Published December, 2011

Good-quality hay is an important component of a horse's diet, particularly in the winter when fresh pasture is not available.

One way to evaluate hay quality is to have your hay analyzed. Many different types of analyses are available, and the cost can range from $10-20 per sample to more than $100. The most extensive analysis will report more than 20 different nutrients or chemical fractions.

Although this detailed information makes sense to nutritionists, it is often not helpful to someone who wants to compare the values of different hays, or who wants an overall assessment of the hay's nutritional value. To simplify the interpretation of hay analyses, nutritionists have come up with equations to give more general estimates of forage quality. One estimate of quality is relative feed value (RFV).

RFV was developed for cattle and takes into account the expected digestibility of the hay as well as the expected level of consumption. Basically, the assumption is the better the hay quality, the more easily the animal will digest it and the more it can consume. Digestibility and intake estimates are calculated from the concentration of neutral detergent fiber (NDF) and acid detergent fiber (ADF) in the hay. As a forage plant matures, it becomes more fibrous and stemmy, which results from increased NDF and ADF concentrations. So, as NDF and ADF increase, RFV decreases.

The table below shows the composition of some common forages used for horses in Kentucky. The RFV is calculated from the equations developed for cattle (no equations currently exist for horses).

Ideally, you would like to know that the second hay provides 25% more value than the first hay, but RFV estimates are not that accurate for horses. Nonetheless, if the two hays are the same price, the one with the higher RFV will usually provide more nutrition for the dollar. RFV does not exactly apply to horses, but it can provide a basis of comparison between different hays, particularly of the same type. For example, if you had a choice between a full-bloom alfalfa hay with an RFV of 100 or a mid-bloom alfalfa hay with an RFV of 125, you could be pretty certain the latter hay would have a higher feeding value.

So what RFV of hay should you purchase?

The answer to this question depends on the type of horse being fed, as well as the price of the hay. Hay with high RFV is most appropriate for horses with high nutrient requirements, such as lactating mares, growing horses, and some performance horses. The higher a hay's RFV, the less you will need to feed. In addition, as the RFV of the hay increases, the amount of concentrate (sweet feed, grain, or pelletized feeds) needed will decrease. So, when choosing between two similarly priced hays, the one with the higher RFV might reduce total feed costs by reducing the amount of concentrate that is fed.

However, hay with a very high RFV isn't necessarily the "best" hay for every horse. If a mature idle horse with low nutrient requirements is given unrestricted access to hay with high RFV, he will probably become very fat. At a normal rate of intake, hay with a moderate RFV might be best for idle, mature horses. Hay with moderate or low RFV value might be desirable for horses with low nutrient requirements, because they can eat enough to satisfy their hunger without gaining too much weight.

Although RFV can provide a general idea about forage quality, it does not give an estimate of how closely the hay will satisfy an animal's nutrient requirements. RFV is calculated only from the amount of fiber in the hay and does not address the amount of protein, calcium, phosphorus, or other important nutrients horses need.

RFV is a calculated value from a laboratory analysis of fiber concentrations in a hay sample. It does not provide any information about the hay's cleanliness. Regardless of RFV, horse hay should be free of dust and mold. Horses are susceptible to respiratory irritation from moldy and dusty hay, and this irritation can affect exercise performance. Therefore, when purchasing hay, both a visual inspection and a laboratory analysis of the composition should be considered in evaluating its value.

Laurie Lawrence, PhD, professor in the department of animal and food sciences at the University of Kentucky, provided this information.