowa strategy for phosphorus indicates that such a change reduces phosphorus loss by 17%.
Increased manure nutrient concentrations and innovations in manure systems
Improved water conservation at swine facilities has led to increased manure nutrient concentrations in manure. Relatively low concentrations of nutrients in manure, compared to commercially available forms of fertilizer, make manure bulky and expensive to apply and transport. Higher concentrations reduce transport costs, allowing manure to move further from the point of generation and be spread over more acres. Water conservation measures at swine farms, primarily using wet-dry feeders, results in less water waste and more concentrated manure. Concentrated manure can be easier to handle and transport as there is less volume to manage, meaning it can be transported to fields further from the facility. The higher nutrient content is also beneficial at the application as it means less soil disturbance is required to fully cover the manure, allowing less tillage. When applied, higher nutrient concentration manure reduces the risk of manure movement as it takes less time to soak into the soil. Iowa data suggests that lower soil disturbance, especially if for quality no-till, can substantially reduce (90%) phosphorus loss. Ensuring farmers minimize tillage after this pass is critical for obtaining potential improvement.
Transitioning beef finishing from open lots to deep pit or bedded pack barns provides a controlled environment for cattle, containing their manure within the barn structure. Moving manure under the roof prevents the dispersion of manure into the surrounding environment, reducing the risk of water pollution. Manure can be exposed to rainfall, runoff and wind in open lot systems, leading to nutrient losses through leaching, volatilization, runoff and erosion. Manure is protected from such weather conditions in deep pits or bedded pack barns, helping preserve the nutrient content. The resulting manure has higher nutrient contents facilitating more efficient manure utilization as a fertilizer.
Transitioning from high-rise egg production systems to manure belts and stacking sheds has significantly improved manure management in the poultry industry. Manure belts and stacking sheds facilitate better storage and management of poultry manure. Once collected, the manure can be stored in dedicated sheds or structures, allowing for composting or improved storage conditions that limit ammonia volatilization. The controlled environment and improved storage capacity make it easier to manage manure and control odors and transformation in the manure. The change has resulted in drier manure that spreads more uniformly and can be transported further. With improved manure management, the nutrient content of poultry manure can be preserved and utilized more efficiently.
All three of these practices have the potential to help us manage soil test phosphorus, which would result in a 17% reduction in the phosphorus load. While these changes allow water quality improvement, making sure these improvements lead to water quality changes through appropriate application decisions is critical.
Equipment improvements and technology
Accurate and controlled manure application can positively impact the environment, crop health, and overall farming efficiency. Advanced manure application equipment often incorporates precision technology such as GPS guidance and mapping systems. These technologies enable precise tracking of the equipment's location and provide accurate application rate control. Farmers can input field boundaries and application rates, allowing the equipment to automatically adjust and apply manure at the desired rate and ensuring equipment is shut off when passing close to water features that need to be protected. Modern equipment has user support and flow monitoring at several points in umbilical manure application systems, leak detection, and automatic shutoff to minimize spills and impacts of manure spills.
New manure application equipment is designed to provide more consistent and uniform spread patterns. Improved uniformity helps distribute manure evenly across the field, ensuring nutrients are applied where needed and more uniform crop growth, helping farmers value manure as a resource. Improved spread patterns also minimize the risk of nutrient concentration in certain areas, which can lead to uneven crop growth and potential environmental issues. Work by Iowa State demonstrated modern manure distributors have reduced knife-to-knife variation from above 35% to around 10%.
Improving Iowa's water quality, and manure's role, still requires further work. Here are a few key areas where there is opportunity for continued and hopefully exponential success.
Manure application timing
Fall application carries a higher risk of nutrient losses through leaching and runoff in winter and early spring. Iowa State research has consistently shown that nutrient loss will be higher in most years if manure is fall applied, and crop yield will be lower with fall application than spring. Despite this, about 75% of Iowa's manure is fall applied. Future work and conservation efforts must look for ways to move manure to a spring application season or to apply in-season to growing crops. Improving manure application timing would reduce the nitrogen load from manured acres by 10-20% and is a crucial challenge moving forward. In our 2023 Manure Applicator Training, we showed a video of in-season manure application and discussed sidedressing corn with liquid manure. As a result, 29% of respondents said they were likely or very likely to consider applying some fraction of their manure in season. This is a stark improvement to 2019 survey data where farmers thought only 1% of manure could be applied in season and suggested that only around 23.4% of manure could be spring applied.
Accurately achieving lower application rates
Traditional manure spreading equipment may need to be redesigned to handle low application rates effectively. The spreader distributors and flow controls may not provide the accuracy required for low-rate applications. Reduced water use and increased nutrient content have been important to farmers getting more value from manure on their farms. Equipment must improve and evolve to capture manure value as application rate challenges increase. Similarly, farmer trust in manure as a fertilizer source that will be plant available (not applied too early and lost) and spread uniformly is critical for improving the rate. It is estimated that rate improvements would reduce nitrogen loading from manured acres by 10-15%.
While also a success, cover crops are more imperative on manure acres, as many have soils with high P testing levels from a long history of manure application. Furthermore, manure application timing challenges that lead to fall manure application increase the potential for nitrogen loss. Manure acres must lead the way in cover crop adoption.
Applying manure to more acres
While swine manure N:P levels have changed, many other manures (dairy, beef, poultry) still have relatively low nitrogen-to-phosphorus ratios, resulting in soil phosphorus accumulation when applied at nitrogen rates. Too often, manure application decisions are based on when the Iowa Phosphorus Index limits application rather than looking for fields where all the nutrients will have agronomic value.
Development of economically viable manure treatment systems that enhance nutrient use and circularity
These include innovations in manure collection and storage that reduce nitrogen loss during storage and limit risks of overflows and leakage, the development of nutrient-conserving composting systems that increase nutrient concentrations and minimize loss to the air, as well as solid and nutrient separation systems that result in saleable fertilizer products that can be transported off-farm.
While it takes time to observe significant improvements in water quality, the Iowa Nutrient Reduction Strategy has laid a foundation for long-term change. The 10th anniversary is an opportunity to acknowledge the collaborative efforts and the positive impact achieved thus far, while recognizing the need for continued commitment and innovation to further reduce nutrient pollution and safeguard Iowa's water resources.
Manure management is improving:
- Equipment accuracy is increasing.
- Manure is moving further from farms.
- Phosphorus buildup in soils is being reduced.
These changes are improving agricultural efficiency and helping to protect water quality; however, all of these changes must be practiced to a greater extent. Moreover, manure application and field management decisions that take advantage of these improvements in manure must be made in every part of our cropping system. While lower disturbance manure injectors are a great innovation – they only make a difference if tillage is reduced or eliminated as part of their use.
The practices we’ve highlighted have laid the groundwork for rapid improvement at field-level nutrient loss, but we must continue to innovate, adapt and evolve. Great possibilities still exist for improvement in manure use. Manure application timing must move closer to periods when growing crops are present. Such a change would reduce nitrogen loss and increase confidence in manure nutrient-supplying power, improving application rate selection. Similarly, cover crops are effective, but larger implementation is needed.
We mention these practices not because they are easy – they are difficult to implement and require better infrastructure, improved support and technology, but they are obtainable. In the next 10 years, we must celebrate the high adoption levels of these practices.
Shareable photo: 1. Injection of manure into the soil reduces odor, runoff, and provides nutrients for uniform crop growth. 2. Testing manure application equipment for the distribution of manure to each outlet where manure is injected into the soil. This improves the uniformity of the manure evenly across the field. 3. Cover crops in a field in central Iowa. Cover crops effectively improve soil health, reduce soil loss, and reduce nitrogen loss to tile lines.