Interpreting hay analysis results
Fiber - Lignin, ADF, and NDF
The fiber, or structural carbohydrate, content of a hay can be divided into three separate components:
Lignin is a complex carbohydrate that is indigestible to the horse and is so slowly broken down by microbes that it makes essentially no contribution to the horse's nutritional needs - although its bulk is essential for digestive health and function. Good quality grass hays typically have a lignin content of 4% to 6% (as sampled).
Acid Detergent Fiber (ADF) includes lignin and cellulose, which is a slowly fermentable complex carbohydrate. Although the microbial fermentation of cellulose in the large intestine does contribute some calories to the horse's daily intake, the ADF content of a forage is inversely proportional to its crude protein content, palatability, and 'chewability', so I usually recommend choosing a hay whose ADF is less than 40% (as sampled).
This threshold is particularly important in senior horses whose teeth aren't as efficient as they once were, and in horses of any age with missing, damaged, or otherwise defective cheek teeth (premolars and molars).
Finding a hay with an ADF less than 40% usually isn't very difficult, as long as you're looking at "horse hay" (hay grown for horses). Hay grown for cattle, particularly beef cattle, or for small ruminants (sheep, goats) often has a higher ADF than horse hay.
Although it's a crude estimate, you can make a rough guess about the hay's ADF in advance of the test results by how stemmy and coarse it is in relation to other hays of that variety. The later in the plant's growth cycle the hay was cut (i.e., the more mature the plant), the thicker its stems and the higher its ADF will be, because lignin and cellulose are structural components which give the stem its strength and resistance to bending and breaking. But that's also what makes it difficult for the horse's teeth to break apart and get to the nutrients inside.
Neutral Detergent Fiber (NDF) includes lignin, cellulose, and hemicellulose (another type of slowly fermentable complex carbohydrate). Put more simply, NDF = ADF + hemicellulose. Hemicellulose is more readily broken down by the gut microbes than ADF,* but it is still a relatively slow process that does not contribute to insulin resistance, laminitis, or colic risk the way the more rapidly fermentable carbohydrates can do, so we want the NDF of a good quality grass hay to be between 50% and 60% (as sampled).
Healthy adult horses can easily manage a NDF content in the low-60s, but I don't like to see the NDF above 60% for seniors or horses with dental problems, nor above 65% for any horse. Palatability and nutrient content tend to decline at that point.
Click here for a chart that makes all this a bit more clear.
* Adult horses on hay-based diets may get more than 80% of their maintenance energy needs met from the microbial fermentation of NDF in the large intestine. [Nutrient Requirements of Horses, 6th edition, National Research Council, 2007.]
What about crude fiber?
Crude fiber (CF) is a measure that varies by feed type, and even by lab (different labs may define it and thus analyze it differently). It typically includes most of the cellulose and some of the lignin in forages, so CF is broadly similar to ADF, although CF values tend to be lower than ADF values for the same feed type.
You'll still see CF values on bagged feeds, including hay pellets, cubes, and chopped forage. By convention, they are all Dry Matter values, so it is easy to compare products when looking for a low-carb forage for a carb-sensitive horse. As a rough guide (although far from foolproof), the higher the CF, the lower the content of nonstructural carbohydrates. When in doubt - and when making a mistake could cause laminitis or colic - test the forage before buying it or choose a product whose starch and sugar content are already known. (Increasingly, manufacturers and suppliers are testing their forage products for the starch and sugar content, so ask and ye may well receive.)
Fiber - Lignin, ADF, and NDF