1392  Thiex et al.: Journal of AOAC International Vol. 95, No.

5, 2012

Agricultural Materials

Determination of Ash in Animal Feed: AOAC Official Method

942.05 Revisited
Nancy Thiex and Lawrence Novotny
South Dakota State University, Olson Biochemistry Labs, Brookings, SD 57007
Andy Crawford

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15737 #5 La Subida Dr, Hacienda Heights, CA 91745

AOAC Official MethodSM 942.05, Ash in Animal Feed, It is a “defining” or “empirical” method, meaning the analytical
has been applied in feed laboratories since its results obtained are defined by the conditions specified for the
publication in the Official Methods of Analysis in method. Any change to the conditions of the method (ignition
1942. It is a routine test with renewed interest due time, ignition temperature, and any other furnace or weighing
to the incorporation of “ash values” into modern conditions) will bias results.
equations for the estimation of energy content of Historically, ash results have posed problems for feed
dairy feed, beef feed, and pet food. As with other laboratories and their clients, in part due to the empirical nature
empirical methods, results obtained are a function of the method. Variability in ash results are well documented
of the test conditions. For this method, the critical in Association of American Feed Control Officials (AAFCO)
conditions are the ignition time, ignition temperature, Check Sample Program method performance reports (3).
and any other furnace or weighing conditions. Renewed interest in tightening the variability of results for
Complete ignition can be observed by the absence of AOAC Official Method 942.05 (4) are due to the incorporation
black color (due to residual carbonaceous material) of “ash values” into modern equations for the estimation of
in the ash residue. To investigate performance energy content of dairy feed (5), beef feed (6), and pet food (7).
of AOAC 942.05, 15 samples were chosen to be In these computations to estimate the energy component of
representative of a wide range of feed materials. feed, a higher ash value computes to a lower energy value.
These materials were tested at the conditions of Since energy is an expensive component of feeds, variability in
AOAC 942.05 (ignition at 600°C for 2 h) and similar or ash results can affect economic value for a given feed product or
more rigorous conditions. The additional conditions ingredient; an incomplete ash will negatively impact economics.
investigated included: 600°C for 4 h; 600°C for 2 h, A perfect residue after ignition of a feed material would be
cool, and ignite 2 additional h; 600°C for 2 h, cool, white, with no hint of residual carbon. A visual examination of
wet, dry, and ignite 2 additional h; 550°C for 6 h; the residue by the analyst can quickly establish if carbonaceous
550°C for 3 h, cool, and ignite 3 additional h; and material has been oxidized and the ignition is complete. Some
550°C for 3 h, cool, wet, dry, ignite 3 additional h. methods call for examination of the residue, while others do not.
Results for all other conditions investigated were The current AOAC Official Method 942.05 does not.
found to be significantly different from the current Today, there are at least four “official” methods for
AOAC Method 942.05. All ignition conditions were determination of ash in animal feed: AOAC Official Method
significantly different from each other except two: 942.05 (1); ISO 5948-2002 (8), Corn Refiners Association
550°C for 3 h, cool, ignite 3 additional h; and 550°C (CRA)-G4 (9); and American Association of Cereal Chemists
for 3 h, cool, wet, dry, and ignite 3 additional h. (AACC) 08-03 (10). Table 1 compares the requirements for
Recommendations for modification to AOAC Official each of the methods.
Method 942.05 are suggested based on statistical
analysis of the data and a review of the literature. Historical Information for AOAC Official Method 942.05

Associate Referee J.L. St. John reported in 1942 (11) that three

A
sh, or crude ash, or more appropriately, “residue on different materials were submitted to each of four laboratories
ignition,” is the inorganic residue remaining after the resulting in 12 total materials. Collaborators determined loss
water and organic matter have been removed by heating on ignition (ash) at the temperature of the then official method
in a muffle furnace. It provides a measure of the inorganic matter (650°C) and also at 500, 550, 600, and 700°C. They were asked
or total mineral content of a feed material. In addition, it can to report results to two decimal places and make observations
provide a quick estimate of contamination or adulteration. For on the appearance of the ash. The materials tested were mash,
example, soil contamination will be reflected in the ash residue barley, alfalfa, oats, mash + 2% shell flour, soybean, wheat,
due to the high silica content of soil (1). Ash is a Type 1 method, tankage, peas, bran and shorts, corn, cottonseed, poultry feed,
as defined by the Codex Alimentarius Commission, Committee herring, and linseed. St. John reported the averages, average
of Methods of Analysis and Sampling Committee scheme (2). differences between duplicates, maximum range, and percent
weight loss due to increase of temperature. Based on these
Received March 25, 2012. Accepted by SG April 15, 2012. calculations, St. John concluded that (1) collaborating analysts
Corresponding author’s e-mail: nancy.thiex@sdstate.edu were able to obtain approximate agreement between their own
DOI: 10.5740/jaoacint.12-129 duplicates; (2) agreement among analysts’ duplicates was good
 Thiex et al.: Journal of AOAC International Vol. 95, No. 5, 2012  1393

Table  1. Comparison of current official methods for crude ash

Method Year Scope Test weight, g Time, h Prechar Temp., °C Surface area, cm2

AOAC 942.05 1942 Feed 2 2 No 600 —

a
ISO 5948-2002 1978; revision 2002 Feed 5 3, or to white ash Yes 550 20
CRA-M4 2009 Feedstuffs 5 4 Yes 525–600 —
AACC 08-03.01 1961; rev 1976 rev 1978 Feedstuffs–rapid method 2 2 No 600 —

a
  If original ash color is black or dark gray, indicating presence of carbon, a repeated ash cycle is specified.

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in many cases, although not “so good” with the mash, mash Experimental
plus added shell flour, and alfalfa and soybean meals; (3) in a
majority of instances, the incremental loss on ignition is greatest Materials
between 500 and 550°C compared to the 50°C steps above
Fifteen samples for study were selected to represent a wide
550°C; (4) with a majority of feed materials, the incremental
range of feed materials and to cover a concentration range from
loss as the temperature is increased by 50°C steps is “within the
2 to 90% crude ash. These reflect the wide range of materials
same general range” between 550 and 700°C (exceptions are
in animal feeds and are representative of the great diversity
mash, mash plus added shell flour, and alfalfa); (5) consistency
of combinations/concentrations of mineral content in feed
of results is essentially the same at temperatures of 550 and
and feed ingredients. The materials studied include fish meal
600°C, as at 650°C and above (a “trifle lower” at 550°C than
(FM), dehydrated alfalfa (DA), mineral vitamin premix (MVP),
at higher temperatures; (6) recovery of mineral elements is
beef cattle conditioner (BC), pork finisher medicated (PFM),
generally better at the lower temperatures than at the higher
chick starter/grower medicated (CSGM), swine feed medicated
temperatures, with better recovery of potassium at 550 and (SFM), corn silage (CS), grass hay (GH), alfalfa hay (AH), horse
600°C than at higher temperatures; and (7) the appearance of feed with molasses (HF), barley (B), adult cat food (ACF), lite
the ash was considered of little or no value in evaluating the loss cat food (LCF), and corn gluten meal (CGM).
of carbon since no consistent variation in color was observed at MVP, BC, PFM, CSGM, SFM, and HF were AAFCO check
any temperature, with some carbon remaining at temperatures as samples used as supplied, with no additional preparation, for
high as 700°C. Based on the conclusions, St. John recommended the proficiency-testing program. All remaining materials were
(1) the temperature of ashing in the official method be reduced prepared in accordance with the National Forage Testing
to 600 or 550°C (from 650°C); (2) careful adherence to the Association (13), AAFCO (14), and AOAC (15) guidelines. CS,
details of the procedure is necessary; and (3) results should be GH, and AH were ground to pass a 1 mm screen in a Tecator
reported to only one decimal place. Cyclotec cyclone mill (Foss North America, Eden Prairie, MN).
In 1943, St. John reported on additional work (12) to compare Other feed materials were ground to pass a 0.75 mm screen
results at 550, 600, and 650°C. Four different feed materials in a Retsch ZM 100 centrifugal mill (Retsch GmbH, Haan,
were submitted to each of four different laboratories, resulting Germany, www.Retsch.com).
in 16 total feed materials. Feed materials selected were
laying mash, dog fish, alfalfa, wheat, starting mash, barley, Apparatus
soybean, straw, developing mash, silage, mill run, dairy feed,
turkey concentrate, corn, linseed, and peas. Averages, average (a)  Muffle furnace.—With programmable controller
differences between duplicates, maximum range, and percent (Thermolyne F6058, Barnstead International, Dubuque, IA).
weight loss due to increase of temperature were reported. (b)  Analytical balance.—Readable to 0.1 mg, electronic
St. John concluded that (1) the results confirmed the 1942 work; (Sartorius, Elk Grove, IL).
(2) precision was generally the same at all three temperatures (c)  Crucibles.—Porcelain, low wide form, 30 mL, numbered
with average difference between duplicates well below 0.1%, with furnace-proof ink (CoorTek, Golden, CO).
with a slight favor of 600°C; (3) maximum ranges were similar (d)  Desiccator.—With stoppered lid (Corning, Acton, MA).
among temperatures, with a slight favor for 550 or 600°C; (e)  Drying oven.—Set to 100°C (Weiss-Gallenkamp,
(4) small progressive decrease in ash residue was observed as the Loughborough, UK).
temperature increases; (5) collaborators noted the importance
of not overloading the muffle furnace; (6) the precision of the Methods
method did not justify utilizing a second decimal place; (7) most
collaborators preferred using 600°C; and (8) there was a critical Fifteen feed materials were tested at the conditions of AOAC
need for a standardized method. 942.05 (ignition at 600°C for 2 h) and similar or more rigorous
As a result of the work of St. John, AOAC Official Method conditions. Two temperatures were chosen, 600 and 550°C,
942.05 (4), Ash in Animal Feed, was adopted as First Action because these temperatures had been studied fairly thoroughly
in 1942. The method simply read “Weigh 2 g test portion into in the past and they had been adopted by AOAC (600°C), AACC
porcelain crucible and place in temperature-controlled furnace (600°C), and ISO (550°C), and the CRA method allowed for
preheated to 600°C. Hold at this temperature 2 h. Transfer either temperature (525–600°C). Additional heating times
crucible directly to desiccator, cool, and weigh immediately. were added to those specified in the AOAC and ISO methods
Report percent ash to first decimal place” (4). to investigate if a longer time would provide more thorough
1394  Thiex et al.: Journal of AOAC International Vol. 95, No. 5, 2012

Table  2.  Average residue on ignition (crude ash), %

Material 600/2 h 600/4 h 600/2 h/wet/2 h 600/2 h/2 h 550/3 h 550/6 h 550/3 h/wet/3 h 550/3 h/3 h

FM 20.08 20.12 20.01 19.97 20.04 20.14 20.00 20.06

DA 8.71 8.62 8.46 8.60 8.87 8.93 8.74 8.87
MVP 89.79 89.42 87.76 85.75 91.33 91.54 91.05 91.15
BC 10.16 10.19 10.10 10.10 10.48 10.42 10.42 10.40
PFM 4.87 4.87 4.86 4.84 5.01 4.97 5.02 4.95

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CSGM 6.96 6.70 6.66 6.61 7.73 7.56 7.58 7.70
SFM 15.49 14.02 12.89 12.61 15.88 15.84 15.52 15.68
CS 3.79 3.74 3.73 3.71 3.84 3.81 3.75 3.74
GH 8.36 8.16 8.28 8.40 8.48 8.42 8.30 8.21
AH 7.64 7.51 7.51 7.55 7.66 7.59 7.43 7.57
HF 8.46 8.42 8.40 8.33 8.80 8.81 8.75 8.80
B 2.41 2.38 2.39 2.41 2.45 2.42 2.39 2.39
ACF 8.68 8.60 8.57 8.64 8.82 8.71 8.73 8.69
LCF 9.03 9.00 8.94 8.99 9.20 9.16 9.11 9.14
CGM 2.45 1.25 1.18 1.23 2.22 1.30 1.29 1.20
LS Mean 13.79 13.53 13.32 13.18 14.06 13.97 13.87a 13.90a
a
  Least square means with same superscript letter are not significantly different (α = 0.05).

ignition. The thoroughness of the ignition was determined by (h)  Carefully add a few mL of distilled H2O to the residue
the color of the ash residue and a corresponding reduction in to break it up. Dry crucible and contents in a 105°C oven
residue weight. The base conditions investigated were ignition (mechanical convection speeds up the process) for a few hours
at 600°C for 2 h and ignition at 550°C for 3 h. The additional until all water is evaporated. The concept is that a crust forms on
conditions investigated included 600°C for 4 h; 600°C for 2 h,
the surface of some materials as they ash. Water is used to break
cool, ignite 2 additional h; 600°C for 2 h, cool, wet, dry, and
ignite 2 additional h; 550°C for 6 h; 550°C for 3 h, cool, and up this crust and allow for a more complete ignition.
ignite 3 additional h; and 550°C for 3 h, cool, wet, dry, and (i)  Reignite crucible and contents in furnace for the specified
ignite 3 additional h. The eight sets of ignition conditions are temperature and time. Repeat steps (e) and (f) before proceeding
referred to later in this manuscript as eight “ignition conditions.” to (k).
(j)  Place crucibles in desiccator with stopper top. Cool
Test Procedure and weigh, recording weight to the nearest 0.1 mg (R). When
opening the desiccator, very slowly loosen the stopper to allow
(a)  Remove crucibles, which have been dried for at least 2 h air to enter slowly, preventing the ash residue from being blown
at 100°C, from oven to desiccator. Cool and weigh, recording
out of the crucibles.
the tare weight to the nearest 0.1 mg (T). Note: New crucibles
(k)  Photograph crucibles containing residue for a visual
need to be ignited at the specified temperature/time of the
method prior to first use. record of residual carbon.
(b)  Program muffle furnace to reach desired temperature (l)  Calculation:
(see Methods above) in 60 min.
(c)  Weigh 1.5–2.0 g test portion into the crucible, recording Residue on ignition (crude ash), % = (R – T) × 100/(W – T).
the weight of crucible and test portion to the nearest 0.1 mg (W).
(d)  Ignite in furnace at time and temperature (see Methods where T = tare (empty) weight of crucible, R = weight of
above) after the furnace reaches temperature. crucible + residue, and W = weight of crucible + test portion
(e)  Allow furnace to cool below 200°C, then transfer
crucibles to a desiccator with stoppered lid. Statistical Analysis
(f)  Cool to room temperature and weigh within 1 h,
recording weight to the nearest 0.1 mg (R). When opening the Statistical analysis was performed using two-way analysis
desiccator, very slowly loosen the stopper to allow air to enter of variance (ANOVA) for the eight sets of ignition conditions
slowly, preventing the ash residue from being blown out of the
and 15 materials (16). The full factorial experimental design
crucibles.
was 8 × 15 treatments with replication in each cell. Significance
(g)  Proceed as follows, depending upon experimental
conditions: was determined with α = 0.05. Having determined a significant
(1)  Skip to (k) if no additional ignition. difference in the methods, a post hoc Tukey HSD test (16) was
(2)  Skip to (i) if reigniting. used to distinguish which ignition conditions were significantly
(3)  Proceed to (h and i) if wetting prior to reigniting. different from others.
 Thiex et al.: Journal of AOAC International Vol. 95, No. 5, 2012  1395

% Ash, Material x ignition condition; ls Means

21 93

19 91

17 89

15 87

% Ash (MVP only)

13 85
% Ash
11 83

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9 81

7 79

5 77

3 75

1 73
FM

DA

MVP

Bc

PFM

csgM

sFM

cs

gH

AH

HF

AcF

lcF

cgM
Materials

Figure  1.  MarkersFigure 1. Markers

represent the mean represent the mean from
% ash estimated % ash estimated
triplicate from triplicate
analysis analysisfor
of each material of each
each set
of ignition conditions. The first
material for four
eachmakers in eachconditions.
set of ignition material group
The represent 600°C conditions
first four markers and thegroup
in each material second
four represent 550°C, so from600
represent the°C
leftconditions
markers represent 600°C/2
and the second h, 600°C/4
four representh, 600°C/2
550 °C, h/wet/2
so fromh,
the600°C/2
left h/2 h,
550°C/3 h, 550°C/6 h, 550°C/3 h/wet/3 h, and 550°C/3 h/3 h. The height of the markers exceeds the 95% confidence interval
markers
derived from the ANOVA represent
error 600°C/2h, 600°C/4h, 600°C/2h/wet/2h, 600°C/2h/2h, 550°C/3h,
term (±0.1).
550°C/6h, 550°C/3h/wet/3h, and 550°C/3h/3h.

Results This is clearly the case for MVP and SFM. CGM does not
have a temperature effect, but does have a clear slope related
Means for 15 materials, expressed as percent crude ash for the to ignition time, flatting at either temperature. This observation
eight experiments, are provided in Table 2. Results were found is confirmed by the black color of the ash residue after the
to be significantly different from the current AOAC Method initial ignition, indicating the presence of significant amounts
942.05 for all additional ignition conditions investigated. All of carbon. Additional ignition greatly lightened the color of the
ignition conditions were also significantly different from each ash residue, corroborating the additional release of carbon. For
other except two: 550°C for 3 h, cool, and ash 3 additional h; FM, B, and CS, there is little impact on results for changes to
and 550°C for 3 h, cool, wet, dry, and ignite 3 additional h. temperature or time.
Figure 1 illustrates the mean percentage of ash estimated Examination of the data indicates that there is no single time
from replicate analysis of each material for each set of ignition and temperature combination suitable for all materials. The ISO
conditions. The eight conditions in each material group are 5948-2002 and AOAC 942.05 methods produce statistically
labeled in Figure 1. The height of the markers exceeds the 95% different results for most feed materials. Examination also raises
confidence interval derived from the ANOVA error term (±0.1). concerns about the 600°C temperature for some materials, such
The presence of carbon was noted in the residues for CGM, as MVP and SFM, that indicate continual loss of weight with
SFM, GH, and B after the initial ignition cycle. The “whiteness” time, suggesting loss of more volatile elements. For these
of the ash improved with additional ignition, as reflected by materials, 550°C appears to be a preferred ignition temperature
lower residue weights. CGM was nearly carbon-free after the for a “constant weight” ignition residue. While a second ash
second heating cycle; however, the ash was still not entirely cycle greatly improved results, there was no apparent advantage
white. to wetting the ash residue between ignitions.
Concerns about ignition temperature lead to a review of
Discussion crude ash methods for all food/feed materials. Most of the
methods were adopted in the early 20th century. Thirty-seven
The results of this study emphasize the empirical nature of of 44 AOAC Official Methods of Analysis methods for crude
the method, and that statistically significant bias will result ash in botanical matrixes (food or products that could be
from any change in ignition temperature or time for most feed considered animal feed ingredients) specify temperatures below
materials. There is no “perfect” time/temperature combination 600°C (17). Similarly, eight of 12 AACC methods for similar
for all feed materials. Some, such as CGM, are resistant to products specify ignition temperature below 600°C (18). Dry
complete ignition, while other high mineral content feed ashing is a common mechanism for removal of organic material
premixes or ingredients may be susceptible to loss of volatile from food and feed materials prior to elemental analysis. Since
elements. full recovery of the elements is critical and loss of inorganics
Figure 1 is an especially revealing visual record of ignition would have been observed, dry ash procedures used in AOAC
temperature and time effects. “Gaps” between the 600 and Official Methods of Analysis methods to remove organic matter
550°C results for a single material are conspicuous, as are from botanical matrixes prior to elemental analysis were
“slopes” with increasing ignition times within a temperature. reviewed. Of the 27 methods, 21 call for ash temperatures of
For example, CSGM, HF, and BC have clearly separated gaps 550°C or below (17), and 10 specify the use of “ashing aids,”
between temperature markers. Sloping lines that continue and such as Mg(NO3)2 and H2SO4, to speed the ashing process and
do not flatten raise a concern for loss of volatile elements. convert elements into less volatile forms. St. John in 1941,
1396  Thiex et al.: Journal of AOAC International Vol. 95, No. 5, 2012

remarked that the effect of ashing temperature on the loss of in 1 h). Allow furnace to cool to below 200°C. Open furnace
elements, particularly potassium, indicates a need for a further door to ensure fresh air supply and repeat the 3 h muffle furnace
study of methods for the determination of potassium in plant heating cycle to carbon-free residue. Final residue should be
material…” (19). The loss of sodium, potassium, and manganese carbon-free (nearly white), not black or gray. Transfer crucible
at ignition temperatures over 500–550°C has been reported in directly to desiccator, cool, and weigh within 1 h, reporting
the years since 1942, when AOAC Official Method 942.05 was results to the appropriate number of significant digits (generally
adopted (20–22). two decimal places). (Note 1: Open desiccator cautiously to
Since the decision on an ignition temperature between 550 and allow air to enter slowly, preventing the ash residue from being
600°C for AOAC Official Method 942.05 was long debated and blown out of the crucibles. Note 2: Care should be taken to not
a somewhat subjective decision by St. John (11, 12), the authors

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overload the furnace).”
suggest that the temperature be reconsidered, or that the method (3) Calculation to appear as follows:
conditions be modified to reduce the scope of application for
the 600°C ignition. It has been recently suggested that a fresh Residue on ignition (crude ash) at 550°C, % =
air supply to the muffle furnace will provide a more thorough (residue mass, g ×100)/(test portion mass, g)
ash in a single 2 or 3 h ignition, but this modification was not
tested in this study.
AOAC Official Method 942.05 specifies adding crucibles Acknowledgments
containing test portions to a hot furnace, a practice that is
susceptible to problems that may lead to loss of material We wish to acknowledge Renata Wnuk and Christelle
from the crucible due to rapid ignition. Placing crucibles into Guillaume for the ash analyses and many contributions to the
a cold furnace and ramping to the final ignition temperature project.
(either manually or with a programmable controller) resolves
issues related to potential loss of residue from crucibles due References
to sputtering or flaming as a result of rapid ignition. AOAC
Official Method 942.05 does not mention prior ignition of new   (1) Thiex, N. (2012) in Distillers Grains: Production, Properties, and
crucibles nor drying of crucibles prior to use. These practices Utilization, K. Liu & K.A. Rosentrader (Eds), CRC Press, Taylor
are commonly known to be necessary to avoid weighing errors and Francis Group, Boca Raton, FL, pp 193–217
associated with residual moisture or loss of crucible weight   (2) Codex Alimentarius Procedural Manual, 2010. http://www.aafco.
during ignition. org/Directory/CommitteePages/CheckSampleProgram.aspx
(accessed July 30, 2010)
  (3) AAFCO Method Performance Reports, AAFCO Collaborative
Recommendations for Modification of AOAC Official
Check Sample Program. http://www.aafco.org/ (accessed July 30,
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  (4) Official Methods of Analysis (2005) 18th Ed., AOAC
Based on the results of this study, a review of methods INTERNATIONAL, Gaithersburg, MD, Method 942.05
and literature, and the omissions mentioned in the preceding   (5) Nutrient Requirements of Dairy Cattle (2001) National Academy
paragraphs, the authors recommend several modifications to Press, Washington, DC, pp 13–18.
AOAC Official Method 942.05. This method should be modified   (6) Tedeschi, L.O., Fox, D.G., & Doane, P.H. (2005) Prof. Anim. Sci.
to allow the ramping of furnaces to the desired temperature to be 21, 403–415
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recommend weighing to the nearest 0.1 mg, and to specify the Publication of the American Feed Control Officials Inc., Oxford,
use of preignited, predried crucibles. The authors recommend IN, p. 141
  (8) ISO 5948-2002, Animal Feeding Stuff–Determination of Crude
lowering the ignition temperature to 550°C; however, they also
Ash, International Organization for Standardization, Geneva,
recommend two 3 h ignition cycles to ensure release of carbon
Switzerland
from the feed material. To emphasize the empirical nature of   (9) CRA-M4, Ash (Residue on Ignition). http://www.corn.org/wp-
the method, the authors suggest revising the title to include the content/uploads/2009/12/M-4.pdf (accessed January 24, 2012)
ignition temperature and labeling the method as “Residue on (10) AACC International Approved Methods of Analysis (1978) 11th
Ignition (Crude Ash).” This title change is consistent with the Ed., Method 08-03.01, AACC International, St. Paul, MN. DOI:
titling of moisture methods as “Loss on Drying.” 10.1094/AACCIntMethod-08-03.01
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(2) The text to read as follows:
National Forage Testing Association, Omaha, NE
“Weigh approximately 2 g test portion into preignited,
(14) Thiex, N., Novotny, L., Ramsey, C., Latimer, G., Torma, L., &
predried, preweighed porcelain crucible, recording all weights Beine, R. (2000) Guidelines for Preparing Laboratory Samples,
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may be programmed. Manual temperature-controlled furnaces (16) StatSoft, Inc. (2004) STATISTICA (data analysis software
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(17) Official Methods of Analysis (2005) 18th Ed., AOAC Y.P. (Ed.), Soil and Plant Analysis Council, Inc., CRC Press, Boca
INTERNATIONAL, Gaithersburg, MD. Raton, FL, p. 53
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