Two characteristics distinguish alfalfa hay from grass hay. First, alfalfa hay typically has lower concentrations of neutral detergent fiber (NDF) than grass hay. This trait places alfalfa hay in a better position relative to grass hay. On the flip side, alfalfa hay generally has a greater concentration of undegraded NDF (uNDF) on an NDF-basis than grass hay, which means that a unit of fiber from alfalfa hay is less digestible than a unit of fiber from grass hay. Therefore, the latter trait places grass hay in a better position than alfalfa hay. In the end, then, which hay is better?
A few years ago, we performed a feeding trial in which we included alfalfa hay or mixed-grass hay into diets of high-producing dairy cows. Not surprisingly, cows on the alfalfa-based diets consumed more dry matter and produced more milk than cows consuming grass-based diets. How could a better performance be possible when alfalfa hay had a higher concentration of uNDF than grass hay? The answer to this question has to be beyond uNDF.
Motivated by this intriguing question, we evaluated the relationship between the rate of fiber digestion and the rate of passage in a recent study. For this, we also fed dairy cows with diets containing either alfalfa hay or grass hay. The challenging part of the study (at least for us) is that we measured the fiber passage rate. Interestingly, cows consuming alfalfa-based diets had a faster passage rate of uNDF than cows eating grass-based diets. However, cows consuming alfalfa-based diets digested more of the potentially digestible NDF (pdNDF) than cows on the grass-based diets, despite the faster passage rate. Noticeably, cows consuming alfalfa-based diets also ate more dry matter than cows consuming grass-based diets.
As a concluding message, alfalfa forages pass faster than grass forages. However, the digestion of the potentially digestible fiber is much faster for alfalfa than for grasses. Therefore, cows consuming alfalfa can perform better than cows consuming grasses, regardless of the higher uNDF concentration typically observed in alfalfa.
