Nutrition: Revisiting Forage NDF

June 5, 2011 08:11 PM
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JIM LINN is an Extension dairy nutrition specialist at the University of Minnesota.

**Extended comments are highlighted in blue.

Forage quality and digestibility are important factors in keeping the cost of feeding dairy cows low and milk production high. Forages provide both structural and chemical neutral detergent fiber (NDF) in dairy cow diets. Structural NDF is important for cud chewing and good rumen health, while chemical NDF provides energy for milk and milk fat production. Therefore, I thought it would be good to review some guidelines and principles for NDF in dairy diets and look at some nutritional factors that influence fiber balancing in diets.

For lower feed costs and highest milk production, forages should be above 50% of diet dry matter. Higher-quality forages require higher feeding amounts than low-quality forage to meet fiber requirements. For example, 1 lb. of straw with an NDF of 76% can substitute for 2.25 lb. of 190 RFV hay with an NDF of 34%.

Substituting straw for hay creates a void of 1.25 lb. that needs to be filled by another feed. High-fiber byproduct feeds should be the first choice. If corn grain is used, the starch and nonfiber carbohydrate content of the diet can become too high, causing acidosis and other problems.

Bonus Content

A minimum of 28% of the diet dry matter should be NDF. The optimum level varies; if quality and particle size of the forages are compromised, NDF should be closer to 32%, with more forage and/or fibrous byproducts contributing a significant portion of the NDF. Because the NDF in byproduct feeds and high-quality forages is highly digestible, a high total dietary NDF concentration will have little to no impact on DM intake.

Forages are the major source of long fiber particles that stimulate cud chewing and maintain the rumen mat. Use a particle size box to calculate a physical effective NDF (peNDF). Excessively long forage particles (over 3") increase rumen fill and decrease DM intake. Conversely, very short or fine particle size forages (less than 1") can contribute to rumen acidosis. A peNDF of about 22% to 25% of dry matter is recommended for lactating cows.

Many nutritionists use forage NDF as a reference for forage amount in diets. Numerous studies have shown that as forage NDF in the diet increases, DM intake decreases. Total NDF in hay crop forages is highly correlated with maturity; as NDF increases, dry matter intake and digestibility of NDF usually decrease. This relationship isn’t as true for corn silages. Thus, forage NDF in high hay crop forage diets will be better related to DM intake than in high corn silage diets.

For most diets, forage NDF should be 20% to 21% of dry matter, with a 18% to 22% range preferred. When feeding less than 20% forage NDF diets, total NDF should be increased, with digestible fiber byproduct feeds substituting for forages.

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