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J. Dairy Sci. 88:748-756
© American Dairy Science Association, 2005.

Feeding Micronized and Extruded Flaxseed to Dairy Cows: Effects on Blood Parameters and Milk Fatty Acid Composition

C. Gonthier1, A. F. Mustafa1, D. R. Ouellet2, P. Y. Chouinard3, R. Berthiaume2 and H. V. Petit2

1 Department of Animal Science, McGill University-Macdonald Campus, Ste-Anne-de-Bellevue, QC, Canada H9X 3V9
2 Agriculture and Agri-Food Canada, Dairy and Swine Research and Development Centre, Lennoxville, QC, Canada J1M 1Z3
3 Département des Sciences Animales, Université Laval, Pavillon Paul-Comtois, QC, Canada G1K 7P4

Corresponding author: A. F. Mustafa; e-mail: Arif.Mustafa{at}mcgill.ca.


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSION
 ACKNOWLEDGEMENTS
 REFERENCES
 
Four lactating Holstein cows fitted with ruminal and duodenal cannulas were used in a 4 x 4 Latin square design to determine the effects of feeding micronized and extruded flaxseed on milk composition and blood profile in late lactation. Four diets were formulated: a control (C) diet with no flaxseed, a raw flaxseed (RF) diet, a micronized flaxseed (MF) diet, and an extruded flaxseed (EF) diet. Flaxseed diets contained 12.6% flax-seed (dry matter basis). Experimental periods consisted of 21 d of diet adaptation and 7 d of data collection. Feeding flaxseed reduced milk yield and energy-corrected milk by 1.8 and 1.4 kg/d, respectively. Yields of milk protein and casein were also lower for cows fed flaxseed diets than for those fed the C diet. Milk yield (1.6 kg/d) and milk fat percentage (0.4 percentage unit) were lower for cows fed EF than those fed MF. Plasma cholesterol and nonesterified fatty acid concentrations were higher for cows fed flaxseed diets relative to those fed the C diet. Flaxseed supplementation decreased plasma concentrations of medium-chain (MCFA) and saturated (SFA) fatty acids and increased concentrations of long-chain (LCFA) and monounsaturated fatty acids. Feeding flaxseed reduced the concentrations of short-chain fatty acids (SCFA), MCFA, and SFA in milk fat. Consequently, concentrations of LCFA and unsaturated fatty acids were higher for cows fed flaxseed diets than for those fed the C diet. Flaxseed supplementation increased average concentrations of C18:3 and conjugated linoleic acid by 152 and 68%, respectively. Micronization increased C18:3 level, and extrusion reduced concentrations of SCFA and SFA in milk. It was concluded that feeding raw or heated flaxseed to dairy cows alters blood and milk fatty acid composition. Feeding extruded flaxseed relative to raw or micronized flaxseed had negative effects on milk yield and milk composition.

Key Words: flaxseed • milk composition • fatty acid • micronization

Abbreviation key: C = control, CLA = conjugated linoleic acid, EF = extruded flaxseed diet, LCFA = long-chain fatty acid, MCFA = medium-chain fatty acid, MF = micronized flaxseed, RF = raw flaxseed, SCFA = short-chain fatty acid, SFA = saturated fatty acid


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSION
 ACKNOWLEDGEMENTS
 REFERENCES
 
Research has shown several health benefits of n-3 fatty acids to humans including a decrease in the incidence of cancer, cardiovascular diseases, hypertension, and arthritis and an improvement of visual acuity (Simopoulos, 1996; Wright et al., 1998). Milk fat contains low concentrations of n-3 fatty acids and high levels of saturated fatty acids (SFA), particularly C16:0, which has hypercholesterolemic properties (Kennelly, 1996). Increasing the level of {alpha}-linolenic acid, an n-3 fatty acid, and other polyunsaturated long-chain fatty acids (LCFA) while reducing the proportion of C16:0 can there-fore be considered an attractive way to modify milk composition, which would increase human consumption of milk and dairy products. Flaxseed contains a high oil level (40% of total seed weight), with {alpha}-linolenic acid constituting approximately 55% of total fatty acids of the oil (Mustafa et al., 2002; Petit, 2002, 2003).

Feeding flaxseed to dairy cows reduces the concentrations of short-chain fatty acids (SCFA) and medium-chain fatty acids (MCFA) and increases that of LCFA in milk fat (Mustafa et al., 2003; Petit, 2003). However, the increase in {alpha}-linolenic acid content in milk is small, suggesting that most of the dietary {alpha}-linolenic acid is subjected to extensive biohydrogenation by ruminal bacteria. If sufficient protection from ruminal biohydrogenation is provided to dietary fat, concentration of {alpha}-linolenic acid in milk fat can be increased up to 20% of total fatty acids (Chilliard et al., 2000).

Heat treatment is commonly used to protect oilseeds from ruminal degradation (AbuGhazaleh et al., 2002; Mustafa et al., 2002; Mustafa et al., 2003). Kennelly (1996) suggested that the application of heat to high-fat products such as flaxseed can denature the protein matrix surrounding the fat droplets and, therefore, protects fat from ruminal biohydrogenation. Micronization and extrusion are heat treatments that can be used to protect oilseeds from ruminal degradability (Pena et al., 1986; Chouinard et al., 1997; Mustafa et al., 2002). Data on milk and plasma fatty acid profiles and on transfer efficiency of unsaturated LCFA from cows fed unheated and heat-treated flaxseed are limited. Therefore, the objectives of this study were to determine the effects of flaxseed supplementation and heat treatment of flaxseed on milk and plasma fatty acid composition and to estimate transfer efficiency of these fatty acids from feed to milk fat of lactating dairy cows.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSION
 ACKNOWLEDGEMENTS
 REFERENCES
 
Animals and Treatments
The present study is part of a larger project with portions of the results reported elsewhere (Gonthier et al., 2004; Gonthier et al., accepted). Heat treatments and experimental procedures were described in detail by Gonthier et al. (2004). Briefly, ground flaxseed was micronized at 115°C for 90 s or extruded at 155°C with a retention time of 43 s. Four multiparous lactating Holstein cows (BW, 595 ± 32 kg; 225 ± 17 DIM) fitted with ruminal and closed-T duodenal cannulas were used in a 4 x 4 Latin square design with experimental periods consisting of 21 d of diet adaptation and 7 d of data collection. Dietary treatments consisted of a control (C) diet with no flaxseed added, a ground raw flax-seed (RF) diet, a micronized flaxseed (MF) diet, and an extruded flaxseed (EF) diet. All diets were formulated to meet nutrient requirements of late lactating dairy cows for milk production at 20 kg/d with 4% fat (NRC, 2001). Flaxseed diets consisted of a 55:45 forage:concentrate ratio (DM basis); the C diet had a 64:36 forage:concentrate ratio (DM basis). The forage part of the diets consisted of 60% grass silage and 40% corn silage (DM basis). Ingredients and chemical composition of the 4 dietary treatments are shown in Table 1Go. Diets were fed as TMR twice daily at 0830 and 1530 h for ad libitum intake. Flaxseed was added as top-dress for the RF, MF, and EF diets. The amount of flaxseed was adjusted daily for each cow based on the amount of TMR offered so that flaxseed was offered at a constant proportion of the dietary DM (12.6%). Cows were milked twice daily at 0930 and 2130 h, and milk yield was recorded from d 22 to 28. Milk samples were collected at afternoon milking on d 26, both milkings on d 27, and morning milking on d 28. Milk samples were then pooled by proportion according to milk yield at each milking.


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  		Table 1. Ingredient and chemical composition of the diets (DM basis).
 
Diets were sampled twice during each collection week and were composited by period. The composited samples were oven-dried at 55°C for 48 h, ground through a 1-mm screen using a Wiley mill (Arthur H. Thomas, Philadelphia, PA), and stored at room temperature for later analysis. Orts were measured every day to determine daily intake for each cow.

Sample Collection and Chemical Analyses
Chromic oxide was used as an inert external marker to determine duodenal flow of fatty acids. Gelatin capsules containing 8 g of Cr2O3 were inserted into the rumen of each cow twice daily starting on d 15 of each period. Duodenal (400 mL) and fecal (350 g wet basis) samples were collected on d 25 (0800, 1600, 2400 h), d 26 (1200 and 2000 h), and d 27 (0400 h) of each period to represent samples every 4 h over a 24-h period. Samples were then stored frozen (–20°C) for later analysis. Duodenal and fecal samples were later thawed, mixed thoroughly, composited by period and cow, and then refrozen (–20°C). Duodenal samples were freeze-dried, and fecal samples were dried at 55°C for 72 h. Dried samples were ground through a 1-mm screen and stored for later analysis.

Total fatty acids in feed, fecal, and duodenal samples were extracted and methylated by the one-step procedure (Sukhija and Palmquist, 1988) using hexane instead of benzene. Methyl esters of fatty acids were separated and quantified by gas chromatography (Hewlett Packard model 5890 series II, equipped with flame ionization detector at 250°C and model 7673 auto injector; Hewlett Packard, Palo Alto, CA) fitted with a fused silica capillary column (SP-2380, 100 m x 0.25 mm; Supelco, Inc., Bellefonte, PA). The carrier gas was H2, and the flow rate was 3.0 mL/min (linear flow rate, 34.4 cm/s). Injector and detector temperatures were 250°C, and the split ratio was 100:1. Column temperature was set at 140°C for 1 min, then it increased by 4.0°C/min until it reached 240°C, where it was maintained for 29 min; therefore, total run time was 55 min. The internal standard used was heptadecanoic acid (C17:0; Nu Check Prep, Inc., Elysian, MN).

Milk samples were analyzed for fat, total protein, and lactose by infrared analysis (Program d’Analyze des Troupeaux Laitiers du Québec, Ste-Anne-de-Bellevue, QC) according to AOAC (1990). Milk noncasein N and NPN were determined according to Rowland (1938) and analyzed for N using a Leco Nitrogen Analyzer (FP-428; Nitrogen Determinator System, Leco Corporation, MI). Total solids content was determined according to the procedure of AOAC (1990). Additional milk samples were kept frozen at –80°C for fatty acid analysis.

Blood samples were collected by venipuncture of the jugular vein using heparinized Vacutainer tubes (Becton Dickinson, Franklin Lakes, NJ). Samples were collected once per period on d 24 at 1100 h from all cows. Following collection, blood samples were immediately placed on ice. Plasma was harvested following centrifugation at 2060 x g for 15 min at 5°C and was then stored at –20°C until analysis. Plasma was thawed and analyzed by colorimetric methods for glucose (kit 510-A; Sigma Diagnostics, Inc., St. Louis, MO), NEFA (kit 99075401; Wako Pure Chemical Industries, Osaka, Japan), and total cholesterol (kit no. 401 to 25P; Sigma Diagnostic, Inc.).

Plasma fatty acids were extracted according to the procedure of Delbecchi et al. (2001) and preparation of plasma fatty acids methyl esters was carried out as described by Park and Goins (1994). Methyl esters of fatty acids were separated and quantified by GLC (Hewlett Packard model 6890 chromatograph; Hewlett Packard Ltée, Montréal, QC, Canada) equipped with a flame ionization detector, an autosampler, and a split-splitless injector (Delbecchi et al., 2001).

For the analysis of milk fatty acids, milk fat was extracted according to the Röse-Gottlieb method (AOAC, 1990) with a single extraction and evaporation under N followed by transesterification with sodium methoxide (Chouinard et al., 1997). Fatty acid concentrations were then determined as described by Chouinard et al. (1997) using GLC (HP 5890; Hewlett Packard Co.) equipped with a 60-m x0.32-mm capillary column. The carrier gas was He, and the temperature of the flame ionization detector was maintained at 250°C. The initial oven temperature was 150°C and increased at 5°C/min up to 200°C.

Statistical Analyses
Statistical analyses were conducted using the MIXED procedure of SAS (1999) for a 4 x4 Latin square design with the following model:


where

Yijk=observation,

m=population mean,

Ti=treatment (i = 1, 2, 3, or 4),

Pj=period (j = 1, 2, 3, or 4),

Ck=random effect of cow (k = 1, 2, 3, or 4), Ck ~N (0, s2cow), and

eijk=residual error, eijk ~N (0, ).

Treatment sums of squares were partitioned to provide contrasts and compared: 1) C diet vs. RF, EF, and MF diets, 2) no heat treatment vs. heat treatment, and 3) MF diet vs. EF diet. Significance was declared at P < 0.05.


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSION
 ACKNOWLEDGEMENTS
 REFERENCES
 
Feed Intake, Milk Yield, and Milk Composition
Dry matter intake was not affected by dietary treatments and averaged 15.6 kg/d (Table 2Go). As expected, cows fed the flaxseed diets consumed more (P < 0.05) fat than cows fed the C diet. Others found no negative effects on DMI when feeding flaxseed to dairy cows up to 17% of the dietary DM (Petit et al., 2001; Mustafa et al., 2003). Kennelly (1996) suggested that the addition of fat to ruminant diets in the form of oilseeds will have less detrimental effects on DMI than if a similar amount was fed as free oil. Cows fed the flaxseed diets produced 1.8 kg less milk than those fed the C diet, and cows fed the EF diet produced 1.6 kg less milk than those fed the MF diet (Table 2Go). Differences in energy-corrected milk between dietary treatments were similar to those reported for total milk yield. Flaxseed had no effect on percentage of milk fat, protein, and lactose (Table 2Go). However, feeding EF relative to MF reduced milk fat percentage by 0.4 percentage units and reduced milk fat yield by 0.14 kg/d. These results suggest that feeding EF relative to RF or MF might have detrimental impact on milk yield and milk composition. However, our inability to detect statistical differences in milk yield and composition between dietary treatments is likely due to a small number of animals used in the study. This makes it difficult to discuss and compare our yield and composition data with those reported in the literature. The main emphasis of our study was to determine the effects of the different flaxseed treatments on blood and milk fatty acid composition.


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  		Table 2. Dry matter intake, milk yield, and milk composition of late lactating cows fed raw and heat-treated flaxseed.
 
Yield of milk protein was lower (P < 0.05) for cows fed the flaxseed diets than for those fed the C diet (Table 2Go). This result is in agreement with other studies, which showed that fat supplementation decreases milk protein yield (DePeters et al., 1989; DePeters and Cant, 1992). Feeding EF relative to MF had no effect on milk composition and yield of components except for milk protein yield, which tended to be lower (P = 0.07) for cows fed EF relative to those fed MF. The decrease in milk protein yield could have resulted from the significant increase in ruminal protein degradability of flax-seed following extrusion (Gonthier et al., 2004). A reduced duodenal flow of RUP may reduce amino acid availability for protein synthesis in the mammary gland (Piepenbrink et al., 1998).

Distribution of milk N showed that flaxseed supplementation had no effect on percentages of true protein, CN, NPN, and whey (Table 3Go). However, yield of true protein, particularly CN, was lower (P < 0.01) for cows fed the flaxseed diets than for those fed the C diet. Others also reported similar effects of fat supplementation on milk N distribution (DePeters et al., 1989; DePeters and Cant, 1992).


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  		Table 3. Nitrogen fractions in milk of late lactating cows fed raw and heat-treated flaxseed.
 
Plasma Profile
Plasma cholesterol and NEFA concentrations were greater (P < 0.01) for cows fed flaxseed than for those fed the C diet (Table 4Go). However, dietary treatments had no effect on plasma glucose concentration. Fat supplementation increased the concentrations of cholesterol and/or NEFA in blood of dairy cows (Delbecchi et al., 2001; Petit et al., 2001, 2002). Reasons for the increase in NEFA concentration as a result of fat supplementation are unclear. However, it is unlikely to be due to fatty acid mobilization from the adipose tissue. Drackley (1999) suggested that a 1-kg/d increase in dietary fatty acid intake would result in an increase in NEFA concentration of only 81 µM in lactating cows compared with 1 mM or more during the transition period. From our fatty acid intake values (Table 2Go), we would expect an increase in plasma NEFA concentration of 46 µM, assuming a linear relationship between plasma NEFA level and fatty acid intake. The calculated NEFA concentration difference between flaxseed treatments and the C diet is close to the actual value obtained, which averaged 42 µM (Table 4Go).


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  		Table 4. Plasma parameters of late lactating cows fed raw and heat-treated flaxseed.
 
Feeding flaxseed diets decreased (P < 0.03) plasma concentrations of C16:0, C16:1, C18:2, and {gamma}-C18:3 and increased (P < 0.03) those of trans-11 C18:1, cis-9 C18:1, and {alpha}-C18:3 (Table 5Go). Consequently, concentrations of MCFA and SFA were lower (P < 0.03), and concentrations of LCFA and monounsaturated fatty acids were higher (P < 0.03) for cows fed the flaxseed diets relative to those fed the C diet. However, plasma concentrations of polyunsaturated fatty acids were not affected by flaxseed supplementation. In agreement with our results, Romo et al. (2000) reported similar effects on plasma fatty acid profile of cows duodenally infused with a mixture of C18:1 isomers. Differences in plasma fatty acid among treatments reflect the profiles of fatty acids reaching the duodenum (Gonthier et al., 2004b).


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  		Table 5. Plasma fatty acid profile of late lactating cows fed raw and heat-treated flaxseed (mg/g of fatty acids).
 
Feeding EF relative to MF increased (P < 0.02) plasma concentrations of C16:1, cis-9 C18:1, and monounsaturated fatty acids and decreased (P < 0.02) those of C18:2, C18:3, and polyunsaturated fatty acids (Table 5Go). These results reflect the differences in the level of protection of fatty acids from ruminal biohydrogenation provided by the 2 heat treatments (Gonthier et al., accepted).

Milk Fatty Acid Composition
Concentrations of all SCFA and MCFA in milk were reduced (P <0.01), and those of LCFA were increased (P <0.02), as a result of flaxseed supplementation (Table 6Go). Cows fed flaxseed diets produced milk with lower (P < 0.01) saturated and higher (P < 0.01) monounsaturated and polyunsaturated fatty acid concentrations than cows fed the C diet. Changes in milk fatty acid composition reported in this study were similar to those reported in the literature (Ward et al., 2002; Mustafa et al., 2003; Soita et al., 2003). The absence of milk fat depression (Table 2Go) would suggest that the reduction in de novo synthesis of fatty acids is equal to the increase in concentration of polyunsaturated fatty acids.


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  		Table 6. Milk fatty acid profile of late lactating cows fed raw and heat-treated flaxseed (mg/g of fatty acids).
 
Concentrations of SCFA and C18:3 in milk were lower (P < 0.03), and those of trans C18:1 were higher (P < 0.01) for cows fed EF than for those fed MF (Table 6Go). Extrusion of flaxseed significantly increased ruminal biohydrogenation of C18:3 and reduced the amount of C18:3 reaching the duodenum (Gonthier et al., accepted), which would explain the lower milk concentration of C18:3 for cows fed EF relative to those fed MF. At the same time, partial biohydrogenation increases the synthesis of trans isomers of fatty acids (including trans 10 C18:1) and their concentrations in milk fat, which may then inhibit de novo synthesis of SCFA (Bauman and Griinari, 2001; Chilliard et al., 2001a). Furthermore, the higher rate of biohydrogenation of extruded relative to MF increased (P = 0.02) monounsaturated fatty acid concentration in milk fat of cows fed EF compared with those fed MF (Table 6Go). Feeding EF relative to MF also reduced (P = 0.05) the concentration of SFA in milk fat (Table 6Go).

Our results indicate that feeding unheated flaxseed to dairy cows can increase C18:3 and conjugated linoleic acid (CLA) concentration by 193 and 51%, respectively. These values are similar to those reported by others (Ward et al., 2002; Mustafa et al., 2003; Soita et al., 2003). A recent review illustrated the potential of feeding flaxseed to increase CLA content of milk fat by >8 fold (Chilliard et al., 2000). Trans-11 C18:1, or vaccenic acid, is formed during biohydrogenation of C18:3 and is desaturated in the mammary epithelial cell by the {Delta}9-desaturase enzyme to produce CLA (Chilliard et al., 2001b). Voigt and Hagemeister (2001) suggested that 33% of trans-11 C18:1 taken up by the mammary epithelial cell is desaturated to cis-9, trans-11 C18:2, the predominant isomer of CLA in milk (90% of all conjugated fatty acid isomers). The {Delta}9-desaturase enzyme is not specific to vaccenic acid. Chilliard et al. (2000) suggested that 40% of C18:0 acid is converted to C18:1 acid by {Delta}9-desaturase, contributing to 50% of total C18:1 in milk.

Differences in daily yield of milk fatty acids among dietary treatments (Table 7Go) were similar to those reported for milk fatty acid profiles (Table 6Go). Feeding flaxseed increased (P < 0.01) daily yield of C18:3 from 3.5 to 8.3 g/d. Daily yield of C18:3 was also higher (P <0.01) for cows fed MF than for those fed EF. Although not statistically different, daily yield of CLA from cows fed EF was 20% higher than that from cows fed MF. Chemical treatments such as formaldehyde seem to be more effective than heat treatment in providing protection of fatty acids from ruminal biohydrogenation. Goodridge et al. (2001) reported an 8-fold increase in C18:3 concentration (from 8 to 64 mg/g of fatty acids) by feeding formaldehyde-treated flaxseed to dairy cows.


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  		Table 7. Milk fatty acid yield (g/d) of late lactating cows fed raw and heat-treated flaxseed.
 
Apparent transfer efficiency of unsaturated fatty acids was estimated by dividing the amount of individual unsaturated fatty acids in milk fat by the amount of individual unsaturated fatty acids consumed or flowing into the duodenum (Table 8Go). Estimates of transfer efficiency of fatty acids from feed to milk were low and decreased as the degree of unsaturation of the fatty acids increased. Our values of transfer efficiency for C18:3 (average 2.0%) were similar to those reported for other n-3 fatty acids (Chilliard et al., 2001a). Flaxseed supplementation reduced (P <0.01) transfer efficiency of C18:1cis and C18:3 from feed to milk but not from duodenum to milk. Feeding EF relative to MF reduced (P <0.01) transfer efficiencies of C18:3 from feed and duodenum into milk. This suggests that the proportion of C18:3 reaching the duodenum of cows fed EF diet is less available for intestinal digestion and/or absorption. Because the duodenal flow of C18:3 was lower for cows fed the EF diet than for those fed MF diet (Gonthier et al., accepted), it is possible that most of it escaping ruminal biohydrogenation in cows fed the EF diet is bound to indigestible material, which would impair proper micelle formation and, therefore, impairs absorption.


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  		Table 8. Efficiency of transfer of selected milk fatty acids in milk of late lactating cows fed raw and heat-treated flaxseed.
 

   CONCLUSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSION
ACKNOWLEDGEMENTS
 REFERENCES
 
Feeding extruded flaxseed at 12.6% of dietary DM to dairy cows in late lactation had detrimental effects on milk yield and milk fat and protein percentages. The lack of significant differences in milk data between dietary treatments is likely due to the small number of animals used in the present study. There was a reduction in milk concentrations of SCFA, MCFA, and SFA and an increase in the concentrations of LCFA and unsaturated fatty acids as a result of flaxseed supplementation. Micronization increased C18:3 level, and extrusion reduced the concentrations of both SCFA and SFA in milk fat. However, the overall influence of feeding either micronized or extruded flaxseed on milk composition compared with feeding unheated flaxseed was minimal. Heat inputs higher than the ones used in our study are likely required to obtain appreciable increases in C18:3 in milk from cows fed flaxseed.


   ACKNOWLEDGEMENTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSION
 ACKNOWLEDGEMENTS
REFERENCES
 
This study was funded by the Flax Council of Canada, Matching Investment Initiative of Agriculture and Agri-Food Canada, and the Research Partnership Program of the Natural Sciences and Engineering Research Council of Canada. The authors gratefully acknowledge Nathalie Plourde, Sylvie Dallaire, and Liette Veilleux for assistance in laboratory analysis and the employees of the dairy unit of the Dairy and Swine Research and Development Centre, Lennoxville, QC, for feeding the cows and assisting with sample collection.

Received for publication April 2, 2004. Accepted for publication September 13, 2004.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSION
 ACKNOWLEDGEMENTS
 REFERENCES
 


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