Effective Fiber: Differences in Actual and Predicted peNDF

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The goal in formulating and feeding dairy rations is to match the ration on paper to the ration the cow actually eats. Moisture changes in forages, forage changes, inaccurate scales, and lax mixing procedures are some of the reasons they are not the same. We can send in a TMR sample to the lab to measure protein , fiber, starch, mineral content, etc. to see how close the formulated ration and the TMR actually are. But how effective is this fiber in the rumen? Most ration programs give an estimate of peNDF (physical effective NDF). This estimation uses fiber levels of our feed ingredients to predict the ration contribution to an adequate rumen mat in the rumen. PeNDF predictions gives us an idea of changes in fiber contribution, but not a good measure of actual peNDF.

We can use the Penn State Particle Separator to give use an actual peNDF value. There are three different versions with different calculations for each one.

Measuring actual peNDF in TMRs

3 “screen” – 2 Sieves and a pan
Add calculated percentage of top two sieves  plus 20% of bottom pan  and multiply by  the NDF of ration

4 “screen” – 3 sieves  (3rd sieve is a fine screen) and a pan
Add calculated percentage of top two sieves plus 30% of 3rd sieve and multiply by NDF of the ration

4 “screen” with new 3rd sieve having 0.16 inch (4 mm) openings. This new screen was designed to measure peNDF.
Add calculated percentage of top 3 screens and multiply by NDF of ration

How close is the actual to the predicted? 

If the actual peNDF is significantly lower than predicted peNDF, cow performance may be enhanced by raising the actual peNDF. For example; If our minimum goal is a predicted peNDF (using ash free NDF) of 18 and our actual is 16, we may want to try to increase our actual peNDF.

Possible reasons for differences in actual and predicted peNDF:

  • Chopped too short at harvest
  • TMR mixing time too long or wrong mixing order
  • Type/variety of forage
  • Inaccurate Scale
  • Difference in forage moisture on paper versus actual

Possible solutions:

  • Adjust chop length at harvest – long term solution
  • Reduce mixing time and/or change mixing order
  • Add straw or hay
  • Make forage moisture adjustments
  • Check TMR scale accuracy

Differences in actual and predicted peNDF are more significant when fiber levels are marginal and/or very digestible forages are in rations. The key is to use both calculations to measure improvement in effective fiber. If you’re concerned about improving effective fiber in your cow’s diets, contact your ANC consultant to help you find solutions to improving digestible fiber.

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