NDF Digestibility: A New Analysis to Evaluate Forage Quality
Advances in forage analysis are improving how we estimate the energy value of feeds and balance rations. Two newer concepts are becoming increasingly important:
- NDF Digestibility (NDFD)
- Relative Forage Quality (RFQ)
This summary focuses on NDFD, a measure that plays a major role in predicting animal performance more accurately than traditional fibre tests.
Why NDFD Matters
Energy is the largest nutrient requirement for livestock, but it cannot be directly analyzed. Traditionally, energy values such as TDN and NEL have been estimated from ADF, but this approach is limited because:
- Equations vary widely
- ADF-based predictions do not explain differences in animal performance
- Forages with similar ADF can differ significantly in digestible energy
The 2001 NRC Nutrient Requirements of Dairy Cattle recommended moving away from ADF-based energy prediction toward a summative system that calculates TDN and NEL using:
- Digestible NDF
- Digestible crude protein
- Digestible fatty acids
- Digestible non‑fibre carbohydrates
This makes NDFD a critical parameter.
Variation in NDF Digestibility
Research shows that NDFD can differ greatly between forages with similar NDF content and across species. Several factors influence NDFD:
1. Forage Species
- Legumes: Lower %NDF but lower NDFD due to higher lignin content
- Grasses: Higher %NDF but higher NDFD, often outperforming legumes in milk production
- Corn silage: Narrower NDFD range because harvest maturity is more consistent
2. Maturity
As plants mature:
- Lignin increases
- NDF increases
- NDFD decreases
Thus, maturity is a key determinant of quality and intake.
3. Genetics
Plant breeding programs now select for higher fibre digestibility.
Example: Pioneer corn silage varieties (2002–03) used a rating scale of 1–9 for NDFD.
Example: Pioneer corn silage varieties (2002–03) used a rating scale of 1–9 for NDFD.
4. Environment
Cooler temperatures and northern climates improve NDFD.
Example: Northern-grown forages (e.g., Manitoba) often perform exceptionally well in U.S. dairy herds.
Example: Northern-grown forages (e.g., Manitoba) often perform exceptionally well in U.S. dairy herds.
Impact of NDFD on Intake and Milk Production
Higher NDFD leads to higher dry matter intake and higher milk yield.
A well‑known study (Oba & Allen, 1999) reported:
- +0.37 lb/day DMI per 1‑unit increase in NDFD
- +0.55 lb/day 4% FCM per 1‑unit increase in NDFD
This makes NDFD one of the strongest predictors of dairy performance.
Availability of NDFD Testing
Many commercial labs now offer NDFD as part of routine analysis or at a modest additional cost:
- DHIA (Sauk Centre, MN)
- Dairyland Laboratories (Wisconsin)
Typical cost: $18–$20 USD/sample, about $8 more than a basic NIR analysis.
Results also include updated TDN and NEL calculations incorporating NDFD.
Results also include updated TDN and NEL calculations incorporating NDFD.
Practical Example: Why NDFD Changes Energy Predictions
University of Wisconsin research (Hoffman et al., 2001):
Two forage lots:
- 17% CP
- 35% ADF
- 50% NDF
Using traditional ADF-based equations:
- Both appear to have 61% TDN and 0.63 Mcal NEL/lb
- They seem nutritionally identical
But their NDFD differs:
- Lot 1: 60% NDFD
- Lot 2: 40% NDFD
Revised energy calculations reveal:
- Lot 1: 57.5% TDN, 0.59 Mcal NEL/lb
- Lot 2: 50% TDN, 0.51 Mcal NEL/lb
Lot 1 contains 15% more energy, enough to support ~5 lb more milk per cow per day.
Conclusion:
Two forages that appear identical on paper can have major differences in animal performance when NDFD is considered.
Two forages that appear identical on paper can have major differences in animal performance when NDFD is considered.
Conclusion
NDF digestibility is one of the most important advances in forage evaluation. It:
- Improves prediction of energy content
- Better explains differences in intake and milk yield
- Allows more accurate and profitable ration formulation
- Enhances understanding of forage quality beyond basic fibre tests
Using NDFD is the next step forward in precise ration balancing and feed quality assessment.