What Are the Five Most Important Things to Measure in Hay Crops?

The most important analytical measurements for evaluating hay crops are those that are used to establish economic value or to meet the nutritional requirements of the animals to which it is fed. The objectives of this review are to identify and discuss the most important analyses that are important for determining the economic and nutritional value of hays for dairy cows. It is proposed that the five most important nutritional measurements for hay crops are: dry matter (DM), ash, amylase-treated neutral detergent fiber (aNDF), some measure of digestibility or energy value, and crude protein (CP). The DM of a hay crop measures the proportion of the as-is weight that is nutrients. Each percentage-unit of DM in a ton of hay represents 20 lb of nutrients that has economic and nutritional value. In general, we purchase and feed hay crops to provide energy, roughage, and to a lesser extent CP. Ash not only measures the portion of hay crops that provides no energy to the animal, it is an excellent indicator of soil contamination, which also dilutes the energy in hays. Fiber is the major organic component of hay crops that affects intake and digestibility. Of the fiber measurements that are available, aNDF is the best method for measuring the insoluble fiber that is most important in ruminant nutrition. The simple summative equation illustrates that aNDF and its digestibility (NDFD) are the primary factors determining DM digestibility. Estimates of energy value (total digestible nutrients – TDN or net energy – NE), which are calculated from chemical composition, are secondary measurements that provide no additional information about hay crop quality, although they are often needed for ration formulation. Calculated TDN or NE values are nothing more than a mathematical transformation of the original chemical measurements, especially when based on one constituent. They provide no additional information and should be left to the user to estimate so that they are calculated and used in the appropriate manner. Unlike calculated values, in vitro measures of DM, OM, or aNDF digestibility provide information beyond chemical composition that is useful in ranking hay crops and assessing energy value. Although the major contribution of hay crops is to the energy in the total ration, they also can provide significant CP. The amino acid balance of ruminally undegraded protein from forages does not have the biological value of many protein supplements, but the CP in hay crops provides value in the production of microbial protein during ruminal fermentation. Mathematical models can and will be used to account for the variability in intake and digestion among high producing cows. These models need inputs for the rates and extents of digestion of fiber and non-fiber carbohydrates in forages. In the future, measurements of digestion kinetics will become more valuable in the assessment of forage quality. Because indigestible NDF provides no energy and limits intake, it will probably have the greatest and most immediate impact on evaluating hay crops.