The Art of Sample Collection: Basic Principles
Dr. Duarte Diaz
USU Extension Dairy Specialist
This is the first of a three part series on sample collection. In the future Dairy Newsletters I will write about proper sampling of forages and sampling of concentrates.
Let�s face it, nutrient analysis is a considerable expense in modern dairying. Unfortunately, many of us pay little or no attention to our sampling procedures. In general our submitted sample should be representative of our entire lot of the specific ingredient or of our ration (for TMR samples). If our sample is not representative, the analysis is invalid. In other words, our money is wasted since the information obtained from our sample is only representative of a portion of the feed, and not the entire amount. Whether it is you or your nutritionist collecting feed samples, a representative sample is the only way to be able to make correct inferences about the overall quality and nutrient value of that feed.
Accuracy and precision are two important points associated with any sampling and test analysis. Accuracy is defined as the closeness of measured values to the true value. Let�s imagine shooting a bow and arrow at a bull�s eye target. The center of the target or the bull�s eye is equivalent to the true value and the holes from the arrow represent the measured values. An accurate target practice will be one where most of the arrow holes are clustered near the bull�s eye. Precision (or variability) on the other hand is defined as the closeness of measured values to each other. Using the same target example to illustrate precision, the closeness of the holes to each other, independent of their closeness to the bull�s eye is a measure of precision. In an ideal test procedure we would like to increase both your precision and the accuracy of your test in relevance to the mean value of the entire lot by collecting the most representative sample.
Random sampling is the most important part of your sample collection. Every individual item in the lot should have an equal chance of being chosen. This means that just simply hand grabbing a sample as you walk past the feed bin on your way to some other task is insufficient. Let�s use cracked corn grain as an example of a hand grab sample. It will probably exclude most of the fine particles that have both settled to the bottom of the bin and the ones that filter out between your fingers as you close your hand. The biases associated with not sampling all sized particles equally greatly influences your accuracy. Because of biases like this, a sample should be an accumulation of many small portions taken from many randomly selected locations and utilizing a device (e.g., a shovel or a type of probe) that does not reduce the chances of any item (i.e., different size particles) in the lot from being chosen. The accumulation of many small samples is called a bulk sample.
In most cases your bulk sample will be larger than desired so it is recommended that you mix the bulk sample well and collect a sub-sample of the desired size utilizing the quartering method. In the quartering method you spread the mixed bulk sample (mixing well prior to quartering is very important) on a clean sheet of plastic or paper to form an even layer. Take a sample from two opposite quarters or all quarters depending on the size of the bulk sample until the desired sample size is obtained. Place your sample in a heavy plastic bag (zip-lock bags are good and even an A.I. glove will work when nothing else is available) and label samples with date, sample number and the information on the contents. Let�s not forget the sample preservation is also very important, so make sure that you either freeze (for high moisture ingredients like silage or green forage), or keep under cool and dry conditions (the trunk of the car in July is NOT a cool and dry location).
Because of the great importance of the information obtained from our feed samples, we must make sure that they offer us the most accurate and precise information. Remember a non-representative sample will invalidate any analytical method.
In the next issue I will discuss forage sampling recommendations.
Dr. Diaz may be contacted at (435) 797-2163 or Duarte.Diaz@usu.edu.