The Art of Sample Collection: Sampling Forages

Dr. Duarte Diaz
USU Extension Dairy Specialist

In the previous article in this series (May, 2006 Dairy Newsletter) we discussed the basic principles of sample collection and the importance of collecting a representative sample. Now we will discuss techniques for sampling forages.
In general, forage sample collection is more difficult than sampling concentrates or TMR�s. Tools like silage and hay samplers are available to facilitate the sample collection process and to help minimize sampling errors.


Pasture sampling is the most complex of all forage sampling procedures. Samples should be collected randomly in either a Z or N pattern, with samples taken from at least 8-10 locations. Remove the forage at grazing height, mix all samples thoroughly and collect a sub-sample for analysis. If there is selective grazing occurring you may need to select samples from the locations where animals are actively grazing, rather than at random. These samples should be either dried quickly or frozen to minimize fermentation. Careful attention should be taken when drying the samples since drying at too high a temperature or for too long will affect the composition of the sample.


On a previously opened trench silo samples should be collected as silage is being removed, preferably as it is being mixed in the mixer wagon (if the silage is the only feed in it). Otherwise 6 to 10 spots should be selected in the face of the silo (depending on the size of the silo) and samples should be collected after digging some 8 to 10 inches in to the face of the silo. Samples should then be mixed thoroughly and sub-samples collected and frozen until analysis.

If the silo is not opened, samples should be collected from the top. Several holes (4 to 10 depending on the size of the silo) should be dug from the top with a silage probe making sure you go deep enough to get a representative sample. Samples should be mixed thoroughly and a sub-sample collected and frozen until analysis. Holes should be plugged with the left over silage and the plastic repaired. If the silo is large and feeding from it takes a long time, it is advisable that the silo be tested every four to six weeks. Bag silo samples can be sampled utilizing this same method with samples taken along the length of the bag and making sure that any holes made in the bag are repaired after the sample is collected. Upright silos can be difficult to sample, so ideally samples should be collected as silage is being removed for feeding. Again, several samples (8-10) should be collected, mixed thoroughly, sub-sampled and frozen until analysis.

When samples are being sent for mold and yeast counts and/or for mycotoxin analysis more sample spots are required. Since fungi are not distributed evenly throughout the silage, we must increase the number of sample spots in order to collect a representative sample. The same can be said for silo that have different types of forages, or that took a long time to be filled and therefore were exposed to different environmental conditions during this period. Samples should not be collected from the portion of the silage that is not going to be offered to the animals (i.e., top spoiled layer).


Baled hay should be sampled utilizing a commercial forage sampler. Core sub-samples from 15-20 bales, selected randomly, should be collected, mixed and sub-sampled to obtain a representative sample. If the hay will be fed by its cutting (first cutting alfalfa, etc.) each cutting should be analyzed separately. For square bales, collect the sample from the end through the entire length, for round bales collect samples across the bale at the center. Place all core samples in a large container for mixing and sub-sampling.

Samples can be collected without the use of a forage sampler by removing small sections from each bale (15-20 randomly selected bales) and cutting the hay into 3 inch long pieces. These samples should then be mixed and sub-sampled. If this method is chosen, care must be taken to not exclude any part of the forage from being sampled, especially the leaves, which may be left out when hand grabbing the hay sample. The same procedures can be used for sampling loose hay, again making sure that all portions of the plant are included in the sample.

In the next issue I will complete this series of articles with a discussion of techniques for sampling concentrate feeds.

Dr. Diaz may be contacted at (435) 797-2163 or ©