This was a topic recently discussed by Dr. Bill Weiss, from The Ohio State University. Work they recently conducted showed that sampling error can vary widely between feeds and between farms. Work they did in Ohio, using a very strict set of protocols for the sampling process, showed more variation in results than most nutritionists (and producers) would like to see.

Variation in dry matter is something that can be tested on-farm with a Koster Moisture Tester, microwave, or other method. As fed weights (when feeding by weight with TMR feeding systems) can easily be adjusted to account for those changes. But, what do you do when the actual nutrient content changes? Do you have your nutritionist adjust your ration every week? That answer really depends on whether or not your forages did actually change or was the variation a result of sampling error?

Actual change is often easily recognized by producers because they know feed came from a different field or was a different variety. How the feed is stored can impact changes at feed out. Feed differences in upright silos and especially Ag. bags will be more apparent than with bunker silos, where differences between fields are often masked because the feed is blended or layered when filling occurred.

So, is this something you should be concerned with? When feed obviously changes it should be resampled. How about when the feed is all the same? Based on the work in Ohio it seems like a good practice to follow (to be sure your results are accurate) is to sample two times a week for two weeks and then average the results. A bit extreme? Maybe, but take a few samples of the same feed in a short period of time and see what you get.