Applying livestock manure based on nutrient content is one factor involved in using manure more effectively. There are two main challenges to sampling manure for a nutrient analysis, determining when to sample and then collecting a representative sample. Ideally, a manure sample is submitted before application and the results are used in calculating the field application rate. In practice, this is difficult, especially for liquid manure systems that require agitation before application. In reality, manure is easiest to sample at the time of application, when it is being loaded and hauled to the field.The main disadvantage of this time period is that the results are not available to guide the present application. However, manure nutrient values typically remain fairly consistent and constant within a farm, provided the livestock production system does not change significantly between years. In this case, the analysis results can serve to guide future applications. Annual manure sampling across manure types will allow the farm to establish baseline nutrient values.
The second challenge is collecting a representative sample to send to the lab. The small sample sent in to the lab must accurately represent many tons of manure in a solid system or thousands of gallons of manure in a liquid system. In order to accurately represent the quantity of manure being applied, it may be necessary to collect several different samples throughout the hauling and application period. Manure sampling guidelines are similar to recommendations for soil and forage sampling. Take several subsamples, combine them together, mix, and take a composite sample to send to the lab. Typically, samples are sent to the lab in either plastic bottles (liquid) or one-gallon heavy-duty zip-lock bags. Often, labs will provide the containers. The next question is how is the sample to be collected? What is the sampling procedure and what tools are needed?
For solid manure systems, sample while the spreader is being loaded or when the manure is being spread in the field. Collect samples that represent the beginning, middle, and end of the process. If sampling during loading, use a plastic bucket to collect a representative sample of what is going into the spreader. Try to collect at least five samples (more is better) during the application process. As each bucket is collected, empty it on to a tarp or a clean surface. Mix all the samples together thoroughly and take a subsample from the composite mix that will be sent to the lab. To sample during spreading, lay out a tarp or sheet of heavy plastic in the field. Collect the manure from the tarp after the spreader has passed over or by it and place the manure in a bucket. Repeat this for at least five loads. Once again, mix the different samples together and then collect a representative subsample from the composite mix to send to the lab.
In liquid manure systems, the pit or lagoon must be agitated to get a uniform sample. Depending upon the size of the pit or lagoon, the agitation equipment, and the objective of the manure application, the agitation process can take several hours or even the better part of an entire day. Without adequate agitation, nutrients are stratified. This has implications for both field application rates and sample results. Sampling directly from the storage structure is usually more difficult and causes more variation in the nutrient analysis results than when sampling directly during loading into the spreader. University of Massachusetts Extension has a publication entitled “Sampling Dairy Manure” that describes how to sample dairy manure, including how to make a PVC sampling probe that can be used to sample directly from manure lagoons. The key is to sample from multiple locations around the lagoon and to the full depth of the lagoon. Mix those subsamples in a bucket and collect a representative sample to send to the lab.
Sampling during loading is similar to the procedure for solid manure. Collect at least five samples during the process of loading the spreader. Save these samples in a separate bucket and when finished collecting samples, mix thoroughly and obtain a representative sample to send to the lab. For liquid samples, if the sampling process is going to occur over a period of hours, keep the subsamples on ice to prevent ammonia losses. To sample during spreading for surface application spreaders, place buckets around the field to collect samples. Place buckets to collect samples from multiple spreader loads. Collect samples after each load, keep them on ice to prevent ammonia loss, combine samples, mix thoroughly, and obtain a representative sample to send to the lab.
Penn State University Extension has a very good publication entitled “Manure Sampling for Nutrient Management Planning” that provides information on sampling procedures, sending a sample for analysis, lab test results, and use/interpretation of results. It is available online. Manure sampling requires some forethought and effort to get a reliable nutrient analysis, but it is an important component of a nutrient management plan.