By Samantha Teten
There are many challenges associated with variable rate manure applications such as variable nutrient content within the manure source and nutrient availability, application equipment, and prescription methodology. However, in many cases, these challenges are outweighed by environmental and economic benefits from variable rate manure applications. The following will explore some potential solutions to these challenges in fields of wide soil spatial variation and demonstrate how variable rate manure applications can be beneficial.
Manure Impacts on Soil and Water Quality
Manure is a great source of P which is needed by crops. Manure contains high levels of phosphate which can have negative environmental implications if not correctly managed. Phosphorus is not very mobile in the soil, which reduces the risk of P leaching into ground water, but high soil P increases the ability for P to leave the field with eroding soil particles, thereby polluting surface water. Many years of over-application of manure can create an environmental risk of phosphorus water contamination. In addition, over application of manure over time can have potentially negative impacts on crop productivity as excessive amounts of phosphorus can limit the availability of micronutrients to the crop.
The nitrogen found in manure can also cause environmental concerns. Manure contains organic nitrogen that can be released over time. If nitrogen supplied through manure is not accounted for in subsequent N applications, over-application of nitrogen will occur. Nitrate is easily leached through the soil profile, especially in sandy soils, which pollutes ground water. Nitrogen can also run-off the soil surface if conservation methods are not in place and if precipitation is not easily infiltrated into the soil. High levels of nitrates in water are a health concern to humans if not maintained below 10 ppm and an ecosystem concern.
Advantages of Variable Rate Manure Applications
With over-application being a concern for the environment and a concern for a producers’ profitability, and under application limiting crop growth and production, the site-specific application of manure provides a significant amount of value. In a study at Iowa State, the variable rate in comparison to the flat rate manure applications in the corn and soybean rotation had no significant changes in yield but resulted in other benefits. The variable rate application resulted in a greater total amount of phosphorus being applied to the field. For many operations, this is beneficial as there is a demand for land close to the manure source that can have manure applied and remain at or below appropriate P thresholds. Varying rates of manure are often determined by dividing a field into management zones and then recommending a rate per management zone based on soil test values within that sub-field area. There are also cases where the total amount of manure applied to the field would decrease with variable rate applications; however, this means there are fewer areas in the field that are over appropriate nutrient thresholds that could be harmful to the environment. Overall, the study found that the variable rate approach allowed manure applications to better meet crop phosphorus needs (Mallarino, A. P., Wittry, D. J. 2010).
Strategies for Developing Variable Rate Manure Applications
Management zones for manure applications can be determined by several different methods. Some common methods include creating zones based on phosphorus soil-test levels, yield-goals, or soil-type. Phosphorus soil-test levels is one of the most common ways to determine the amount of manure that can be applied to a field and provides a very accurate method to making a variable rate prescription. However, this method requires high resolution soil samples (2.5 acre grids), which can be costly and a logistic challenge in the busy time of year between other field operations and manure applications.
Phosphorus grid sampling can also be used to ensure that levels do not exceed the maximum threshold. Another method is a yield-based method where historical yield data is used to delineate management zones. This is a better option if the manure rate determination method is based upon how much phosphorus the crop is estimated to remove with average yields, also known as the crop removal estimate. A potential challenge with this method is that fertilizing based on what the previous crop removed will only maintain soil nutrient levels. Because this methodology uses estimates based upon yield and how much P was removed with the grain, if the soil P levels are historically high or low this recommendation will not correct these imbalances values and will keep them the same over time.
Challenges with Variable Rate Manure Applications
Often cited challenges with variable rate manure applications are logistic limitations and equipment. Updating equipment with the additional sensors and flow meters necessary for a variable rate application is a considerable cost that must be considered for adoption. As for logistics, taking the time to soil test and make prescription maps may take time, but with careful planning or hiring a consultant to assist, this challenge can be overcome.
The precision application of variable rate manure is a challenge as the nutrient content within the manure and the nutrient availability from decomposition is inconsistent. The nutrient content in manure can also vary based upon feeding diets and weather conditions (Koelsch, R. K. et al., 2018). Variable manure nutrients are especially challenging for manures harvested from open lots. One potential solution to this is compositing the manure prior to the application; composting decreases the manure nutrient variability, decreases the biological processes occurring after application, and ultimately reducing variability in the manure nutrient content (Modderman, C. 2019).
In addition to varying manure nutrient content, soil conditions also influence nutrient availability due to varying moisture conditions. The same factors that can create different management zones also influence the decomposition rate of the organic compounds in manure, such as soil type, water drainage, and slope. This makes the estimation of nutrient demand to apply very difficult and decreases the reliability of the variable rate recommendation. A possible solution to this challenge is to use manure to apply a base nutrient rate and then supplement the field’s nutrient requirement with a side-dress variable rate fertilizer based upon in-season plant or soil testing.
Figure 2: Comparison of fixed rate and variable rate manure applications in difference management zone scenarios
Another agronomic concern with variable rate manure applications is the variability this can introduce in other nutrients or soil organic matter. For instance, if the manure rate is varied based upon soil-test phosphorus levels, the rate of nitrogen supplied in that manure will also be varied and potentially not aligning with the crop demands. An example of this is shown in Figure 2 where areas of low soil test P can have more manure applied and areas of high soil test P can have less manure applied in a variable rate application. As a result, this varied nitrogen supplied across the field from the different manure rates can be difficult to estimate and makes a second supplemental application of nitrogen fertilizer challenging to prescribe. However, there are some promising technological developments in sensor-based nitrogen applications that could help estimate the crop’s nitrogen demand in-season to account for this variability. With this technology, areas with a slight nitrogen deficiency will trigger the sensor system to apply more nitrogen (Maresma, Á. et al., 2016, Aranguren, M. et al., 2019, Schmidt, J. P. et al., 2009).
Conclusions and Resources
In conclusion, variable rate manure application technology has come a long way, but adoption remains low due to many challenges. However, the benefits of variable rate manure applications on both the environment and producer profitability provide a value that in many cases overcome these challenges. For more information on manure management strategies, go to manure.unl.edu or consider participating in Nebraska On-Farm Research to test this technology in your operation.
Source : unl.edu