WSDA has a helpful Manure Sampling Guidelines webpage (external link).
Laboratory analysis of manure nutrient content is an essential component of crop management planning that can improve farm profitability and the environment. Manure analysis is also necessary for application rate planning. Estimates of manure nutrient content are available from a number of published sources, but nutrient composition varies widely between farms due to differences in animal species and management, and manure storage and handling. The only method available for determining the actual manure nutrient content for your particular operation is with laboratory analysis. Since such a small amount of material is used for analysis, it is essential that the sample submitted represent the manure that is applied to the fields. The following guidelines outline proper technique for obtaining samples as well as handling and transport.
When You Should Sample
Liquid manure samples can either be taken at hauling time, or directly from the waste lagoon. For safety and best results, it is recommended that manure be sampled during hauling. The sample will be more accurate because it has been mixed and safer to obtain. The only drawback to using this method is that the results of this year's sample cannot be used to plan this year's manure applications. This is because samples are taken during application periods and lab results can take up to a week to be returned. Additionally, manure results will vary throughout the year as dilution or nutrient influx occurs. However, manure test results can be used to adjust application rates made later in the year and estimate next year's manure nutrient content, if all variables (i.e., cow numbers, rainfall, inflow sources, etc.) stay relatively the same.
For systems that are emptied or cleaned out once per year, a good rule of thumb is to sample and analyze manure each time it is applied. This is especially true for uncovered lagoons, pits, basins, and stacking slabs. Variations in temperature and precipitation between years will affect changes in nutrient content due to dilution, evaporation, leaching, and volatilization. Manure from under-barn concrete pits or covered above ground tanks will not vary as much between applications, unless the type of animal or other factors change (i.e., species, size, feed, bedding, confinement). Systems emptied twice per year or more may differ between application times, so a fall analysis may not be accurate for planning spring applications.
Sampling from Tanker Truck
For best results, sample the manure that is in the tank or spreader box being delivered to the field for application. Such samples will be the most representative because the liquid manure is likely to have been agitated in order to load the tank, and semi-solid manure scraped from the barn is moderately mixed after being loaded into a box spreader. Ideally, the sample submitted for lab analysis should be a mixture of manure taken from several loads (5-10) representing the beginning, middle, and end of the application process. This will help ensure that manure samples accurately represent the manure being applied.
For liquid manure, retrieve samples with a plastic pail or a coffee can on a pole from the top of the spreader if possible. Otherwise, a sample can be taken from the bottom unloading port. Sampling should be done immediately after filling. After collecting 5-10 sub-samples in a 5-gallon bucket, mix sub-samples thoroughly. From this mixture, fill a clean/sterile sample jar 3/4 full (allowing room for gas expansion), wipe off sample jar, and seal in a bag. Be sure to write the date, sample number, and any other pertinent information on the jar. Manure samples should be stored in a cool, dark place and sent to the laboratory within 12-24 hours of collection. If this is not possible, samples should be frozen prior to mailing in order to slow down any microbial activity, and mailed early in the week so that they will not remain in the post office over the weekend.
Sampling from a Storage Lagoon
An alternative to sampling manure during application is to sample manure from the storage structure prior to application. However, sampling from storage lagoons is not as accurate as sampling during application, and it can be more dangerous. For these reasons it is recommended that manure be sampled during application. If time and management practices prevent this, manure can be sampled from the storage structure.
Lagoon samples should be taken with a plastic pail or a coffee can on a pole from varying locations and depths of the lagoon. If a can or other container cannot be attached to a long pole and dipped into the pit or basin, an alternative method is to use a 1/2" or 3/4", 8-10-ft. PVC pipe. Push the pipe into the manure, avoiding the bottom foot of the structure. Place a hand over the top opening to seal the pipe, pull up, and empty into a bucket. Do not collect samples from areas that have uncharacteristic crusts or floating chunks on the surface. Collect anywhere from 10-20 sub-samples in a 5-gallon bucket, mix sub-samples thoroughly. From this mixture, fill a clean/sterile sample jar 3/4 full (allowing room for gas expansion), wipe off sample jar, and seal in a bag. Be sure to write the date, sample number, and any other pertinent information on the jar. Manure samples should be stored in a cool, dark place and sent to the laboratory within 12-24 hours of collection. If this is not possible, samples should be frozen prior to mailing in order to slow down any microbial activity, and mailed early in the week so that they will not remain in the post office over the weekend.
Solid Manure Sampling
Stacked, piled, or bedded pack manure can also be sampled in storage, and an average nutrient content estimated. Using a pitch fork or shovel, take 10-20 sub-samples from different depths and locations in the pile or pack. Place the sub-samples in a pile, mix, and place a composite sub-sample of the pile (avoid large pieces of bedding) into a large airtight bag. You may want to take two or more composite samples for analysis, and use the average of the results. Write the sample number, date, and other pertinent information on the bag.
Keep track of all manure analyses from a particular storage system over a period of time. This will help you determine both the necessary frequency of sampling and an average nutrient content to use for manure application planning. Remember that manure should always be resampled if you change any component of your animal management or manure storage or handling system.