Forage Fragility as a Measure of Effective Fiber

Published on: 13:08PM Mar 26, 2010

By Rick Lundquist, Ph.D.


Feeding cows is part science and part art. Nutritionists and dairy producers who know and study cows recognize that the art of feeding cows is the intangible that can’t be predicted by a computer program. Nutritional science continues to attempt to explain the art of feeding. Fiber digestion is one area that challenges nutritional science.


Dr. Dave Mertens of the U.S Dairy Forage Research Center defines physically effective NDF (peNDF, neutral detergent fiber) as the fraction of NDF that stimulates chewing and helps form the “rumen mat.” The “rumen mat” retains feed particles in the rumen, which increases their digestibility and can affect rumen pH, butterfat, acidosis and feed efficiency.


Physically effective NDF is measured by dry sieving the forage and measuring the portion of particles retained on the sieve. Diets with 21% to 24% peNDF of dry matter appear to promote the highest efficiency of 4% fat corrected milk production. But peNDF doesn’t explain all of the variation in chewing time. For example, we know that cows have to chew longer to process the same amount of NDF from oat straw compared to alfalfa.


Dr. Rick Grant of the Miner Agricultural Research Institute has attempted to take the prediction of fiber digestibility one step farther by measuring forage fragility. Fragility is defined as the relative rate at which forage is reduced in particle size during chewing. Alfalfa NDF is more fragile than oat straw NDF. Brown midrib corn silage contains highly fragile NDF.  


Fragility is measured in the lab by subjecting the forage to a ball mill filled with ceramic balls that mimic the grinding action of molars. Theoretically, a fragility factor combined with peNDF will better predict cow chewing responses. Their lab has conducted experiments that tend to confirm this. Diets were formulated to supply similar amounts of NDF from either wheat straw or grass hay. The grass hay and straw had similar peNDF’s (similar particle size), but the grass fiber had much higher fragility and had greater NDF digestibility than the straw. The straw diet resulted in about 30 minutes more rumination time per day than the grass diet.


For now, NDF digestibility is probably the best predictor of chewing and rumination time for most forages.  Fragility and peNDF may help improve this prediction. But, there’s still some art in feeding cows.


Rick Lundquist is an independent nutrition and management consultant based in Duluth, Minn. Contact him at [email protected].



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