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* South Dakota State University, Brookings 57007;
and
Texas Tech University, Lubbock 79409
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Abstract |
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 Top Abstract IntroductionAnalytical MethodsMaterials and MethodsResults and DiscussionImplicationsLiterature Cited |
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Key Words: Analytical Methods • Drying Methods • Feeds • Moisture Content • Ovens
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Introduction |
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TopAbstractIntroductionAnalytical MethodsMaterials and MethodsResults and DiscussionImplicationsLiterature Cited |
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Accurate moisture determinations are critical in the feed industry for many reasons: 1) water is weight and must be paid for when grain [feed] is bought and sold (Hunt and Pixton, 1974
); 2) water is weight and must be shipped; 3) moisture content plays a role in storage conditions; 4) moisture in feedstuffs and mixed feeds works as a diluent to energy, protein, minerals, and vitamins in diets for animals. Moisture determinations are used to convert all other nutrients to a dry matter basis; therefore, errors in moisture determination are incorporated into other nutrient concentration calculations; 5) proper moisture concentrations in diets are necessary for optimum intake and performance of animals; thus, an accurate determination of moisture in ingredients is necessary.
Factors affecting accurate determination include range of moisture content, sampling of feedstuffs, transport and storage of laboratory samples, laboratory sample preparation, including grinding, and bias and variability associated with the specific analytical method used. Inaccurate moisture analysis has a direct and negative influence on the precision of diet formulation, in balancing consumption of other nutrients, and in predicting performance.
Feedstuffs have been classified by Kellems and Church (2002) as including eight categories: dry roughages, pasture and range grasses, ensiled roughages, high energy concentrates, protein sources, minerals, vitamins and additives. An animal feed may contain one or all eight categories, complicating the analytical matrix. Methods for determining moisture in these matrices are compared in this study.
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Analytical Methods |
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TopAbstractIntroductionAnalytical MethodsMaterials and MethodsResults and DiscussionImplicationsLiterature Cited |
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. During oven drying, volatile substances other than water are lost and side chemical reactions occur during the heating process. Windham et al. (1987) and Thiex and Van Erem (1999) reinforced Mo and Tjornhom’s findings, suggesting no progress on improvement of moisture testing over two or more decades. Even though oven methods are prone to error, they remain commonly used because the determinations are fast and inexpensive to perform.
A second type of analytical method to determine moisture methods are those that extract the water molecules from feedstuffs and measure water concentration against a calibration standard. This includes the Karl Fischer (KF) method, which was collaboratively studied and received first action status as AOAC Official Method 2001.12 in 2001 (Thiex and Van Erem, 2002). It might also include azeotropic distillation (AOAC, 2000) however, this method is no longer widely used in laboratories.
A third approach for determining moisture is the development of NIR calibrations based on a primary method. The approach was established by Windham et al. (1991) as an alternative to oven methods for forages. A critical aspect of NIR methods is in the choice of the primary method for calibration.
Sources of Error
Hunt (1974) described moisture behavior and measurements related to cereal grains and discussed sources of error. Horwitz (1990) elaborated on sources of error specific to the determination of moisture by evaporation. Sources of error that apply to all moisture methods include: 1) representative-ness of laboratory sample; 2) storage conditions of both laboratory and analytical samples; 3) grinding techniques (exposure to air, generation of heat, contamination, fineness of grind, necessity for two step moisture determination for high moisture feeds); 4) weighing errors; 5) test portion size; 6) room humidity; 7) nonaqueous losses or interferences (specificity/selectivity of the method). Additional sources of error for oven drying methods include: 8) drying time and temperature; 9) oven (temperature stability, heating uniformity, ventilation, airflow, recovery rate, thermometer accuracy); 10) drying pan (area, nature, size and placement in oven); 11) desiccant.
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Materials and Methods |
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TopAbstractIntroductionAnalytical MethodsMaterials and MethodsResults and DiscussionImplicationsLiterature Cited |
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Analytical Methods
A large number of moisture methods are discussed or compared throughout this paper. A letter designation, brief description and the reference for each are provided in Table 1. The methods will be referred to by the letter designation in subsequent tables.
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was approved as first action AOAC Official Method 2001.12, Determination of Water (Moisture)/Dry Matter in Animal Feed, Grain and Forage (Plant Tissue), Karl Fischer Titration Methods, based on an interlaboratory collaborative study published in 2002 (Method KF1) (Thiex and Van Erem, 2002). The principle of this method is to extract water from animal feed or forage material into methanol–formamide (1 + 1) directly in the Karl Fischer titration vessel using high-speed homogenization. The water is then titrated at 50°C with one-component Karl Fischer reagent based on imidazole. The high-speed homogenization ruptures cells, allowing for titration of free and bound water. Comparison from AAFCO Check Sample Program
The American Association of Feed Control Officials (AAFCO) operates the AAFCO Check Sample program for feed laboratories. The program consists of monthly proficiency samples sent to ~300 participating laboratories worldwide. The series includes a variety of complete feeds, premixes, concentrates and supplements with drugs, antibiotics, minerals, and vitamins at levels normally encountered in commercial products. Analysts are asked to perform single determinations on the check sample on two separate days and submit the results, and code data to specific analytical methods. Moisture data was extracted from 20 years of check sample data. Five oven methods, a Karl Fischer (Method G), azeotropic distillation (Method H), and a thermogravimetric (Method F) method were compared to oven drying under vacuum at 95°C for 5 h (Method J). The oven methods included drying at 135°C for 2 h (Method A), drying at 104°C for 3 h (Method B), drying under vacuum at 60°C for 18 h (Method C), drying at 102°C for 16 h (Method D) and drying under vacuum at 70°C (Method E).
Comparisons of Oven Method to Karl Fischer for Forages
Two published studies compared oven methods to Karl Fischer for forages. The first was that of Windham (1987) and the second was that of Thiex and Van Erem (1999).
Thirty forages evaluated by Windham et al. included six bermudagrasses, six temperate grasses, six legumes, six silages, and six silage based rations. All materials were dried by forced air oven at 65°C, and ground in a Wiley mill to pass through a 1-mm screen. Moisture was determined by oven drying at 135°C for 2 h (Method A) and by Karl Fischer titration (Method KF2) using the extraction procedure of Robertson and Windham (1983) and water calculation as reported by Jones (1985).
The forages evaluated by Thiex and Van Erem (1999) included hay, haylage, and corn silage. Hay materials included four each of grass hay, legume hay, and legume-grass mix. Haylage materials included one grass silage, nine legume silages, and one legume/grass mixed silage and ten corn silages. Materials were dried in a microwave oven, and ground allowing to pass a 1mm sieve in a cyclone mill.
Moisture (as water) was determined in triplicate on all materials using the Karl Fischer method, and in duplicate with the oven methods as follows: drying at 135°C for 2 h (Method A), 104°C for 3 h (Method B), and 104°C for 6 h (Method I).
National Forage Testing Association Assessment of Moisture Methods in Forages
The National Forage Testing Association (NFTA) was founded in 1984 as a joint effort of the American Forage and Grassland Council, the National Hay Association and forage testing laboratories in a concentrated effort to improve the accuracy of forage testing and build grower confidence in testing animal feeds. One of the activities of the organization to improve repeatability and accuracy of forage testing among laboratories is a laboratory proficiency testing program. Bimonthly samples are sent to participating laboratories. The series includes three alfalfa hay, one alfalfa-grass mixed hay, one grass hay and one corn silage annually.
Laboratories are evaluated on performance compared to a reference method. The NFTA reference method for moisture was established in 1993 as oven drying at 135°C for 2 h (Method A) (Undersander et al., 1993). In 2000, the NFTA Board of Directors established a moisture task force to investigate the reproducibility, accuracy, and applicability of various oven moisture methods for estimating moisture in forages. Two alfalfa hay, two grass hay and two corn silage materials were sent to each of eight labs. All materials came from previous NFTA Proficiency Testing materials. Each lab ran moisture by three oven methods: 105°C for 3 h (Method B); 105°C for 6 h (Method I); and 135°C for 2 h (Method A). One lab ran Karl Fischer (Method KF1).
Comparisons of Oven Methods to Karl Fischer for Nonurea and Urea Feeds
Six feeds containing urea and ten feeds containing no urea were compared by Thiex and Van Erem (1999). The feed materials were ground to pass a 1mm sieve in a Retsch centrifugal mill, except for the soybeans and cat food, which were ground using a Tecator Knifetec mill. Urea containing feeds represented a range of urea levels, from 0.08 to pure feed grade urea. Nonurea containing feeds represented a cross section of commercial feeds and feed ingredients. Moisture (as water) was determined in triplicate on all materials using Karl Fischer (Method KF1), and with oven methods as follows: drying at 135°C for 2 h (Method A), 104°C for 3 h (Method B), 95°C for 5 h under vacuum (Method J), 104°C for 6 h (Method I), 110°C for 3 h (Method L).
In another study, Shreve et al. (2000) compared four drying methods with Karl Fischer moisture titration (Method KF1) on ten urea supplements and feed grade urea to investigate method effect on urea loss. Supplements ranged from 35.4% to 71.6% crude protein and from 3.2% to 49.3% nonprotein nitrogen. All moisture determinations were made in triplicate. Drying methods included: convection oven at 105°C for 3 h (Method B), vacuum oven 60 mm Hg at 60°C for 20 h (Method N), vacuum oven 60 mm Hg at 95°C for 5 h (Method J), and vacuum oven 30 mm Hg at 70°C for 20 h (Method O). Means were tested at the 
= 0.01 level with a paired t-test.
Comparison of Oven Methods to Karl Fischer Method for Steam-Flaked Corn and Corn Products
Five steam-flaked corn batches produced from two different steam flaking units at Texas Tech University were used to determine moisture addition as compared to whole shelled corn of the same source, and moisture level of the steam-flaked batches as determined by oven drying and Karl Fischer. A small scale pilot steam flaker and a commercial model steam flaker were used to compare moisture levels of corn processed under routine conditions. Flaking units differed in the design of the steam cabinets (round vs. rectangular), but the diameter and corrugation of the rolls were the same.
Moisture was determined in triplicate on all steam-flaked samples and the whole shelled corn sample by oven drying overnight (15 h) at 110°C (Method M), and by Karl Fischer (Method KF1). Oven drying was achieved by weighing samples into aluminum pans and placing them inside a forced air oven without vacuum overnight. Samples were removed from the oven, placed in a desiccator until they were reweighed to determine loss in weight.
The Corn Refiners Association (CRA) is experienced in the measurement of moisture of corn raw material and the products of the corn wet milling process. By the mid 1950’s, CRA industry members had come to rely on azeotrope distillation of water-toluene and water-benzene systems as methods of reference to standardize oven drying methods (Analytical Methods of the member companies of the Corn Refiners Association Inc, 1999), and had standardized two oven methods relative to azeotrope distillation: 80°C, 20 h under vacuum and 100°C, 4 h under vacuum (Method P). CRA member laboratories also had an early interest in the Karl Fischer method and first adopted it for the determination of water in concentrated steep water, which contains water-soluble extractives from the steeping of corn. For steepwater, azeotrope distillation was not practical in routine work and oven-drying methods were not reproducible owing to variable contents of volatile matter other than water in the product. In 1996, a Karl Fischer method was proposed by Corn Products International to measure the moisture contents of whole corn based upon the extraction of water by ball milling a test sample to a paste in the presence of anhydrous methanol. It was subjected to an inter-laboratory evaluation and was found to agree with results obtained by azeotrope distillation (Bernetti et al., 1996). The CRA therefore became interested in the method of Thiex and Van Erem (1998) (Method KF1). A technical committee of the CRA decided that it could be adopted for corn, but it would be necessary to obtain supplementary evidence of applicability to corn gluten meal and corn gluten feed before adoption as a CRA method. The CRA objective was to determine if Karl Fischer Method KF1 could be established as a new primary method for standardizing and monitoring the secondary loss-on-drying methods in corn and corn products. (Raffaele Bernetti and Jennifer Snyder, unpublished data).
The CRA committee chose samples available from internal check studies. Typically, these samples are used to monitor product quality between plants at each company and, therefore have known analytical histories and traceability. Two samples of corn gluten meal were submitted by Corn Products International and samples, respectively, of corn gluten feed and corn gluten meal were submitted by Roquette America for analysis by Karl Fischer (Method KF1). All four samples had been analyzed by oven method P.
Assessing the Potential for NIR Moisture Determinations in Feed
Windham et al. (1987, 1991) had established NIR as a valid technique for moisture determination in forages. The potential for extending an NIR method for moisture in forages to determine moisture in feeds was reported by Thiex and Van Erem (1999). Evaluation was based on a standard error of calibration (SEC), correlation coefficient, and partial least squares calibration.
Partial least squares calibration equations were generated using a 1, 4, 4, 1 math treatment, where the first value indicates the first derivative, the second value indicates a gap of four over which the derivative is calculated, the third value is the number representing the smoothing of points, and the fourth value indicates no second smooth.
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Results and Discussion |
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TopAbstractIntroductionAnalytical MethodsMaterials and MethodsResults and DiscussionImplicationsLiterature Cited |
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The comparison of check sample results submitted by participating laboratories over a 20-year period on eight methods for the determination of moisture is provided in Table 2 (George W. Latimer, AAFCO Check Sample Committee Chair, unpublished data). These comparisons are of special interest because they represent the methods routinely practiced in feed laboratories in the US and other countries. The most widely used methods are Method J (at least 223 check samples), Method B (223 check samples), followed by Method A (157 check samples) and Method D (96 check samples). Recoveries were based on AOAC Method 934.01 (Method J). Mean recoveries of water range from 92.07% for Method G to 113.17% for Method A. However, averaging over all samples masks the extent of the problem. Minimum and maximum percent recoveries within a method for individual check samples varied widely, and were 100.10 and 202.03, respectively, for Method A; 76.45 and 155.04 for Method B; 61.28 and 331.92 for Method C; 79.09 and 153.66 for Method D; 75.45 and 197.05 for Method E; 81.35 and 115.79 for Method F; 53.81 and 153.39 for Method G; and 69.42 and 119.27 for Method H. Few conclusions can be drawn from this comparison about the merits of a particular method. But the wild fluctuations demonstrate the inappropriateness of extending an empirical oven drying method for estimating moisture to matrices for which they have not been validated.
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Percent moisture bias in dry forage materials of oven methods compared to Karl Fischer are illustrated in Figure 1a and average percent recovery for oven methods compared to Karl Fischer methods are illustrated in Figure 1b. Windham et al. (1987) reported Karl Fischer and oven means (Method A) different (P < 0.05) for legume and temperate forage, and no difference for bermudagrass. Likewise, Thiex and Van Erem (1999) reported Oven Method A different (P < 0.05) from Karl Fischer for seven of the twelve hay materials. Both authors observed a 121% recovery of moistures from legume hay for Oven Method A compared to Karl Fischer methods. Windham et al. observed a 109% recovery for temperate grasses, and a 100% recovery for bermudagrass for Oven Method A. Thiex and Van Erem (1999) observed a 111% recovery for legume/grass mixed hay and a 108% recovery for grass hay for Oven Method A compared to Karl Fischer. Observations of the two authors were very similar, even though they were using different extractions prior to the Karl Fischer titration. Both authors concluded that Oven Method A was inappropriate for use with legume and temperate forage materials.
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reported on two additional oven methods: Oven Method B means were different (P < 0.05) from the Karl Fischer mean for just one of twelve hay materials and Oven Method I mean different (P < 0.05) from the Karl Fischer mean for two of twelve hay materials. They observed an average percent recovery for Oven Method B compared to Karl Fischer of 99% for legume hay, 93% for legume/grass mixed hay, and 96% for grass hay. They observed an average percent recovery for Oven Method I compared to Karl Fischer of 100% for legume hay, 93% for legume/grass mixed hay, and 99% for grass hay.
Windham et al (1987) reported Karl Fischer and oven means (Method A) different (P < 0.05) for silages and different (P < 0.05) for silage-based rations. Likewise, Thiex and Van Erem (1999) reported Oven Method A different (P < 0.05) from Karl Fischer for ten of eleven haylage materials and for all ten corn silage materials. Windham et al (1987) observed a 151% recovery for Oven Method A compared to Karl Fischer for silage and a 121% recovery for silage based rations. Thiex and Van Erem (1999) observed a 159% recovery for legume silage, a 191% recovery for mixed haylage, 159% recovery for grass silage and 133% recovery for corn silage. When averaged together, the recovery of 148% on all silage materials compares to the 151% recovery observed by Windham et al.
Thiex and Van Erem (1999) reported on two additional oven methods for silage materials: Oven Method B means were different (P < 0.05) from the Karl Fischer mean for nine of eleven haylage materials and five of ten corn silage materials. They observed an average percent recovery for Oven Method B compared to Karl Fischer of 120% for alfalfa haylage, 139% for mixed haylage, 123% for grass silage, and 113% for corn silage. Oven Method I means were different (P < 0.05) from the Karl Fischer mean for ten of eleven haylage materials, and for seven of ten corn silage materials. They observed an average percent recovery for Oven Method I compared to Karl Fischer of 127% for alfalfa haylage, 148% for mixed haylage, 130% for grass silage, and 117% for corn silage.
Thiex and Van Erem (1999) reported correlation coefficients and slope of Karl Fischer and each oven method for forage materials (Table 3). The correlation coefficients (r) for hay, haylage, and corn silage for Karl Fischer vs. Method A were 0.85, 0.50, and 0.61, and the slopes were 0.76, 0.30, and 0.62, respectively. The correlation coefficients for hay, haylage, and corn silage for Karl Fischer vs. Method B were 0.97, 0.45, and 0.84, and the slopes were 0.95, 0.41, and 0.79, respectively. The correlation coefficients for hay, haylage, and corn silage for Karl Fischer vs. the Method I method were 0.97, 0.42, and 0.78, and the slopes were 0.92, 0.38, and 0.74, respectively.
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For ensiled forages, the loss of volatile substances other than water is drastic with oven heating. This is logical due to the loss of volatile fatty acids that occur in ensiled products. Overall, the Method B oven method most closely approximated Karl Fischer. It appears that with this method only 96% of the water is removed; however, the loss of volatile substances other than water more than compensates for the incomplete removal of water from the ensiled products.
National Forage Testing Association Assessment of Moisture Methods in Forages
The average moisture for the Karl Fischer, Method A, Method B, and Method I methods were 7.08, 8.93, 7.35, and 7.71% respectively (NFTA Moisture Task Force Reports, 2001). Standard deviations for the respective methods were 0.73, 0.81, 0.82, and 0.80. Percent moisture bias as compared to Karl Fischer is illustrated in Figure 1a
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Analysis of variance for each sample revealed that labs and methods all resulted in different oven moisture values. All labs’ results were within ± 3 standard deviations of the mean for each sample. For one of the grass hay samples, removing two outlying labs eliminated the significant effect of lab on oven moisture results. In every case, Method A produced higher moisture results than the other methods. Little difference was observed in reproducibility among the various methods.
The NFTA study grouped all forages (hays and silages) together for statistical analysis. Paired t-tests comparing each oven method to Karl Fischer moisture indicated no difference (P < 0.01) between Method B and Karl Fischer values for the materials studied. Both the Method I and Method A resulted in higher moisture results than Karl Fischer (Method KF1).
The NFTA Moisture Committee concluded that, based upon agreement with Karl Fischer, Method B most closely represents the true moisture content of alfalfa and grass hays. Even for corn silage, this method appears better than other oven methods currently available. Their conclusion is consistent with those of Thiex and Van Erem (1999) and Windham et al. (1987).
Comparisons of Oven Methods to Karl Fischer for Nonurea and Urea Feeds
The average percent moisture obtained for feedstuff and mixed feed materials and average percent recovery of moisture compared to the Karl Fischer method are reported in Table 4. Also provided is a summary of the number of materials which were different (P < 0.05) from the Karl Fischer means for each oven method. The percent recovery of water for the Method A, Method B, Method J, Method I, and Method L means were 116, 88, 83, 90, and 94 respectively, for nonurea feeds.
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) with oven drying methods. For feeds which 
6.6% urea, volatilization of urea during drying for Method B, Method J, Method I and Method L was small enough to keep water recoveries at or below 105%. However, significant urea losses were observed with Method A, even at less than a concentration of 1% urea.
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. The correlation coefficients for feed with urea and feed without urea for Karl Fischer vs. the Method A method were 0.44 and 0.30, and the slopes were -0.30 and 0.86, respectively, indicating Method A is unsuitable to estimate water in either group. The correlation coefficients for feed with urea and feed without urea for Karl Fischer vs. the Method B were 0.96 and 0.99, and the slopes were 1.11 and 1.08, respectively, indicating Method B may be a suitable method to estimate water in feed. The correlation coefficients for feed with urea and feed without urea for Karl Fischer vs. the Method I were 0.75 and 0.99, and the slopes were 1.03 and 1.15, respectively, indicating little or no improvement in performance over Method B. The correlation coefficients for feed with urea and feed without urea for Karl Fischer vs. the Method J were 0.30 and 0.95, and the slopes were 0.80 and 1.00, respectively, indicating Method I is suitable only for nonurea feeds or feeds containing less than 5% urea. The correlation coefficients for feed with urea and feed without urea for Karl Fischer vs. the Method L were 0.76 and 0.98, and the slopes were 1.09 and 1.19, respectively, indicating no general improvement over Method J.
Comparisons made by Shreve et al. (2000), found mean biases of 0.24, 1.10, 0.92, and 0.51 for the Method B, Method N, Method J, and Method O, respectively, as compared to Karl Fischer (See Table 5). The Karl Fischer and Method B produced comparable moisture values ( = 0.01). All vacuum oven methods tested resulted in significantly lower moisture content compared to the Karl Fischer moisture. No relationship of nonprotein nitrogen was observed for any of the methods evaluated (Table 5). The mean biases reported by Shreve et al. tend to mask the large range in minimum to maximum bias and % recovery. Even Method B, which was not statistically different from Karl Fischer had a range in bias from -0.69 to 1.73 (range in recovery from 112% to 73%).
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Comparison of Oven Methods to Karl Fischer Method for Steam-Flaked Corn and Corn Products
Previous research by investigators has shown that steam flaking of corn and grain sorghum improves starch utilization and feed conversion by feedlot cattle, and that moisture level and temperature are critical factors in controlling the quality of flakes produced (Richardson, 1996a; Richardson, 1996b). Moisture in whole shelled corn and in the same source of corn after steam flaking with two different flaking units is presented in Table 6. The average bias of percent moisture determined by Method M and by Karl Fischer for steam-flaked corn was small (0.23). However, a greater bias for whole shelled corn was observed (-1.11) than for steam-flaked samples. These data indicate that oven drying slightly overestimated moisture in steam-flaked corn, and underestimated moisture in whole shelled corn by 8.5%, as compared to the Karl Fischer method. The difference between the two methods was greater for whole shelled corn and the use of the Karl Fischer method offers potential for improving accuracy of moisture determination.
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and indicate very good agreement between Karl Fischer method KF1 and method P. The Association concluded that the Karl Fischer method could be adopted as a primary method for the standardization of secondary loss-on-drying methods.
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Results of NIR calibrations for forage materials by four methods are reported in Table 8. The standard error of calibration (SEC) and correlation coefficient (r) were 0.203 and 0.980 for Karl Fischer, 0.504 and 0.958 for Method A, 0.356 and 0.968 for Method B, and 0.412 and 0.962 for the Method I respectively. These results indicate that Karl Fischer is the method of choice for NIR calibrations.
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. Partial least squares calibration equations were generated using a 1, 4, 4, 1 math treatment. The SEC and R2 were 0.358 and 0.980 on feed materials alone for Karl Fischer, and 0.298 and 0.982 on all feed and forage materials combined, respectively. Preliminary NIR calibrations were made for two reasons: 1) to develop an idea of the accuracy of the reference method to measure water (good calibrations cannot be obtained with a poor reference method), and 2) to determine the feasibility of using Karl Fischer as a potential reference method for NIR calibrations for the determination of moisture in a diverse set of animal feed materials.
NIR calibrations for moisture in forages were made with no effort to optimize the calibrations, or to select samples for calibration based on spectra. Simply, the samples used for the experiments were used to determine the feasibility and relative accuracy of NIR calibrations for moisture (dry matter) based on Karl Fischer and the three oven methods. Best calibrations were obtained using Karl Fischer because calibrations could be made directly for water (best calibration data), while calibrations for the oven methods would be based on water and various other volatile components of the forage materials.
NIR calibrations based on Karl Fischer were also attempted for feeds even though the population was small (14 samples). Considering the diversity of the samples, different grinding procedures, and small population, good calibrations were obtained indicating excellent feasibility for robust calibrations for moisture in animal feed. The 33 forage samples and 14 feed samples were combined to determine the feasibility of a mixed calibration. Calibrations were acceptable for combined feed and forage samples, indicating one calibration for forages and feeds is feasible with additional work. For laboratories with NIR capabilities, NIR calibrations for moisture based on Karl Fischer appear to be a vast improvement over oven methods.
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Implications |
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TopAbstractIntroductionAnalytical MethodsMaterials and MethodsResults and DiscussionImplicationsLiterature Cited |
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Footnotes |
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1 Correspondence: Box 2170, ASC 133 (phone: 605-688-5466; fax: 605-688-6295; E-mail: nancy_thiex{at}sdstate.edu).
Received for publication November 26, 2002. Accepted for publication March 31, 2003.
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TopAbstractIntroductionAnalytical MethodsMaterials and MethodsResults and DiscussionImplicationsLiterature Cited |
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