This document discusses the role of physically effective neutral detergent fiber (peNDF) in rumen fermentation and dairy cow production. It defines peNDF and explains how it is calculated based on factors like particle size and NDF content. The document reviews literature showing that maintaining a minimum of 21% peNDF in dairy cow diets is important for rumen pH above 6.0 and prevents issues like milk fat depression. Studies presented found reducing peNDF through finer chop length decreased chewing activity, rumination time and rumen pH, with implications for fermentation and production.

1. Role of Physically effective NDF (peNDF) in
Rumen Fermentation and Animal Production
Saurabh Rajvaidya
ICAR-NDRI Karnal

2. “Fiber can be defined nutritionally as the slowly
digestible or indigestible fraction of feeds that
occupies space in the gastrointestinal tract of
animals”
(Mertens et al., 1985)
Introduction

3. Fiber in
dairy
rations
Chewing
activity
Less
salivary
buffer
secreation
pH
Altered
fermentation
A:P ratio
Milk fat
synthesis
Modified
animal
metabolism
(Mertens, 1997)

4. • Effective NDF: The effectiveness of dietary fibre to
maintain milk fat percentage
Pitt et al., 1996
• Physically effective NDF (peNDF): The effectiveness of
fibre in stimulating chewing activity and is more related
to the physical characteristics of NDF, particularly
particle size
(Mertens, 1997)

5. •The proportion of sample DM >1.18-mm sieve is commonly used
as the physical effectiveness factor (pef)
•The peNDF of a feed is the product of the NDF content of the feed
multiplied by a physical effectiveness factor (pef)
•The pef ranges between 0 and 1 (not effective to 100% effective at
stimulating chewing
(Mertens, 1997)

6. Feed NDF 1.18-mm sieve peNDF
(% of DM) (Fraction
retained)
(% of DM)
Standard 100 1.00 100
Grass hay 65 0.98 63.7
Legume hay 50 0.92 46.0
Legume silage, coarse
chop
50 0.82 41.0
Legume silage, fine
chop
50 0.67 33.5
Corn silage 51 0.81 41.5
Brewers grains 46 0.18 8.3
Corn ground 9 0.48 4.3
Soybean hulls 67 0.03 2.0
Soybean meal 14 0.23 3.2
Estimated peNDF values of feeds
(Mertens, 1997)

7.  The NRC (2001) recommends for total mixed rations a
minimum of 25% NDF in the total dietary dry matter
(DM)
 The physical characteristics of NDF from long forages
have important effects
 Maintenance of normal ruminal pH
 Stimulation of chewing and rumination activity
 Forming the floating ruminal mat
 Stimulating ruminal motility
(Balch 1971; Beauchmin 1991; Allen 1997).

8. • It is estimated that salivary buffers account for about 30
to 40% of the neutralization of fermentation acids in the
rumen
(Allen et al., 1997)
• Hence, it is usually assumed that increasing chewing time
through manipulating dietary particle size improves
ruminal pH status and thus potentially reduces the risk of
ruminal acidosis
(Yang et al., 2001; Krause et al., 2002)

9. • Fibousness characteristic: Chewing time per kilo
gram of DM, measure of physical characteristics of
forage
(Balch et al, 1971)
• Chewing time is positively associated with ruminal
motility and pH (acid base balance) and has been
proposed as an indicator of the physical effectiveness
of fibre
(Norgaard 1989, Armentano and Pereira 1997, Tafaj et al., 2007)

10. • DMI may be increased if fibre length is decreased:
however, chewing time and rumen pH are also
decreased, which leads to an increased risk of SARA
• Longer fibre length can reduce the risk of SARA,
however, this longer fibre can lead to reduced rumen
turnover and DMI, thereby reducing growth rate
Bhandari et al., 2008

11. • By plotting ruminal pH by dietary peNDF derived from a review of literature,
Mertens (1997) proposed that to ensure a ruminal pH of 6.0, dietary peNDF should
be maintained at or above 21%
• This has become the “gold standard” reference value for dietary peNDF

12. peNDF>1.18
required for
ruminal pH
Rumen pH regressed on peNDF>1.18
Kg/100 kg of BW Kg/d % of ration DM
5.8 0.35 2.8 13
5.9 0.47 3.4 16
6.0 0.58 4.1 19
6.1 0.80 5.1 23
Physically effective NDF (peNDF>1.18) required to maintain a specified ruminal
pH of cows in early lactation fed TMR
(Zabeli et al., 2006)

13. • PSPS>8 : The peNDF>8 content of TMR is determined by
multiplying the proportion of DM retained by the 19- and 8-mm
screens of PSPS (Penn State Particle Separator) by dietary NDF
content (DM basis)
(Lammers et al., 1996)
• PSPS>1.18: The peNDF>1.18 content is DM proportion retained on a
1.18-mm sieve, obtained either through the vertical oscillating
sieving technique or the new version of PSPS was multiplied by
NDF content (DM basis) of TMR
(Kononoff et al., 2003)
• Z-Box system: It uses 3.18- and 2.38-mm sieves
Grant and Cotanch (2005)
Methods for estimating physically effective factor (pef)

14. A B
A: Z box
B: Feed material separated by Penn State Particle Separator (PSPS)

15. Particle length
Ingredients Long (28.6) Medium (15.9) Short (4.8)
pefps-2s 0.72 0.68 0.41
pefps-3s 0.96 0.96 0.93
Pef1.18 0.91 0.90 0.82
peNDFps-2s, % of
DM
35.2 31.5 17.4
peNDFps-3s,% of
DM
47.0 44.3 44.4
peNDF1.18, % of
DM
45.0 41.8 39.0
Particle size distribution, physical effectiveness factors (pef) and physically effective
fiber (peNDF) contents of corn silage based TMR (C:F, 54:46)
Yang and Beauchemin, (2006)

16. Holstein cows=6
BW=664±62 kg
Particle length
Ingredients Long (28.6) Medium (15.9) Short (4.8)
pH
Mean 6.08 6.08 5.99
pH<5.8, h/d 3.0 4.1 4.8
pH<5.5, h/d 0.8 1.0 2.0
VFA
Total, mM 120.4 119.7 120.7
Acetate, mol/100
mol
57.6 57.8 58.5
Propionate,
mol/100 mol
22.8 22.1 22.5
Butyrate, mol/100
mol
15.5 15.6 14.6
A:P 2.60 2.66 2.62
Effects of reducing dietary peNDF on ruminal pH and fermentation of
lactating dairy cows
Yang and Beauchemin, (2006)

17. Holstein cows=6
BW=664±62 kg
Particle length
Ingredients Long (28.6) Medium (15.9) Short (4.8)
peNDF1.18 45.0 41.8 39.0
DM 65.1 64.5 63.2
OM 66.6 66.0 65.4
NDF 50.5 47.6 45.7
ADF 49.4 45.0 43.9
CP 65.7 66.2 61.3
Microbial protein
synthesis (N, g/d)
291.0 317.3 280.0
Milk yield, kg/d
Actual 32.1 32.4 31.5
4% FCM 30.1 30.7 29.5
Milk fat
% 3.30 3.23 3.24
Kg/d 1.04 1.04 1.01
Effect of decreasing peNDF levels on digestion and production parameters in
cows fed with TMR (C:F, 54:46)
Yang and Beauchemin, (2006)

18. (Akbari-Afjani et al., 2014)
HF Cows (n=15)
BW=576 ±63 kg,
Milk=39 kg
TMR (Concentrate60:Forage40)
(Maize
silage30:Leucerne10)
(Maize
silage20:Leucerne20)
(Maize
silage10:Leucerne30)
peNDF8 20.3 19.1 17.9
peNDF1.18 48.2a 42.4c 46.0b
Eating (min/kg)
DM 16.3bc 17.4ab 12.1c
NDF 40.5bc 44.4ab 31.6c
Ruminating (min/kg intake)
DM 22.1 18.3 17.4
NDF 51.1 47.2 45.5
Chewing (min/kg intake)
DM 38.4 35.7 29.5
NDF 95.6 91.9 77.2
Chewing behavior of cows eating total mixed rations (TMR)
Values within a row followed by the same letter are not significntly
different, n.s.=p<0.15

19. (Akbari-Afjani et al., 2014)
HF Cows (n=15)
BW=576 ±63 kg, Milk=39
kg
TMR (Concentrate60:Forage40)
(Maize silage30:Leucerne10) (Maize silage20:Leucerne20) (Maize silage10:Leucerne30)
peNDF8 20.3 19.1 17.9
peNDF1.18 48.2a 42.4c 46.0b
Rumen pH 6.64a 6.24b 6.49ab
Total VFA 75.90 65.58 71.40
Acetate 61.46 52.68 57.75
Propionate 9.34 8.02 8.60
Butyrate 3.62 3.46 3.94
A:P 6.74 6.63 6.93
DM 38.4 35.7 29.5
NDF 95.6 91.9 77.2
Concentrations of pH and volatile fatty acids (VFAs) in rumen fluid of cows
eating total mixed rations (TMR)
Values within a row followed by the same letter are not significntly
different, n.s.=p<0.15

20. TMR offered
(53%
Concentrate+47%
BS)
High (Barley silage
long cut
( 46.6%)
Medium (Barley
silage long cut
23.3%+short cut
23.3%)
Low (Barley silage
short cut 46.6%)
DM retained on
screens
19 mm 2.0 1.4 0.7
8 mm 39.2 35.6 32.0
pan 68.8 63.1 67.4
pef 0.41 0.37 0.33
peNDF, % of DM 13.8 11.8 10.5
Physical effectiveness factor (pef) and physically effective fiber (peNDF) contents of
the diets
Long cut=9.5 mm
Short cut=4.8 mm

21. High (Barley silage
long cut 46.6%)
Medium (Barley
silage long cut
23.3%+short cut
23.3%)
Low (Barley
silage short
cut 46.6%)
Eating
Chews/kg of DM 597 636 604
Min/d 279.6 272.1 259.4
Min/kg of DM 14.9 14.1 13.6
Ruminating
Chews/kg of DM 1418 1536 1361
Min/kg of DM 23.2 22.8 18.8
Total chewing
Chews/kg of DM 2015 2165 1966
Min/kg of DM 38.2 36.4 32.5
pH 5.62 5.65 5.68
VFA
Total, mM 128.3 138.7 128.7
Acetate, mol/100 mol 53.3 52.4 53.3
Propionate, mol/100 mol 26.9 23.1 25.8
Butyrate, mol/100 mol 14.6 18.3 15.7
A:P 2.1 2.5 2.2
NH3 8.5 9.8 10.6
Effects of reducing dietary physically effective fiber (peNDF) on chewing,
rumination, ruminal pH and fermentation characteristics of dairy cows

22. • These dietary peNDF values are well below the recommended concentration of 21% of
DM for maintaining rumen pH above 6.0 by Mertens, 1997

23. • Dairy cows should achieve between 59 and 72.8 min of
chewing time per kg of DM from forages to prevent
ruminal disorders and milk fat depression
De Brabander et al. (2002)
• In dairy cows to achieve a chewing time of 74 min/ kg
of DM from a long-chopped hay, diets should contain at
least 10.7% long-chopped hay-crude fiber, which in
turn corresponded to 28% NDF or 19% peNDF and
60% slowly degradable concentrate in the diet
Tafaj et al. (2005)

24. Lactating HF
cows=4
655±79 kg
Diets
(concentrate60:Forage40)
Alfaalfa :Alfaalfa hay (50:50) Alfaalfa :Alfaalfa hay (25:75)
CH GH CH GH
Top, 19-mm 9.4 0.8 11.9 0.2
Middle, 8-mm 32.1 29.9 25.8 21.6
Bottom 58.6 69.3 62.3 78.2
Pefp, 1.18% 73.5 63.6 68.8 59.4
peNDFp>1.18
Kg/d 6.29 4.98 4.99 4.06
% of DMI 26.7 20.9 24.3 19.7

25. Lactating HF
cows=4
655±79 kg
Diets
(concentrate60:Forage40)
Alfaalfa silage :Alfaalfa hay (50:50) Alfaalfa silage:Alfaalfa hay (25:75)
Chopped hay Ground hay Chopped hay Ground hay
Eating activity
Min/kg of DM 12.5 10.8 14.8 15.5
Min/kg of NDF 34.5 32.9 42.2 46.6
Ruminating time
Min/kg DM 19.1 15.7 20.8 13.5
Min/kg of NDF 87.1 81.1 101.3 87.1
Salivation rate,
ml/min
198.9 191.1 173.9 219.6
pH, Mean 5.97 5.78 6.18 5.90
VFA
Total, mM 143.4 145.7 133.3 133.3
Acetate 64.4 65.6 67.9 66.0
Propionate 20.1 18.1 17.5 18.8
Butyrate 11.7 11.7 10.6 11.3
A:P 3.41 3.72 3.95 3.77
NH3-N 3.23 3.55 4.80 4.05
(Beuchemin et al., 2003)

26. • peNDF of the diets were not correlated to
mean ruminal pH but were negatively
correlated to the area below pH 5.8 and the
time during which ruminal pH was below 5.8

27. Lactating HF
cows=4
655±79 kg
Diets
(concentrate60:Forage40)
Alfaalfa silage :Alfaalfa hay (50:50) Alfaalfa silage:Alfaalfa hay (25:75)
Chopped hay Ground hay Chopped hay Ground hay
Milk yield, kg/d
Actual 29.4 29.2 26.2 26.5
4% FCM 28.3 27.9 25.3 25.9
Milk fat (%) 3.92 3.78 3.80 3.93
Milk protein (%) 3.26 3.34 3.31 3.31
Milk lactose (%) 4.62 4.63 4.57 4.60
Milk production and composition of dairy cows fed diets differing in ratio of
Alfaalfa silage to Alfaalfa hay
(Beuchemin et al., 2003)

29. • peNDF plays important role in regulating
ruminal pH
• It increases the chewing activity either by
increasing eating or ruminating time
• peNDF is good indication of the rumination
potential of the feed
Conclusions