All species are most palatable and produce the best rate of gain during what might be called their green growth period-prior to heading out.  Since Montana's livestock industry is primarily a cow-calf operation, species are rated for fall and winter forage also.  Certain species are more palatable to livestock, have higher protein content, remain erect and above the snow, or in other ways are more desirable for fall and winter use than others.  Species are described using the following abbreviations: 

Sp = spring
Su = summer
F = fall
W = winter