Nutritional value of pasture forages (grasses and legumes)

Nutritional value of forages changes rapidly throughout the growing season.

When harvested for hay, the value can be determined by sampling the hay and sending it off to a certified laboratory. See - Testing Hay, List of Labs, and Understanding Your Forage Test Results for details on testing hay. Another helpful fact sheet it Western Oregon Hay (LFC0610).

You could also sample pastures for lab analysis, but the assessment will be outdated by the time you receive the results. Instead, you could visually assess the maturity of the grasses and legumes to estimate the nutritional value.

Young, fast growing grass is in the vegetative stage and has very high nutritional value. As plants mature they become more fibrous (acid and neutral detergent fibers, ADF and NDF, respectively), and less protein (crude protein, CP) and energy (total digestible nutrients, TDN). Once they are fully mature the forage is in reproductive stage (seed heads milky, dough-like, or hard seed) and the value is relatively low. In between these two stages is the transition phase (boot stage) where the developing seed head swells and can be felt within the stems. See Table 1 for grass morphology and nutritional value.

Young legumes (alfalfa and clovers) have higher nutritional value than mature legumes. Flowering characteristics are used to describe the morphology of the plant. Buds are the youngest, then first flower (FF), mid-bloom, full-bloom, and then post bloom are the stages on which to key in on. Table 2 has the values for alfalfa hay and can be used as a guideline for other legumes (clovers).