When To Sample Manure For Analysis

Crop Life. 23 September 2014.  Collecting a representative sample for analysis is the single most important factor affecting the accuracy of manure nutrient content.  Obtaining a representative sample is also the most challenging aspect of the manure sampling process. Keep in mind that the sample must represent the actual manure being spread. If sampling is not done correctly, the results of the analysis can be worse than having no analysis at all.

Because only a small amount of manure is sent to the laboratory for analysis, it is imperative that the sample represent the average composition of the manure being applied. The two critical aspects of sampling are the timing of sampling with respect to the manure’s application and the ease or difficulty of the required sampling procedure. Both of these factors are related to variations in nutrient content that occur over time or are present in manure based on how the manure is stored and handled.

Even on a single farm, both weather and management can affect the nutrient composition of manure. Seasonal variations in temperature and precipitation can change nutrient content through dilution, evaporation and volatilization, particularly in uncovered storage and stacks. Manure stored in barns as bedded pack or litter or stored in covered stacks is typically not affected by weather, but there is often significant variation in nutrient content throughout the manure based on the uneven mixing of bedding, hay and/or spilled feed. Finally, a single liquid manure source can have a large variation in nutrient content if the manure is not thoroughly agitated and mixed before spreading.

When is the ideal time to collect a manure sample? Because the goal is to collect a sample that represents the manure actually being applied, the best time to sample is during loading or field application. The sample can be obtained during loading of manure application equipment or in the field as the manure is being spread. Sampling at this time has several advantages:

  • The time-related changes in nutrient content caused by management and weather are minimized.
  • The non-uniformity due to lack of mixing is reduced. Subsamples can be taken as the manure is loaded, which results in more representative samples.
  • The difficulty of collecting representative samples while manure is in the storage, barn, or stack is reduced.
  • The complexity of the sampling equipment required is reduced.
  • In some cases, the sampling procedure is safer,reducing the risk of falling in or being overcome by gases.

There is one disadvantage to sampling during spreading: the analysis results from samples collected at this time will not be available to calculate manure application rates for that application. However, the results can be used to calculate future application rates. It is recommended that the manure nutrient content values used in calculating manure application rates be based on running averages or baseline values. To obtain these values, each manure group should be sampled annually for three to five years. After the initial period, manure can be sampled periodically to monitor the nutrient values. If there are changes in feeding programs or manure storage and handling, the manure should be re-sampled. As long as no significant changes are made in the production system, the nutrient content of the manure should remain fairly constant.

Some states may have specific regulatory requirements related to manure sampling that may vary from the general guidance provided above and elsewhere in this fact sheet. When developing a manure sampling program, farmers, consultants, and nutrient management planners should learn and integrate their state regulatory requirements with the guidance outlined in this fact sheet.