Randomness Rules: Living with Variation in the Nutrient Composition of Concentrate Feeds

A fundamental rule regarding diet formulation is that you will never know the true value of anything. Although we have reasonably accurate estimates of the average requirements for most nutrients, we have less certainty regarding nutrient requirements of a specific herd or animal under specific circumstances. We have equations that accurately estimate the average dry matter intake for groups of cows but we are not very good at estimating how much dry matter a specific cow is eating. We have developed several good analytical procedures to measure the concentrations of many nutrients in feeds and tables are available that contain the average nutrient composition of all feeds commonly fed to dairy cows. However, biological and manufacturing variation, variation caused by sampling, and variation in analytical measurements can be substantial so that concentrations of nutrients within a specific feedstuff may be quite different from the average. Does all this uncertainty mean that we should give up on ration formulation and feed analysis? The answer to that question is obviously, no. However, the uncertainty associated with feed analysis and ration formulation must be understood and addressed. With proper sampling techniques, adequate number of samples, and appropriate data handling, you can reduce the uncertainty associated with feed analysis data. The objective of this paper is to discuss expected variation in feed composition, factors affecting that variation, and methods you can use to increase the reliability of feed analysis data. Although much of the following discussion is appropriate for all feeds, discussion will be limited to grains and byproduct feeds.