J. Anim Sci. 2008. 86:2310-2320. doi:10.2527/jas.2008-0864
© 2008 American Society of Animal Science
Influence of level of supplemental whole flaxseed on forage intake and site and extent of digestion in beef heifers consuming native grass hay1,2
E. Scholljegerdes3 and
S. Kronberg
Northern Great Plains Research Laboratory, USDA-ARS, Mandan, ND 58554
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Abstract
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The objectives of this study were to evaluate the influence of supplemental whole flaxseed level on intake and site and extent of digestion in beef cattle consuming native grass hay. Nine Angus heifers (303 ± 6.7 kg of BW) fitted with ruminal and duodenal cannulas were used in a triplicated 3 x 3 Latin square. Cattle were given ad libitum access to chopped native grass hay (9.6% CP and 77.5% NDF, OM basis). All animals were randomly allotted to 1 of 3 experimental treatments of hay plus no supplement (control); 0.91 kg/d whole flaxseed (23.0% CP, 36.3% NDF, and 25.5% total fatty acid, OM basis); or 1.82 kg/d whole flaxseed on a DM basis. Supplemental flaxseed tended to decrease (linear, P = 0.06) forage OM intake. However, total OM intake did not differ (P = 0.29) with increasing levels of flaxseed. Total duodenal OM flow increased (linear, P = 0.05) with additional flaxseed in the diet, and no differences (P = 0.29) were observed for microbial OM flow. True ruminal OM disappearance was not affected (P = 0.14) by supplemental flaxseed. Apparent lower tract OM digestibility increased (linear, P = 0.01) with level of whole flaxseed. Apparent total tract OM digestibility was not different (P = 0.41) among treatments. Nitrogen intake increased (linear, P < 0.001) with supplemental flaxseed. In addition, total duodenal N flow tended (P = 0.08) to increase with additional dietary flaxseed. However, true ruminal N digestibility did not differ (P = 0.11) across treatment. Supplemental whole flaxseed did not influence ruminal (P = 0.13) or total tract (P = 0.23) NDF digestibility. Ruminal molar proportion of propionate responded quadratically (P < 0.001) with increasing levels of whole flaxseed. An increase in the duodenal supply of 18:3n-3 (P < 0.001), total unsaturated fatty acids (P < 0.001), and total fatty acids (P < 0.001) was observed with additional dietary whole flaxseed. Apparent postruminal 18:3n-3 disappearance tended to decrease (P = 0.07) as intake of flaxseed increased. Overall, the inclusion of 1.82 kg/d of flaxseed does not appear to negatively influence nutrient digestibility of a forage-based diet and therefore can be used as an effective supplement to increase intestinal supply of key fatty acids important to human health.
Key Words: beef cattle • digestion • fatty acid • flaxseed • forage
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INTRODUCTION
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Feeding beef cattle diets high in n-3 fatty acids is warranted for livestock producers interested in enhancing the human healthfulness of meat products (Weill et al., 2002
) or reducing reproductive losses (Ambrose et al., 2006). Feeding flaxseed, which is high in linolenic acid (56% 18:3n-3), is a viable option for bolstering the n-3 fatty acid concentration in livestock diets and subsequent n-3 fatty acid concentration in ruminant products (Soita et al., 2003; Kronberg et al., 2006; Maddock et al., 2006). Unfortunately, there are limitations on the level of fat that can be added to ruminant diets because of reductions in intake and digestibility (Schauff and Clark, 1992). This is especially true for fats high in PUFA (Palmquist and Jenkins, 1980; Jenkins, 1993). However, flaxseed seems to be unique in this regard because previous work by Zhang et al. (2007) showed an increase in DM digestibility for lactating ewes fed a silage-based diet containing 8% flaxseed compared with diets with 7.3% canola seeds or no oilseeds. In feedlot diets, flaxseed does not influence (Maddock et al., 2006) or increases (Drouillard et al., 2004) dietary intake, and an overall improvement in ADG is observed with inclusions of 5 to 8% flaxseed in the diet. However, no information exists with flaxseed fed in high-forage diets. Nevertheless, others have evaluated the use of oilseeds in forage-based diets and have found varying results for intake and total-tract digestibility of DM (Albro et al., 1993) and OM (Leupp et al., 2006). Specifically, Albro et al. (1993) fed weanling beef steers 1.5 kg/d of whole soybeans and did not observe a difference in intake but did report an increase in total-tract DM digestibility. Leupp et al. (2006) did not observe any differences in either forage intake or total-tract OM digestibility when canola (whole or ground) was offered to steers fed low-quality grass hay. Therefore, our objectives were to evaluate site and extent of digestion in beef heifers fed increasing amounts of whole flaxseed and consuming native grass hay.
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MATERIALS AND METHODS
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All experimental procedures were reviewed and approved by the Northern Great Plains Research Laboratory, Animal Care and Use Committee.
Animals and Diets
Nine Angus heifers (average BW: 303 ± 6.7 kg) fitted with ruminal and J-style duodenal cannulas (manufactured by the USDA-ARS, Northern Great Plains Research Laboratory) were used in a triplicated 3 x 3 Latin square. Animals were housed in a temperature-controlled barn within 3.3- x 2.7-m pens equipped with cup waterers. Before the initiation of the experiment, cattle were offered chopped (5 cm) native grass hay [Kentucky bluegrass (Poa pratensis L.), smooth brome (Bromus inermus Leyss.), blue grama (Bouteloua gracilis (H. B. K.) Griffiths), needle and thread (Stipa comata Trin. and Rupr.), green needlegrass (Stipa viridula Trin.), western wheatgrass, and sedges (Carex spp.); 9.6% CP and 77.5% NDF, OM basis; Table 1] at 5% above the previous day’s intake and were given ad libitum access to whole flaxseed for 21 d in an effort to determine the level at which the animal would freely consume flaxseed. All pens were equipped with large feed bunks for hay, and bucket holders were mounted so that intake of hay and flaxseed could be measured individually. It was determined that the average maximal amount of whole flaxseed consumed was 1.82 kg/d. Therefore, cattle were fed hay at 5% above the previous day’s intake and randomly allotted to 1 of 3 experimental treatments of hay plus no supplement; 0.91 kg/d of whole flaxseed on a DM basis; or 1.82 kg/d of whole flaxseed on a DM basis. Daily sampling of hay, flaxseed, and orts started on d 12 and continued through d 21 of each period. Heifers were fed twice daily at 0600 and 1800 h and given free access to trace mineralized salt (American Stockman Trace Mineralized Salt, North American Salt Co., Overland Park, KS: NaCl >95.5%; Zn >3,500 mg/kg; Fe >2,000 mg/kg; Mn >1,800 mg/kg; Cu >280 mg/kg; I >100 mg/kg; Co >60 mg/kg). Each experimental period was 21 d with a 17-d adaptation to ensure adequate adjustment of the gastrointestinal tract to the new dietary treatment and 4 d of intensive sampling. Starting on d 8 of each experimental period, gelatin boluses (size #11, Torpac Inc., Fairfield, NJ) containing 5 g of TiO2 were placed into the central rumen twice daily at each feeding as an external marker for digesta flow (Myers et al., 2004).
Sampling
Beginning at 0400 h on d 18 of the experimental period, duodenal (200 mL) and fecal (50 mL) samples were collected every 4 h. On d 19 of the sampling period, duodenal and fecal collection times were advanced 2 h so that samples were collected to represent every 2 h in a 24-h period. Fecal samples were composited (equal volume) over time by heifer and dried in a 55°C forced-air oven, ground (Wiley mill, 1-mm screen; Arthur H. Thomas, Philadelphia, PA), and composited. Duodenal digesta samples were composited (equal volumes) within heifer for each period and immediately frozen. Duodenal digesta samples were lyophilized (Freezemobile 25SL Freeze Dryer, The VirTis Co., Gardiner, NY) and ground (Wiley mill; 1-mm screen).
Immediately before the 0600 h feeding on d 20, whole ruminal contents (500 mL) were removed and 200 mL of Co-EDTA (Uden et al., 1980) was dosed intraruminally (0 h). Whole ruminal contents were collected at 3, 6, 9, 12, 15, 18, 21, 24, and 36 h postdosing (samples collected at 24 and 36 h were analyzed for Co only). Immediately after collection, ruminal pH was measured using a combination electrode (Orion Research Inc., Boston, MA), contents were strained through 4 layers of cheesecloth, and a 10-mL aliquot of strained ruminal fluid was acidified with 0.1 mL of 3.6 M H2SO4 and immediately frozen at –20°C. In addition, an unstrained sample of whole ruminal contents was placed in a blender (Hamilton Beach/Proctor Silex, Washington, NC) with an equal volume of 0.9% NaCl (wt/vol) solution and homogenized for 1 min to dislodge particulate-associated bacteria. The homogenate was then strained through 8 layers of cheesecloth and immediately frozen for subsequent bacterial isolation by differential centrifugation (Merchen et al., 1986).
Laboratory Analysis
All feed, orts, microbes, duodenal digesta, and fecal samples were analyzed for DM and ash (AOAC, 1990). Nitrogen content of feed, microbes, duodenal digesta, and feces were determined using a Carlo Erba Model NA 1500 Series 2 N/C/S analyzer (CE Elantech, Lake-wood, NJ). Neutral detergent fiber of feed, duodenal digesta, and feces were determined using an Ankom 200 fiber analyzer (Ankom Technology, Fairport, NY). Duodenal and fecal samples were analyzed for TiO2 according to the procedures of Myers et al. (2004) using a spectrophotometer (DU-640, Beckman Instruments Inc., Fullerton, CA). Microbial and duodenal samples were analyzed for purines as described by Zinn and Owens (1986).
Ruminal fluid samples were centrifuged at 20,000 x g for 20 min and a 2.5-mL aliquot was added to 0.5 mL of 25% (wt/vol) metaphosphoric acid containing 2 g/L of 2-ethyl-butyric acid (Goetsch and Galyean, 1983). These samples were analyzed for concentrations of VFA using a Varian 3800 gas chromatograph (Varian Inc., Palo Alto, CA) equipped with a 15 m x 0.533 mm (i.d.) column (Nukol, Supelco, Bellefonte, PA) with an initial column temperature of 80°C held for 1 min, ramped to 140°C at 20°C/min, and then ramped to 175°C at 30°C/min. Injector and detector temperatures were held at 250°C. The column flow rate was 8 mL/min and H2 was the carrier gas. Approximately 100 mg of duodenal digesta was reconstituted to 3% DM using 0.1 N HCl for subsequent analysis of NH3 concentration. Concentration of NH3 in ruminal fluid and reconstituted duodenal digesta was determined using the phenol-hypochlorite procedure of Broderick and Kang (1980). Ruminal fluid Co concentrations were determined by atomic absorption spectroscopy using an air/acetylene flame (Model 3110, Perkin Elmer Inc., Norwalk, CT).
Feed was analyzed for fatty acids via direct trans-esterification (Whitney et al., 1999) with methanolic-HCl (Kucuk et al., 2001), and duodenal digesta fatty acids were analyzed for fatty acid using the procedures of Lake et al. (2006). Separation of fatty acid methyl esters was achieved by GLC (Model CP-3800, Varian Inc.) with a 100 m x 0.25 mm (i.d.) x 0.2 µm (film thickness) capillary column (SP-2560, Supelco) and H2 gas as the carrier at 1.0 mL/min for feedstuffs and 1.5 mL/min for duodenal digesta. Initial oven temperature was maintained at 120°C for 2 min, ramped to 210°C at 6°C/min, and then ramped to 250°C at 5°C/min. Injector temperature was 260°C and flame-ionization detector temperature was 300°C. Identification of peaks was accomplished using purified fatty acid standards (Sigma-Aldrich, St. Louis, MO; Nu-Chek Prep, Elysian, MN; Matreya, Pleasant Gap, PA). Furthermore, based on the chromatogram published by Loor et al. (2004), a peak appearing between peaks confirmed to be 18:1 trans-11 and 18:1n-9 was putatively identified as 18:1 trans-13+14.
Calculations and Statistical Analysis
Nutrient flows, fluid passage rate, and microbial efficiency were calculated as described by Scholljegerdes et al. (2004b). Furthermore, biohydrogenation of 18C unsaturated fatty acids was calculated by using the equations of Tice et al. (1994) and Scholljegerdes et al. (2004a).
All data were analyzed using the MIXED procedure (SAS Inst. Inc., Cary, NC) as a triplicated 3 x 3 Latin square experiment. The model included animal as the random variable. In addition, time course data included the effects of animal, period, treatment, time, and treatment x time. Autoregression order one was determined to be the most desirable covariance structure according to the Akaike’s information criterion. Orthogonal contrasts were used to compare linear and quadratic responses to level of flax intake (Steel and Torrie, 1980). There was a treatment x time interaction for ruminal NH3 (P < 0.001) and the molar proportion of isovalerate (P = 0.02), whereas no other treatment x time interactions were observed (P 
0.27) for other time course data; therefore, only main effects are presented. A significance level of 0.05 was used to separate treatment effects.
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RESULTS AND DISCUSSION
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Intake and Digestibility of OM
The addition of whole flaxseed tended to reduce forage OM intake (linear, P = 0.06), but total OM intake did not differ (P = 0.29) because of the inclusion of whole flaxseed (Table 2). The substitution rate was a 0.65-kg decrease in forage intake per 1-kg increase in flaxseed consumed (forage intake = –0.647 flaxseed intake + 6.35, R2 = 0.13; P = 0.07; data not shown). Leupp et al. (2006) did not report any differences in OM intake of low-quality grass hay or total intake with supplemental canola seeds. Pavan et al. (2007) reported a decrease in both forage and total DM consumption when level of corn oil increased for beef steers grazing endophyte-free tall fescue. Despite no differences being observed for total OM intake, duodenal OM flow increased (P = 0.05) linearly with supplemental flaxseed. However, no differences (P = 0.29) were observed for microbial OM flow to the duodenum. When ruminal digestibility is presented as a percentage of intake, no differences (P = 0.14) were observed for true ruminal OM digestibility across flaxseed intake levels. This is contrary to other reports (Doreau and Chilliard, 1997b; Scholljegerdes et al., 2004a) that reported a decrease in true ruminal OM digestibility, yet agrees with Leupp et al. (2006) who did not observe a depression in ruminal OM digestibility of forage-based diets supplemented with canola. It is not clear why these differences occurred between experiments. It is not likely due to differences in dietary fatty acid percentage, because Scholljegerdes et al. (2004a) fed diets ranging from 3.2 to 8.0% total fatty acids and Doreau and Chilliard (1997b) fed diets that ranged from 0.77 to 3.5% total fatty acids. Leupp et al. (2006) fed canola at 8% of DMI, which provided animals with diets that were approximately 4.2% total fatty acids. In the current experiment, feeding heifers 0.91 or 1.82 kg/d of whole flaxseed led to dietary contributions from the flaxseed of 13.0 and 24.8% of the daily OM intake. Therefore, dietary total fatty acid percentage ranged from 0.96 to 7.4. Because our highest level of fat inclusion was greater than either of the previously mentioned studies, it is difficult to conclude that fat intake level was the cause of discrepancy among these previous reports (Doreau and Chilliard, 1997b; Scholljegerdes et al., 2004a; Leupp et al., 2006) and the current experiment. It should be noted that Doreau and Chilliard (1997b) and Leupp et al. (2006) only reported crude fat; therefore, dietary total fatty acid percentage was calculated from crude fat using the equation of Allen (2000):
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Table 2. Influence of supplemental whole flaxseed level (kg/d) on OM intake, flow, and digestibility in beef heifers consuming native grass hay
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One could attribute these differences to availability of supplemental fat. In the current experiment, flaxseed was fed as the whole seed, whereas Doreau and Chilliard (1997b) fed fish oil and Scholljegerdes et al. (2004a) fed cracked safflower seeds, which would allow greater availability of fatty acids in the rumen. However, Leupp et al. (2006) fed both whole and ground canola and again found no differences in ruminal OM digestibility. Therefore, processing does not seem to be the reason for the inconsistencies across these experiments.
Apparent lower tract OM digestibility (% of duodenal flow) linearly increased (P = 0.01) when flax was added to the diet (Table 2
). These findings agree with previous work in which feeding flaxseed to dairy cows consuming a 55:45 forage:concentrate ratio also increased lower tract OM digestibility (Gonthier et al., 2004). Despite the increase in lower tract digestibility in the current experiment, apparent total-tract OM digestibility (% of intake) did not differ (P = 0.41) across dietary treatments. This is contrary to previous results that reported an increase in total-tract digestibility in dairy cows fed linseed oil (Ueda et al., 2003) and flaxseed (Gonthier et al., 2004) in a total mixed ration.
Intake and Digestibility of N
Overall, N intake increased linearly (P < 0.001) with additional flaxseed (Table 3). However, supplemental flaxseed only tended (linear, P = 0.08) to increase the supply of N reaching the duodenum. Duodenal supply of microbial N and NH3 did not differ (P 0.22) across treatments. Others have indicated that duodenal microbial N supply (g/d) was unaffected by fat feeding (Brokaw et al., 2001; Scholljegerdes et al., 2004a) with forage-based diets. Nonammonia, nonmicrobial N flow responded quadratically (P = 0.03), with heifers fed 0.91 kg/d of whole flaxseed being lower than heifers fed 1.82 kg/d, which had the greatest duodenal N supply. This quadratic effect indicates that feeding 0.91 kg/d of whole flaxseed may increase ruminal degradability of dietary N. A quadratic tendency (P = 0.10) was observed for true ruminal N digestibility. Likewise, Murphy et al. (1987) were unable to detect a linear response in ruminal N digestibility when lactating dairy cows consuming a 59:41 forage:concentrate were fed 0, 1, or 2 kg of rapeseed and although not reported as a significant quadratic effect, cattle fed 1 kg of rapeseed had numerically greater ruminal N digestibility than those fed 2 kg of rapeseed. Apparent lower tract N digestibility increased (P = 0.003) with level of flaxseed. These data would indicate that whole flaxseed did not hinder ruminal N metabolism and that N escaping ruminal degradation is available at the small intestine. Apparent total-tract N digestibility was greater (P < 0.001) for heifers supplemented with whole flaxseed.
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Table 3. Influence of supplemental whole flaxseed level (kg/d) on N intake, flow, and digestibility (OM basis) in beef heifers consuming native grass hay
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Intake and Digestibility of NDF
No differences were observed (P 0.20) for NDF intake or duodenal and fecal NDF flow (Table 4). In turn, the inclusion of whole flaxseed had no effect on ruminal, lower tract, or total-tract NDF digestibility (P 0.13). Inclusion of flaxseed in the current experiment was 3.2 and 5.8% of added dietary fatty acids (DM basis) for the 0.91 or 1.82 kg/d treatments, respectively. Therefore, the fat offered in this experiment is below the amount of added fat (6.3%) found to inhibit fiber digestibility (Moore et al., 1986). An alternative explanation would be that by feeding whole oilseeds we provided partial protection of the fatty acids, which can be an effective way to avoid the negative effects of fat on fiber digestion (Murphy et al., 1987). However, this is unlikely because of the relatively extensive biohydrogenation of 18C fatty acids observed in this trial.
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Table 4. Influence of supplemental whole flaxseed level (kg/d) on NDF intake, flow, and digestibility (OM basis) in beef heifers consuming native grass hay
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Ruminal Fermentation Patterns
Ruminal pH tended (linear, P = 0.06) to increase linearly with increasing levels of whole flaxseed (Table 5). There was a treatment x time interaction (P < 0.001) for ruminal NH3 (data not shown). This interaction occurred due to a reduction in the difference between the 0.91 and 1.82 kg/d inclusion of flaxseed at 15 h postdosing but similar treatment differences at other time points. Overall, the main effects of dietary treatment showed that increasing levels of whole flaxseed increased (P < 0.001) ruminal NH3 concentrations. According to in situ analysis, 67.3% of flaxseed CP is effectively degradable (Mustafa et al., 2003). Ruminal NH3 levels reported herein were above the minimum level (1.4 mM) suggested by Satter and Slyter (1974) to support microbial growth. No differences (P = 0.19) were observed for fluid passage rate across dietary treatments. This appears logical due to the lack of differences in total intake and ruminal OM digestibility, both of which can affect ruminal passage rate (Galyean and Owens, 1991). A quadratic tendency (P = 0.07) was observed for total ruminal VFA concentrations with cattle receiving 0.91 kg/d having the greatest concentrations and 1.82 kg/d of flaxseed being lowest in VFA concentration. Of the VFA measured, only isovalerate exhibited a treatment x time interaction (P = 0.02) at 3 h postdosing, which was due to a reduction in magnitude of the difference between 0.91 kg/d and control treatments (data not shown). The molar proportions of acetate decreased (quadratic, P < 0.001), whereas propionate increased (quadratic, P < 0.001) with increasing levels of dietary whole flaxseed. Ruminal molar proportions of isobutyrate, isovalerate, and valerate increased linearly (P < 0.001) as intake of whole flaxseed increased from 0 to 1.82 kg/d. The tendency of ruminal pH to increase with the addition of flaxseed is due to the trend for lower ruminal total VFA concentration as cattle consumed more flaxseed. Reduced total VFA production has been observed previously by Murphy et al. (1987) when increasing levels of rapeseed were fed to dairy cows. As with our results, Ikwuegbu and Sutton (1982) also observed an increase in propionate with increasing levels of linseed oil. The increased molar proportion of branched-chain VFA (isobutyrate, isovalerate) observed in this experiment with whole flaxseed supplementation is due to the increased ruminal supply of branched-chain AA (valine and leucine; Maeng and Baldwin, 1976). Mustafa et al. (2003) reported that leucine and valine in ground flaxseed had a ruminal digestibility of 83.5 and 79.9%, respectively, which would increase ruminal branched-chain VFA concentrations.
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Table 5. Influence of supplemental whole flaxseed level (kg/d) on ruminal pH, NH3, fluid passage rate, and VFA in beef heifers consuming native grass hay
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Fatty Acid Intake and Disappearance
Due to experimental design, fatty acid intake increased linearly (P < 0.001) for all dietary fatty acids measured (Table 6). Overall, total unsaturated fatty acid intake (18:1n-9, 18:2n-6, and 18:3n-3) increased from 33.4 to 442.1 g/d as whole flaxseed supplementation increased from 0 to 1.82 kg/d.
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Table 6. Influence of supplemental whole flaxseed level (kg/d) on fatty acid intake (g/d) in beef heifers consuming native grass hay
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Ruminal biohydrogenation (percentage of 18C intake) of 18:1n-9 increased quadratically (P = 0.004) across treatments yet did not differ (P 0.33) for 18:2n-6 and 18:3n-3 (Table 7). Furthermore, total 18C fatty acid biohydrogenation was not different (P = 0.51) when cattle were fed increasing amounts of whole flaxseed. Wu and Palmquist (1991) observed an increased in vitro biohydrogenation of 18:1n-9 when diets were supplemented with 3% Ca-soaps and biohydrogenation decreased when dietary Ca-soaps increased to 6%. Biohydrogenation intermediates expressed as either the proportion of dietary or duodenal 18C fatty acids responded quadratically (P = 0.01 and 0.001, respectively) with cattle consuming 0.91 kg/d of whole flaxseed having the greatest proportion of biohydrogenation intermediates compared with 0 or 1.82 kg/d. This response was unexpected given the fact that Harfoot et al. (1973) observed an irreversible inhibition of biohydrogenation and subsequent accumulation of biohydrogenation intermediates with high levels of linoleic acid. Likewise, Beam et al. (2000) found that for every percentage unit increase in linoleic acid there was a 1.2%/h decline in the rate of biohydrogenation. The reduction in biohydrogenation intermediates was due to the observed reduction in biohydrogenation of 18:1n-9, which led to a reduction in the accumulation of biohydrogenation intermediates when cattle were fed 1.82 kg/d of whole flaxseed. Although this experiment was not focused on examining the effects of feeding whole seeds as it relates to processing per se, we had hoped to provide some protection from biohydrogenation by feeding the whole seed, which did not appear to be the case as evidenced by the fairly extensive biohydrogenation of dietary 18C fatty acids. This agrees with Keele et al. (1989) who concluded that feeding whole cottonseed did not provide unsaturated fatty acids with much protection from biohydrogenation.
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Table 7. Influence of supplemental whole flaxseed level (kg/d) on ruminal biohydrogenation1 in beef heifers consuming native grass hay
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Duodenal supply of 18C fatty acids increased (P < 0.001) with increasing levels of whole flaxseed (Table 8). Duodenal flow of 17:0, 18:0, and 24:0 increased quadratically (P 
0.04), whereas 12:0, 16:0, and 20:0 increased linearly (P 0.05) with increasing supply of whole flaxseed. The approximate 20-fold increase in intestinal supply of 18:0 compared with intake was due to biohydrogenation of 18C fatty acids averaging 82.1% across treatments. Significant increases in SFA may be concerning at first glance, especially because diets high in saturated fats are considered a detriment to human health. However, 18:0 has been shown to be as beneficial as 18:1n-9 in reducing plasma cholesterol levels compared with other SFA (Bonanome and Grundy, 1988). Duodenal supply of all 18:1 isomers increased (P < 0.001) in flax-fed cattle compared with unsupplemented controls and there was a quadratic increase (P < 0.01) for 18:1 trans-9, 18:1 trans-11, and 18:1 trans-13+14, whereas 18:1n-9 increased linearly (P < 0.001) with increasing levels of whole flaxseed. This agrees with Loor et al. (2004) who reported an increase in duodenal supply of 18:1 isomers when linseed oil was added to either a low concentrate or high concentrate dairy-type diet. Flax feeding increased (P < 0.001) the intestinal supply of 18:3n-3. Intestinal supply of MUFA responded quadratically (P = 0.02) to increasing levels of flaxseed. Intestinal PUFA and total unsaturated fatty acid supply increased linearly (P < 0.001) with flaxseed supplementation. These data agree with those of others (Murphy et al., 1987; Wu et al., 1991; Kucuk et al., 2004) in which intestinal supply of MUFA and PUFA increased with a greater fatty acid intake.
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Table 8. Influence of supplemental whole flaxseed level (kg/d) on duodenal fatty acid flow (g/d) in beef heifers consuming native grass hay
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Although our primary objective was to evaluate intestinal supply of fatty acids, it is important to know the extent to which those fatty acids are available postruminally. Apparent postruminal disappearance of 14:1, 18:1 trans-13+14, and 18:3n-3 tended (P = 0.06 to 0.08) to decline, whereas 12:0 increased in digestibility with level of whole flaxseed (Table 9). Apparent postruminal disappearance of 18:0, 18:1n-9, 18:2n-6, total SFA, and MUFA decreased linearly (P = 0.001 to 0.04) across dietary treatments. The decrease in intestinal digestibility with increasing levels of dietary fat has been reported previously by others (Doreau and Ferlay, 1994; Plascencia et al., 2003; Kucuk et al., 2004). Intestinal digestibility is generally greater for unsaturated fatty acids compared with SFA due to their ability to easily form micelles in the small intestine (Ockner et al., 1972). Values for intestinal digestibility of 18:0 were within range of others (Aldrich et al., 1997; Wachira et al., 2000; Scholljegerdes et al., 2004a) who have fed oilseeds. According to the extensive review by Doreau and Chilliard (1997a), intestinal digestibility of fatty acids ranged from 55 to 92%. These differences among fatty acids are generally due to chain length and number of double bonds (Doreau and Chilliard, 1997a). In the current study, apparent intestinal digestibility of 18:3n-3 ranged from 65.7 to 48.6% as intake of flaxseed increased from 0 to 1.82 kg/d. Wachira et al. (2000) reported much greater intestinal digestibility of 18:3n-3 (91.0% of duodenal flow) in sheep fed a 71.5% forage diet supplemented with whole flaxseed. Those diets were formulated to provide 4.8% total fatty acids, which is similar to that of cattle fed 0.91 kg/d whole flaxseed (4.1% total fatty acids) in the current experiment, ruling out dietary fatty acid concentration as a reason. The large discrepancy between Wachira et al. (2000) and the current experiment may be the inherent differences in extent of mastication between sheep and cattle. As with our data, Aldrich et al. (1997) noted relatively low values for postruminal supply of 18C fatty acids when steers were fed whole canola seed. These relatively low values for postruminal digestibility could be due to some protection of fatty acids by the seed coat, which caused fatty acids to be less available in the small intestine. In addition, hind gut fermentation and microbial fatty acid production may have elevated fatty acid concentration in feces, thus lowering postruminal fatty acid digestibility.
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Table 9. Influence of supplemental whole flaxseed level (kg/d) on apparent postruminal fatty acid disappearance (% of duodenal flow) in beef heifers consuming native grass hay
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Several reviews have outlined positive effects of supplemental fatty acids on the composition of ruminant products (Palmquist and Jenkins, 1980; Wood and Enser, 1997; Demeyer and Doreau, 1999) or as precursors for reproductive hormones (Staples et al., 1998; Abayasekara and Wathes, 1999; Funston, 2004). However, these advantages often come at the expense of intake, diet digestibility, or growth performance. Therefore, it is important to consider trade-offs of fat-supplemented diets between known physiological functions and animal performance. In the current trial, flaxseed increased the supply of intestinal fatty acids and may increase tissue supply of unsaturated fatty acids when fed under the conditions described herein. However, more work is needed in this area to elucidate the efficacy of flaxseed for improving growth performance, tissue fatty acid composition, and reproductive success in forage-based production systems.
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Footnotes
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1 Mention of a proprietary product does not constitute a guarantee or warranty of the product by USDA or the authors and does not imply its approval to the exclusion of another product that may also be suitable. USDA, ARS, Northern Plains Area, is an equal opportunity/affirmative action employer. All agency services are available without discrimination. 
2 Acknowledgment: Faye Kroh, Clay Erickson, Lindsey Voigt, and Gordon Jensen for their assistance with this project. North Dakota Oilseed Cooperative for their generous donation of flaxseed.
3 Corresponding author: Eric.Scholljegerdes{at}ars.usda.gov
Received for publication January 11, 2008.
Accepted for publication May 6, 2008.
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LITERATURE CITED
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Abstract
INTRODUCTION
