Q. I recently bought tested hay, so I got to review the nutritional analysis. The report includes two data columns. One is titled “As Fed” and the other “Dry Matter.” What do these terms mean, and how should they affect what I feed my horse?

A. Lucky you for being able to purchase tested hay. Typically, labs report analysis results on an as-sampled, as-received, or as-fed, as well as a dry-matter basis. As-fed or as-sampled mean exactly that: They are results describing the sample in its natural state, or in the form you would feed it. This means the sample contains moisture. On the other hand, samples on a dry matter basis have been analyzed after having moisture removed. The analysis sheet should tell you the feed’s moisture or dry matter content.

Water dilutes the results of a test, so the feed’s nutrient value always looks higher when reported on a dry matter basis. Comparing results from different feeds on a dry matter basis is more accurate because the amount of water in a feed can vary, meaning the results are diluted by different amounts. When looking at a feed on a dry matter basis, the levels of various nutrients will always be more concentrated or higher than the same sample on an as-fed basis.

The guaranteed analysis on a feed tag is given on an as-fed basis because we’re interested in the feed’s nutrient content in the form in which we feed it, which includes moisture. Most of the time when creating equine rations, we work on as-fed basis because this is what gets fed to the horse. Many feeds, such as hay, are relatively high in dry matter, so in reality the dry matter and as-fed values are similar. If providing feeds with high moisture content, such as fresh pasture that might only have a dry matter content of 15%, consider focusing on dry matter values, because horses need to consume a certain amount of dry matter each day to maintain gut function.

The typical rule of thumb is that horses should consume 1.5-2% of their body weight per day on a dry matter basis. This means the wetter the feed, the more of it they need to consume to meet this target. For example, if an 1,100-pound horse needs to consume 2% of body weight as dry matter each day, this would equal 22 pounds of dry matter. He might meet this requirement by eating 24.4 pounds of a hay with a dry matter content of 90%, or 146 pounds of pasture with a dry matter content of 15%.

If you’re interested in what your recently purchased hay will provide your horse based on an amount you are feeding each day, focus on the as-fed values, because that correlates most closely with what you are doing.