acidogenic value of feeds. i
DESCRIPTION
FDASFSFTRANSCRIPT
Acidogenic value of feeds. I. The relationship between theacidogenic value of feeds and in vitro ruminal pH changes
B. Rustomo1, J. P. Cant1, M. Z. Fan1, T. F. Duffield2, N. E. Odongo1, and B. W. McBride1,3
1Department of Animal and Poultry Science, 2Department of Population Medicine, University of Guelph, Guelph,Ontario, Canada N1G 2W1. Received 21 October 2004, accepted 28 December 2005.
Rustomo, B., Cant, J. P., Fan, M. Z., Duffield, T. F., Odongo, N. E. and McBride, B. W. 2006. Acidogenic value of feeds. I. Therelationship between the acidogenic value of feeds and in vitro ruminal pH changes. Can. J. Anim. Sci. 86: 109–117. The objec-tive of this study was to use an in vitro technique (i) to assess the acidogenic value (AV) of feed ingredients, (ii) to evaluate the rela-tionship between the AV of feed and ruminal pH changes, and (iii) to determine the relationship between the AV of feeds andchemical constituents of feeds. Assessments of AV were based on 24 and 48 h in vitro incubation in rumen liquor. A series of feeds,ranging from energy, fibre and protein sources were evaluated. Ruminal fluid pH changes in the incubation medium were measuredat the end of 24 and 48 h incubation in buffered rumen liquor (60% buffer, 40% rumen liquor). The chemical constituents of the feedingredients were determined using standard procedures. There were no differences (P > 0.05) between 24 and 48 h incubations onapparent AV and rumen fluid pH changes. The best predictors of AV for all classes of feed were non-fibre carbohydrate (NFC) frac-tion and acid detergent fibre (ADF; R2 = 0.81; P < 0.001). The best predictor of AV for energy sources were NFC and ADF (R2 =0.70; P < 0.027); neutral detergent fibre (NDF) for fibre sources (R2 = 0.84; P < 0.027) and crude protein (CP) for protein sources(R2 = 0.73; P < 0.014). The rumen fluid pH changes had stronger relationships with apparent AV of all feeds after 24 h (R2 = 0.74, P< 0.0001) than starch (R2 = 0.35, P = 0.04) or NFC (R2 = 0.56; P < 0.0001). The results indicate that 24 h AV measurements andrumen fluid pH changes are acceptable measures for qualitatively describing or ranking feed ingredients.
Key words: Acidogenic value, dairy cow, feed chemical composition, rumen pH
Rustomo, B., Cant, J. P., Fan, M. Z., Duffield, T. F., Odongo, N. E. et McBride, B. W. 2006. Valeur acidogène des aliments dubétail. I. Relations entre la valeur acidogène des aliments du bétail et la variation in vitro du pH du rumen. Can. J. Anim.Sci. 86: 109–117. Par le biais d’une technique in vitro, l’étude devait : (i) évaluer la valeur acidogène (VA) des aliments du bétail,(ii) établir les liens entre la VA des aliments du bétail et les changements de pH dans le rumen et (iii) préciser la relation entre laVA des aliments du bétail et leur composition chimique. La VA a été évaluée par incubation in vitro dans du fluide du rumen pen-dant 24 h ou 48 h. Les auteurs ont examiné plusieurs types d’aliments représentant diverses sources d’énergie, de fibres et de pro-téines. Ils ont mesuré l’écart du pH du fluide du rumen dans le milieu d’incubation au terme de la période d’incubation de 24 h oude 48 h dans du fluide du rumen tamponné (60 % de solution tampon, 40 % de fluide du rumen). La composition chimique desaliments du bétail a été établie de la manière usuelle. La VA apparente et la variation de pH du fluide du rumen sont les mêmes(P > 0,05) après 24 ou 48 h d’incubation. La fraction d’hydrates de carbone non cellulosiques (FNC) et les fibres au détergentacide (FDA) prédisent le mieux la VA, peu importe le type d’aliment (R2 = 0,81, P < 0,001). La FNC et les FDA prédisent le mieuxla VA pour les sources d’énergie (R2 = 0,70, P < 0,027); pour les sources de fibres, il s’agit des fibres au détergent neutre(R2 = 0,84, P < 0,027) tandis que pour les sources de protéines, la protéine brute donne les meilleurs résultats (R2 = 0,73,P < 0,014). La variation de pH du fluide du rumen présente des liens plus étroits avec la VA apparente des aliments du bétail après24 h (R2 = 0,74, P < 0,0001) que l’amidon (R2 = 0,35, P = 0,04) ou la FNC (R2 = 0,56, P < 0,0001). Les résultats indiquent qu’onpourrait recourir à la quantification de la VA après 24 h d’incubation et à la variation du pH dans le fluide du rumen pour décrireou classer les aliments du bétail qualitativement.
Mots clés: Valeur acidogène, vaches laitières, composition chimique des aliments, pH du rumen
Sub-acute rumen acidosis (SARA) represents lowered rumi-nal pH as a result of carbohydrate fermentation (Nocek1997). Effective fibre intake and carbohydrate digestion rateinteract to determine ruminal pH (Armentano and Pereira1997). Rumen acidosis is therefore related to the amount ofacid produced as feed is fermented and the ability of the feedto encourage chewing and production of salivary buffers.Although animals suffering from SARA do not typicallyexhibit clinical signs of illness and often go undetected(Owens et al. 1998), SARA can reduce feed intake, nega-
tively affect rumen fermentation, growth, performance, andcontribute to laminitis (Nocek 1997). Sub-acute ruminal aci-dosis can also damage ruminal and intestinal epithelial tis-sue, leading to bacterial infection and subsequent liverabscesses (Underwood 1992).
Wadhwa et al. (2001) have developed a simple laboratory-based technique for evaluating acid production from feedstuffs
109
3To whom correspondence should be addressed (e-mail:[email protected]).
Abbreviations: AV, acidogenic value; ADF, acid deter-gent fibre; BC, buffering capacity; CP, crude protein; DMI,dry matter intake; NDF, neutral detergent fibre; NFC, non-fibre carbohydrate; NPN, non-protein nitrogen; SARA, sub-acute rumen acidosis
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110 CANADIAN JOURNAL OF ANIMAL SCIENCE
based on the dissolution of Ca from CaCO3. This method hasbeen used to rank feeds in terms of the acid-load accumulatedwithin the rumen during fermentation. The concept of effec-tive neutral detergent fibre (eNDF), which integrates theeffects of diet on chewing, saliva production, ruminal acid pro-duction and neutralization, uses milk fat composition as aresponse variable to predict acidosis (Armentano and Pereira1997). However, Pereira et al. (1999) showed that milk fatdepression was not a reliable indicator of peNDF or SARA.The concept of physically effective NDF (peNDF) to describethe impact of physical effectiveness of NDF in stimulating cudchewing has also been described (Mertens 1997). Althoughthe concept of peNDF was developed to avoid the problem ofexcess production of acid in the rumen (Mertens 1997), thisapproach does not consider acid production from the feed.Other in vitro studies have determined acid production fromfeeds by measuring the final pH after incubation (de Smet etal. 1995; Malestein et al. 1982). However, pH measurementsalone do not include aspects of buffering (Stewart 1983). Inthe acidogenic value (AV) approach, the dissolved Ca repre-sents how much total acid is produced and neutralized duringfermentation (Wadhwa et al. 2001).
However, Wadhwa et al. (2001) did not measure the rela-tionship between AV and ruminal fluid pH changes in theincubation medium for different classes of feeds.Additionally, Wadhwa et al. (2001) did not measure therelationship between AV and the chemical componentsfrom different classes of feeds. The objectives of this study,therefore, were: (i) to measure the AV of feeds, (ii) to eval-uate the relationship between AV of feeds and ruminal pHchanges, and (iii) to examine the relationship between AVor ruminal fluid pH changes and feed chemical composition.
MATERIALS AND METHODSThe AV of the feed was determined using an in vitro techniquedeveloped by Tilley and Terry (1963) and modified byWadhwa et al. (2001). Feeds were freeze-dried and groundthrough a 1-mm screen in a laboratory mill (Thomas Wiley,Philadelphia, PA) before being returned to the freeze drier toremove any moisture that had been picked up during grinding.One-gram (DM basis) samples were weighed directly from thefreeze drier into 100-mL incubation tubes held at 39°C in awater bath. The samples were incubated in duplicate with 30mL of buffered rumen liquor comprising 60% buffer and 40%rumen liquor. The buffer (5.880 g L–1 NaHCO3; 5.580 g L–1
Na2HPO4; 0.282 g L–1 NaCl; 0.342 g L–1 KCl; 0.028 g L–1
CaCl2.2H2O and 0.036 g L–1 MgCl2) was made up at 20% thestrength of the Tilley and Terry (1963) buffer (Wadhwa et al.2001). Rumen fluid was collected from two rumen-fistulatedcows fed alfalfa hay ad libitum 3 h after morning feeding. Allexperimental procedures using the fistulated cows were donewith the approval of the University of Guelph Animal CareCommittee in accordance with the guidelines of the CanadianCouncil on Animal Care.
Cysteine hydrochloride monohydrate (0.025% wt/vol)was added into the 100-mL incubation tubes just beforeincubation and the tubes were closed with gas release valvesand shaken continuously. After 24 and 48 h of incubation, 2-mL samples were withdrawn from each tube and transferred
in to 8-mL centrifuge tubes containing 50 mg of CaCO3powder (Catalogue No. C6763; Sigma Chemical Co., St.Louis, MO). The mixture was shaken manually for 5 s andthen centrifuged at 4000 × g for 10 min and the Ca contentin the supernatant determined using a test kit (SigmaDiagnostics Inc., Calcium Procedure No. 587; Sigma-Aldrich Co., St. Louis, MO) in a laboratory spectropho-tometer (HACH, DR/4000, Loveland, CO) set at 575 nm.The absorbance was read to zero using water as reference.All samples were run in duplicates in two separate runs.
Apparent AV was calculated as the product of Ca concen-tration from the analysis and fluid volume (30 mL) divided bythe sample weight (1 g; Wadhwa et al. 2001). To eliminate thecontribution of Ca from the feed, basal AV was calculatedafter correcting for Ca dissolved before the addition of CaCO3.True AV was calculated as apparent AV (after CaCO3) lessbasal AV (before CaCO3). Blank (with no feed sample) sam-ples and standards (wheat and straw) were included in eachrun for calibration but these were not used to adjust the AV.The pH was measured before (0 h) and after incubations (24and 48 h). Feed ingredients chosen for assessment included arange of energy, protein and fibre sources.
The chemical composition of the feed ingredients wasdetermined using standard Association of OfficialAnalytical Chemists [(AOAC) 1990] procedures. The DMcontent was determined by oven drying at 60oC for 48 h.Crude protein (CP), soluble protein and non-protein nitro-gen (NPN) were analyzed using macro-Kjeldahl (method984.13). Starch using IKA analyzer technick (C5000) andCa and P by inductively coupled plasma spectroscopy(method 945.46). The samples were also analysed for fat,non-fibre carbohydrate (NFC), acid detergent fibre (ADF;method 973.18c) and lignin (AOAC 1990) and neutraldetergent fibre (NDF) (Goering and Van Soest 1970).
Statistical Analysis The relationship between AV and rumen fluid pH and AV andfeed chemical composition was determined using simple andmultiple linear regression analysis within the SAS Institute,Inc. (2004). Comparison between 24 and 48 h incubationswere conducted using paired t-test procedures in SAS. Effectswere considered significant at a probability P < 0.05.
RESULTS AND DISCUSSIONThe chemical composition of the feed ingredients is present-ed in Table 1. Acidogenic values for feed ingredients after 24and 48 h of incubation are shown in Table 2. The basal AV ofsome feed ingredients (e.g. sugar beet pulp and alfalfa hay)were relatively high (6.15 ± 0.29 and 5.44 ± 0.21, respective-ly; Table 2), which suggests contribution of inherent Ca with-in these feed ingredients to the measured apparent AV.Energy sources had the highest AV; fibre sources had inter-mediate AV and protein sources had the lowest AV.Acidogenic values were not different (P > 0.05) between the24 and 48 h incubation for all feed classes (Table 3). This sug-gests that there was little further fermentation after 24 h. It hasbeen shown that the rate of degradation of feed ingredients ishigher during the first 9 to12 h than during subsequent incu-
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RUSTOMO ET AL. — ACIDOGENIC VALUE OF FEEDS. I 111T
able
1. C
hem
ical
com
posi
tion
of
the
feed
ingr
edie
nts
(% D
M).
Sta
ndar
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of t
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ean
(SE
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bas
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arch
NFC
Ca
P
Suga
r be
et p
ulp
88.3
11.6
2.5
21.3
2.3
91.2
0.9
5.7
45.2
24.4
4.0
0.6
41.2
0.7
0.1
(0.0
2)(0
.06)
(0.0
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.49)
(0.0
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)(0
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ats
89.1
12.4
2.7
22.0
0.8
30.0
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29.3
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heat
88.1
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44.8
1.8
1.7
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62.0
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Hig
h m
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corn
58.0
9.0
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Cor
n si
lage
38.6
9.6
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(0.0
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.02)
(0.2
5)(0
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(0.0
8)(0
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(0.0
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Hay
lage
31.9
18.2
9.8
53.7
8.7
88.9
3.4
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58.8
37.6
5.2
0.3
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0.7
0.3
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(0.2
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(0.6
2)(0
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(0.8
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(0.0
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(0.2
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(0.2
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(0.0
9)(0
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Whe
at s
traw
92.2
4.0
0.7
16.9
0.2
35.4
2.1
5.0
83.3
67.7
18.5
1.9
10.2
0.1
0.1
(0.0
4)(0
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(0.0
6)(2
.65)
(0.0
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93.6
42.3
3.0
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33.8
23.4
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0.2
0.6
(0.0
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ybea
n m
eal
87.2
56.0
5.5
9.9
1.5
26.7
3.5
6.4
8.6
4.6
0.4
1.1
24.9
0.3
0.7
(0.1
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(0.0
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(0.4
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(1.4
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(0.0
0)C
anol
a m
eal
86.3
42.1
3.5
8.3
2.2
64.4
4.9
8.1
26.5
19.1
9.7
1.3
15.8
0.8
1.1
(0.0
9)(0
.38)
(0.0
1)(0
.10)
(0.1
1)(3
.43)
(0.4
0)(0
.00)
(0.0
0)(0
.20)
(0.1
3)(0
.13)
(2.3
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r m
eal
89.5
86.0
6.9
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(1.1
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(0.3
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(3.3
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(0.0
3)(0
.33)
(0.1
5)(0
.49)
(0.0
7)(N
D)
(0.0
1)(0
.01)
Cor
n gl
uten
87.4
71.4
4.7
6.6
3.9
82.4
1.0
2.5
9.2
3.4
1.4
13.3
18.8
0.0
0.59
(0.0
2)(0
.10)
(0.0
9)(0
.11)
(0.0
5)(0
.37)
(0.1
5)(0
.04)
(0.5
7)(0
.00)
(0.0
5)(1
.35)
(0.0
1)(0
.00)
(0.0
1)B
lood
mea
l95
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63.
81.
234
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32.
1N
DN
DN
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.03)
(0.0
7)(0
.08)
(0.0
8)(0
.04)
(1.8
6)(0
.09)
(0.0
0)(N
D)
(ND
)(N
D)
(0.1
5)(N
D)
(0.0
0)(0
.00)
Her
ring
mea
l94
.377
.57.
29.
35.
779
.215
.410
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D0.
60.
60.
3N
D2.
21.
9(0
.04)
(0.3
0)(0
.02)
(0.0
1)(0
.10)
(1.2
0)(0
.20)
(0.0
4)(N
D)
(0.0
2)(0
.15)
(0.0
3)(N
D)
(0.0
4)(0
.01)
z % s
olub
le p
rote
in/C
P.
y % N
PN/S
ol. P
. x N
D =
not
det
erm
ined
.
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bation (de Smet et al. 1995). The differences in AV were like-ly related to the rapid initial fermentation of the feeds. deSmet et al. (1995) also showed that the rate of degradationvaried with the type of feed ingredient, being higher for feeds
with high cell content and lower for feeds with high cell wallcontent. Energy source feeds resulted in similar AV with fibresources (Table 2) but had the greatest rumen fluid pH changes(Table 3 and Fig. 1a).
Table 2. Acidogenic value [AV; dissolved Ca (mg Ca g–1 feed DM)] of feed ingredients after 24 and 48 h incubations. Values are given for Ca dissolvedbefore (basal AV) and after the addition of CaCO3 (apparent AV) and the difference (true AV). Standard error of the mean is given in parentheses
24 h 48 h
Feed ingredients Apparent AV Basal AV True AV Apparent AV Basal AV True AV
Energy sourcesSugar beet pulp 14.68 (0.43) 6.15 (0.29) 8.53 (0.66) 13.14 (0.34) 5.37 (0.06) 7.77 (0.38)Barley 12.67 (0.36) 3.60 (0.34) 9.06 (0.54) 11.28 (0.11) 3.07 (0.06) 8.21 (0.09)Oats 12.41 (0.22) 2.98 (0.21) 9.43 (0.41) 11.13 (0.27) 2.91 (0.12) 8.22 (0.16)Wheat 11.65 (0.62) 3.28 (0.28) 8.37 (0.87) 11.06 (0.12) 2.74 (0.13) 8.32 (0.21)Wheat middling 10.84 (0.94) 3.52 (0.23) 7.31 (0.17) 10.56 (0.37) 3.92 (0.19) 6.64 (0.54)High moisture corn 9.59 (0.24) 2.14 (0.23) 7.44 (0.31) 11.53 (0.25) 2.43 (0.22) 9.10 (0.45)Corn distillers 8.38 (0.22) 2.67 (0.30) 5.71 (0.29) 9.26 (0.17) 2.58 (0.20) 6.68 (0.24)Wheat shorts 8.04 (0.81) 3.17 (0.34) 4.87 (0.78) 9.67 (0.41) 2.97 (0.11) 6.70 (0.31)Wheat bran 5.70 (0.19) 2.84 (0.27) 2.86 (0.26) 8.35 (0.17) 2.70 (0.18) 5.64 (0.20)
Fibre sourcesCorn silage 12.86 (0.25) 3.60 (0.22) 9.24 (0.29) 12.20 (0.18) 3.73 (0.11) 8.46 (0.11)Alfalfa pellet 12.63 (0.67) 4.92 (0.42) 7.71 (0.26) 10.41 (0.28) 4.01 (0.39) 6.41 (0.25)Alfalfa hay 11.71 (0.66) 5.44 (0.21) 6.09 (0.50) 11.46 (0.15) 4.27 (0.31) 7.18 (0.31)Haylage 9.30 (0.50) 5.28 (0.34) 4.02 (0.28) 7.95 (0.36) 3.86 (0.17) 4.09 (0.21)Wheat straw 6.27 (0.90) 3.04 (0.27) 3.22 (0.64) 6.78 (0.56) 2.69 (0.17) 4.09 (0.40)
Protein sourcesRoasted soybean 7.62 (0.68) 2.34 (0.34) 5.28 (0.39) 5.45 (0.32) 1.57 (0.18) 3.88 (0.17)Soybean meal 6.43 (0.39) 3.54 (0.33) 2.89 (0.39) 5.48 (0.40) 2.69 (0.08) 2.78 (0.37)Canola meal 5.13 (0.43) 3.65 (0.13) 1.48 (0.32) 4.09 (0.16) 2.54 (0.30) 1.55 (0.46)Feather meal 3.03 (0.20) 0.79 (0.08) 2.24 (0.17) 1.40 (0.18) 0.34 (0.03) 1.06 (0.16)Corn gluten 2.40 (0.24) 2.13 (0.30) 0.27 (0.21) 3.10 (0.16) 2.07 (0.28) 1.02 (0.34)Blood meal 1.66 (0.11) 1.33 (0.12) 0.33 (0.16) 1.31 (0.19) 0.57 (0.05) 0.74 (0.15)Herring meal 1.42 (0.22) 0.97 (0.09) 0.46 (0.28) 1.87 (0.24) 0.63 (0.07) 1.24 (0.29)
Table 3. Effects of incubation time on basal, apparent and true acidogenic value [AV; dissolved Ca (mg Ca g–1 feed DM)] and rumen fluid pH changesduring 24 and 48 h incubations
24 h 48 h SEMz P value
All feed classesApparent AV (after the addition of CaCO3) 8.30 7.97 0.31 0.461Basal AV (before the addition of CaCO3) 3.21 2.75 0.11 0.003True AVy 5.09 5.23 0.24 0.680Rumen fluid pH changesx 1.30 1.39 0.07 0.478
Energy sourcesApparent AV (after the addition of CaCO3) 10.44 10.66 0.27 0.570Basal AV (before the addition of CaCO3) 3.37 3.19 0.13 0.338True AVy 7.07 7.47 0.24 0.250Rumen fluid pH changesx 1.75 1.90 0.05 0.107
Fibre sourcesApparent AV (after the addition of CaCO3) 10.52 9.76 0.42 0.207Basal AV (before the addition of CaCO3) 4.46 3.71 0.17 0.003True AVy 6.06 6.05 0.35 0.986Rumen fluid pH changesx 1.36 1.53 0.07 0.199
Protein sourcesApparent AV (after the addition of CaCO3) 3.96 3.24 0.27 0.086Basal AV (before the addition of CaCO3) 2.11 1.49 0.15 0.005True AVy 1.85 1.75 0.22 0.757Rumen fluid pH changesx 0.68 0.65 0.09 0.884zStandard error of the mean.yTrue AV was calculated as apparent AV (AV after addition of CaCO3) – basal AV (AV before addition of CaCO3). xRumen fluid pH changes = decrease in rumen fluid pH from the start of incubation to the end of incubation.
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Fig. 1. (a) Rumen fluid pH changes from 0 h to 24 and 48 h after incubation for the energy sources. (b) Rumen fluid pH changes from 0 h to 24 and 48 h after incubation for the fibre sources. (c) Rumen fluid pH changes from 0 to 24 h and 48 h after incubation for the pro-tein sources.
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Rumen fluid pH changes for different classes of feedingredients after 24 and 48 h of incubation are shown inFig. 1. The rumen fluid pH decreased significantly duringthe first 24 h of incubation, but remained relatively constantafter that to 48 h of incubation (Fig. 1 a, b). The rate ofchange of rumen fluid pH changes appears to follow similarpatterns as the AV changes. Energy source feed ingredientsshowed the greatest pH decrease during the first 24 h ofincubation (Fig. 1a). This result is in agreement with thoseof de Smet et al. (1995) and Hall (2002), that highly fer-mentable carbohydrates, such as sugars, soluble fibre andsome starches, have the capacity to decrease ruminal pH ina relatively short period of time (1 to 5 h). Rumen fluid pHchanges were lowest for protein feed sources (Fig. 1c). Thedifferences in rumen fluid pH changes between 24 and 48 hof incubation were also smallest for protein feeds (Table 3).These results suggest that protein feed sources have differ-ent effects on ruminal pH changes. It has been shown thathigh protein feeds generally have higher buffering capacity(McBurney et al. 1983; Jasaitis et al. 1987) due to theammonia produced during fermentation (Crawford et al.1983). High protein feeds are therefore capable of neutraliz-ing acid to maintain constant pH in continuous culture(Dewhurst et al. 2001).
The relationship between apparent AV and rumen fluid pHchanges after 24 h of incubation are presented in Table 4.There was a positive correlation between AV and rumen fluidpH change for all classes of feed (R2 = 0.74; P < 0.0001;Table 4). High AV was associated with a greater decrease inrumen fluid pH change. Furthermore, the significant relation-ship between apparent AV and rumen fluid pH changes sug-gests high AV diets would be expected to increase the risk ofrumen acidosis in cows. Several studies have showndecreased ruminal pH when more rapidly fermentable carbo-hydrates were included in the diet (Keunen et al. 2002;Krause et al. 2002b; Plaizier et al. 2001; Reinhart et al.1993).It has also been suggested that increased fermentation ofstarch might overload the absorptive capacity of the rumenthus exacerbating the reductions in ruminal pH (Krause et al.2003). Furthermore, a decrease in ruminal pH was reported todecrease appetite (Briton and Stock 1987), fibre degradabili-ty (Hoover 1986; Krajcarski-Hunt et al. 2002), microbial pro-tein synthesis (Strobel and Russell 1986; de Veth and Kolver2001; Calsamiglia et al. 2002), DMI and subsequent milk pro-duction (Aldrich et al. 1993; Krause et al. 2002a). Predictionof potential rumen fluid pH changes based on the AV of feedswould be highly beneficial with regard to diet formulation inorder to balance for diets that would optimize ruminal pH andrumen function. However, in vivo studies are needed to clar-ify the effect of feed AV on actual ruminal pH changes indairy cows.
The relationships between the AV of the feeds and feedchemical composition are presented in Table 5. There was apositive correlation between AV and starch content (R2 = 0.20;P = 0.042) of feed and AV and NFC content (R2 = 0.43; P <0.001) of the feed. Although the starch contents of sugar beetpulp and corn distillers were low, their NFC contents werehigh (Table 1). Sugar beet pulp contains a significant amount
of soluble fibre (17.4 to 30.0% DM), and sugars (12.8 to24.7% DM), while corn distillers contain soluble fibre in therange of 7.8 to 11.6% DM, and sugars from 3.2 to 14.5% DM(Hall et al. 1999). Sugars and starch ferment to lactic acid,which has a lower pKa than the volatile fatty acids. Solublefibre, such as pectin, ferments rapidly, but its rate of fermenta-tion declines at low pH (Ben-Ghedalia et al. 1989; Strobel andRussel 1986). Although the starch and NFC content of high-moisture corn were the highest among the energy feeds (Table1), the AV was intermediate (Table 2). Different types of NFCdiffer in their rate of fermentation and their effects on ruminalpH (de Smet et al. 1995; Hall 2000). Rooeny and Pflugfelder(1986) reported that the starch granules in corn are almostcompletely contained within a protein matrix, which decreas-es the availability of starch to hydrolysis. For all classes offeed, the relationship between rumen fluid pH changes andAV were stronger (R2 = 0.74; P < 0.001) than the relationshipbetween starch and rumen fluid pH changes (R2 = 0.35; P =0.004) or rumen fluid pH changes and NFC (R2 = 0.56; P <0.001; Table 4). Crude protein content was negatively corre-lated with AV (Table 5). High protein feeds would thereforebe expected to have low fermentability and thus produce lessrumen acid load. As noted above, it has been shown that highprotein feeds generally have higher buffering capacity(McBurney et al. 1983; Jasaitis et al. 1987) due to the ammo-nia produced during fermentation (Crawford et al. 1983). It istherefore, important to consider feed ingredient AV when for-mulating rations rather than just using the starch content or theNFC content.
Among the forages tested, corn silage had the highest AVand wheat straw the lowest (Table 2). Corn silage wasalmost as acidogenic as the high-energy feeds despite itslower starch content and higher NDF content (Table 1). Thehigh AV for corn silage might suggest high levels of freeacids (Thomas and Wilkinson 1975) and low CP content(Table 1). Additionally, it has been shown that corn silagerequired higher alkali to stabilize high rumen fluid pHchanges in continuous culture (Crawford 1983; Dewhurst etal. 2001; Wadhwa et al. 2001). The low AV for wheat strawmight be explained by its poor NDF degradability.According to Weiss et al. (1989) and de Smet et al. (1995),feedstuffs rich in cell wall content have low degradationrates. These results support the use of an optimum foragemixture when feeding large amounts of starch-rich concen-trates to dairy cows (Phipps et al. 1995).
Table 6 shows apparent AV predictions from chemicalcomposition of the dietary ingredients based on stepwisemultiple linear regression analysis. Greater variation wasexplained when the factors were included in combinationdemonstrating the importance of the interactions amongchemical constituents in a diet. This suggests that acid pro-duction and neutralization in the rumen is a complex phe-nomenon and involves the interaction between feedcomponents, ruminal environment and microbial popula-tions. More species of microorganisms grow in the rumenwhen the substrates consist of a more complex composition(Malestein et al. 1982). de Smet et al. (1995) postulated thata synergism between different bacterial species may occurwhen a substrate is more complex and fermentation prod-
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ucts of one species are used as substrate by another species.However, some feed ingredients have a poor balance of sub-strates for rumen microbes. Wadhwa et al. (2001) found thatsome feed ingredients such as wheat (high starch content),wheat straw (high fibre), and feather meal (high protein con-tent) had deviations from a linear response on AV.Furthermore, the extent of ruminal fermentation for bothfibre and non-fibre carbohydrates among feedstuffs isextremely variable and is influenced by interactionsbetween diet composition and rumen microbial activity(Allen 1997). Measures of feed chemical composition aloneare therefore inadequate for estimating acid-load in therumen and may not be useful as AV indicators.
CONCLUSIONSThis study shows that the acid loads from feeds can be esti-mated by an in vitro technique. Energy sources and fibresources had the highest AV whilst protein sources had thelowest AV. There was a positive correlation (P < 0.001)between AV and rumen fluid pH changes for all feeds. A highAV was associated with a greater decrease in rumen fluid pHchange. The rumen fluid pH changes had stronger relation-ship with apparent AV of all feeds after 24 h (R2 = 0.74, P <0.0001) than with starch (R2 = 0.35, P = 0.04) or with NFC(R2 = 0.56; P < 0.0001). This relationship may allow predic-tions of ruminal fluid pH change from feed AV. The AV esti-mates of a range of feed ingredients can be used to formulate
Table 4. Predictionz of rumen fluid pH changesy (Y) from apparent acidogenic value [AV; dissolved Ca (mg Ca g–1 feed DM)] of feed ingredients (x)after 24 h of incubation
Factor (x) b0 b1 b2 b3 R2 P value
All feedsY1 = b0 + b1 x1 (starch ) 1.046 0.014 0.35 0.004Y2 = b0 + b2 x2 (NFCx) 0.795 0.018 0.56 <0.0001Y3 = b0 + b3 x3 (AV) 0.313 0.119 0.74 <0.0001
Energy sources Y1 = b0 + b1 x1 (starch ) 1.71 0.001 0.02 0.72Y2 = b0 + b2 x2 (NFC) 1.57 0.004 0.17 0.26Y3 = b0 + b3 x3 (AV) 1.19 0.054 0.47 0.04
Fibre sources Y1 = b0 + b1 x1 (starch ) 1.23 0.015 0.61 0.118Y2 = b0 + b2 x2 (NFC) 0.99 0.018 0.62 0.114Y3 = b0 + b3 x3 (AV) 0.42 0.089 0.80 0.040
Protein sourcesY1 = b0 + b1 x1 (starch ) 0.59 0.037 0.17 0.351Y2 = b0 + b2 x2 (NFC) 0.29 0.036 0.82 0.005Y3 = b0 + b3 x3 (AV) 0.17 0.130 0.58 0.048z Rumen fluid pH changes after 24 h incubations (Y1) = b0 + b1 (x1); (Y2) = b0 + b2 (x2); where b0 = intercept, b1 = slope (increase in pH changes per unitincrease in starch); b2 = slope (increase in pH changes per unit increase in NFC); x1 = starch content and x2 = NFC content ; and b3 = slope (increase in pHchanges per unit increase in apparent AV); x1 = starch content; x2 = NFC content, and x3 = apparent AV.yRumen fluid pH change = rumen fluid pH at the start of incubation – rumen fluid pH at the end of 24 h of incubation. xNFC = non-fibre carbohydrate R2 = coefficient of determination
Table 5. Single predictor of apparent acidogenic valuez from dietary chemical composition variable
Factor (x) b0 b1 R2 P value
CP (% DM) 12.221 –0.119 0.690 <0.0001Sol. P (% DM) 11.376 –0.769 0.163 0.070Sol. P/CP (%) 5.466 0.157 0.228 0.029NPN (% DM) 9.596 –0.481 0.058 0.291NPN/Sol P (%) 6.821 0.024 0.019 0.550Fat (% DM) 9.607 –0.229 0.107 0.148Ash (% DM) 9.269 –0.197 0.017 0.575NDF (% DM) 6.039 0.071 0.141 0.093ADF (% DM) 5.976 0.187 0.248 0.021Lignin (% DM) 8.242 0.022 0.0001 0.953Starch (% DM) 6.930 0.077 0.200 0.042NFC (% DM) 5.111 0.115 0.425 0.001Calcium (% DM) 8.624 –0.784 0.011 0.652Phosphorus (% DM) 10.597 –4.006 0.231 0.027
zApparent acidogenic value (AV) (mg Ca g–1 feed DM) = b0 + b1 (x); where b0 = intercept, b1 = slope (increase in AV per unit increase in dietary chemicalcomposition variables), and x = dietary chemical composition variables. R2 = coefficient of determination.
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diets that would present a low rumen acid load in dairy cowsand therefore prevent rumen acidosis. There were no differ-ences in apparent AV and rumen fluid pH change measure-ments at 24 and 48 h of incubation suggesting that 24 h ofincubation may provide an acceptable measure of feed AVand rumen fluid pH change. However, further studies areneeded to examine the effect of feed AV on in vivo ruminalpH changes in dairy cows.
ACKNOWLEDGEMENTSThe authors wish to acknowledge the support of DairyFarmers of Ontario, Ontario Ministry of Agriculture, Foodand Rural Affairs and the Natural Sciences and EngineeringResearch Council (BWM) for financial support. The techni-cal assistance of Qian Zhang in the laboratory work is alsogratefully acknowledged.
Aldrich, J. M., Muller, L. D., Varga, G. A. and Griel, L. C.1993. Nonstructural carbohydrate and protein effects on rumen fer-mentation, nutrient flow, and performance of dairy cows. J. DairySci. 76: 1091–1098.Allen, M. S. 1997. Relationship between fermentation acid pro-duction in the rumen and the requirement for physically effectivefiber. J. Dairy Sci. 80: 1447–1462.Armentano, L. and Pereira, M. 1997. Measuring the effective-ness of fiber by animal response trial. J. Dairy Sci. 80: 1416–1425.Association of Official Analytical Chemist. 1990. Official meth-ods of analysis. 15th ed. AOAC Arlington, VA.Ben-Ghedalia, D., Yosef, E., Miron, J. and Est, Y. 1989. Theeffects of starch- and pectin- rich diets on quantitative aspects ofdigestion in sheep. Anim. Feed Sci. Technol. 24: 289- 295.Britton, R. A. and Stock, R. A. 1987. Acidosis, rate of starchdigestion and intake. Pages 25–137 in Okla. Agric. Exp. Stn.Mp–121.Calsamiglia, S., Ferret, Plaixats, A. J. and Devant, M. 2002.Effect of pH and pH fluctuations on microbial fermentation andnutrient flow from a dual-flow continuous culture system. J. DairySci. 85: 574–579.
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Table 6. Stepwise multiple linear regression analysis prediction of apparent acidogenic valuez from dietary chemical composition variables
b0 b1 b2 R2 P value
Model 1 (all feed classes) 0.81 <0.0001Intercept 1.628 0.076NFC (% DM) 0.135 <0.0001ADF (% DM) 0.236 <0.0001
Model 2 (energy sources) 0.701 0.027Intercept 0.805 0.772NFC (% DM) 0.130 0.011ADF (% DM) 0.356 0.022
Model 3 (fibre sources) 0.844 0.027Intercept 18.738 0.003NDF (% DM) –0.146 0.027
Model 4 (protein sources) 0.730 0.014Intercept 10.686 0.002CP (% DM) –0.100 0.014zApparent acidogenic value (AV) (mg Ca g–1 feed DM) = b0 + b1 (x1) + b2 (x2); where b0 = intercept, b1, b2, b3 = slope (unit increase in AV per unit increasein dietary chemical composition variable), x1 and x2 = dietary chemical composition variables (% DM). R2 = coefficient of determination).
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