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ANIMAL NUTRITION |
Department of Animal Science, University of Wyoming, Laramie 82071-3684
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Abstract |
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 Top Abstract IntroductionMaterials and MethodsResults and DiscussionImplicationsLiterature Cited |
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0.02) for Control than Linoleate and Oleate. True ruminal N degradability (% of intake) was not different (P = 0.38) among treatments. Apparent ruminal biohydrogenation of dietary 18:2 was greatest (Linoleate vs. Oleate, P < 0.001) for Linoleate, whereas biohydrogenation of dietary 18:1 was greatest (Linoleate vs. Oleate, P = 0.02) for Oleate. Duodenal flow of 18:0 was least (P < 0.001) for Control but did not differ (P = 0.92) between Oleate and Linoleate. Total flow of unsaturated fatty acid to the duodenum was greatest (P < 0.001) in cattle fed safflower seeds, and was greater with Linoleate (P < 0.001) than with Oleate. Duodenal flow of 18:1 and 18:2 increased (P < 0.001) in Oleate and Linoleate, respectively. Duodenal flow of 18:1trans-11 was greater (P < 0.001) in cattle fed safflower seeds and in Linoleate than in Oleate. Postruminal disappearance of saturated fatty acids was greatest (P < 0.001) for Control; however, postruminal disappearance of total unsaturated fatty acids was greater (P = 0.002) for Linoleate vs. Oleate. Supplemental high-linoleate or high-oleate safflower seeds to cattle fed forage-based diets may negatively affect ruminal OM and fiber disappearance but not N disappearance. Provision of supplemental fat in the form of safflower seeds that are high in linoleic acid increased intestinal supply and postruminal disappearance of unsaturated fatty acids, indicating that the fatty acids apparently available for metabolism are affected by dietary fat source.
Key Words: Beef Cattle • Digestion • Fatty Acids • Safflower • Supplementation
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Introduction |
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TopAbstractIntroductionMaterials and MethodsResults and DiscussionImplicationsLiterature Cited |
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) and on the quality of ruminant-derived food products (Bauman et al., 2000). Our previous results (Bottger et al., 2002) suggested that lactating beef cows fed high-linoleate safflower seeds were more capable of maintaining body condition, whereas cows fed high-oleate safflower seeds had greater total milk fat. Cattle fed supplemental oilseeds have greater concentrations of CLA in meat (Wood et al., 1999) and milk (Griinari and Bauman, 1999), which may constitute a health advantage to consumers of these food products (Bauman et al., 2000). Griinari and Bauman (1999) reported that milk fat 18:1trans-11 is the major predictor of milk fat 18:2cis-9,trans-11 CLA concentration. Duodenal flow of 18:1trans-11 was greater for dairy cattle fed 60% forage diets supplemented with high-linoleate or high-oleate sunflower oil than for nonsupplemented controls (Kalscheur et al., 1997). It may be inappropriate to extrapolate results of research conducted with dairy cows to beef cows because beef cows are typically fed greater proportions of forage. Ruminal cellulolytic bacteria are primarily responsible for biohydrogenation (Harfoot and Hazelwood, 1988); therefore, duodenal flow of biohydrogenation products and intermediates could be very different in ruminants consuming markedly different levels of forage (Kucuk et al., 2001). Our hypothesis was that supplemental fat high in linoleate or oleate would alter unsaturated fatty acid supply to the small intestine with minimal effects on site and extent of digestion in cattle consuming high-forage diets. Therefore, our objectives were to evaluate site and extent of nutrient disappearance, ruminal biohydrogenation, and ruminal fermentation patterns in cattle fed bromegrass hay and supplemental high-linoleate or high-oleate cracked safflower seeds. |
Materials and Methods |
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TopAbstractIntroductionMaterials and MethodsResults and DiscussionImplicationsLiterature Cited |
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Nine Angus x Gelbvieh heifers (4 yr old; 641 ± 9.6 kg) cannulated in the rumen and proximal duodenum were used in a triplicated 3 x 3 Latin square experiment in accordance with an approved University of Wyoming Animal Care and Use Committee protocol. Heifers were housed in individual pens (2 m x 3.3 m) in a temperature-controlled room (20°C) under continuous lighting.
Diets
Heifers were fed (OM basis) 9.1 kg of chopped (2.54 cm) bromegrass hay (1.3% N, 58% NDF) and one of three supplements: 1) 1.8 kg of cracked corn and 0.20 kg of soybean meal (Control); 2) 0.13 kg of soybean meal and 1.5 kg of cracked high-linoleate (postexperiment analysis = 67.2% 18:2 and 3.0% N on an OM basis) safflower seeds (Linoleate); or 3) 1.5 kg of cracked high-oleate (postexperiment analysis = 72.7% 18:1 and 1.9% N on an OM basis) safflower seeds (Oleate). Safflower seeds were cracked with a roller mill to increase safflower seed DM availability within the rumen (Lammoglia et al., 1999). Dietary forage and safflower seeds were provided at the levels previously reported by our laboratory (Bottger et al., 2002). Dietary TDN contribution from safflower seeds was estimated using IVOMD (Bottger et al., 2002), assuming 177% TDN (NRC, 1996) for the fat within the safflower seeds. The Control supplement was provided at a level that should have provided quantities of dietary TDN and N similar to that of the safflower seed supplements (Table 1). Half the respective diets were fed twice daily at 0600 and 1800. Heifers had ad libitum access to fresh water and trace mineral salt (Champions Choice, Akzo Salt Inc., Georgetown, SC; guaranteed analysis [percentage of DM]: NaCl, 95 to 99; Co, Cu, I, Mn, Zn, and Fe, less than 1).
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As a marker of digesta flow, boluses (No. 10 lock ring gelatin capsules, Torpac Inc, Fairfield, NJ) containing 5 g of Cr2O3 were dosed intraruminally at each feeding. Each 17-d experimental period included 14 d of adaptation to the new dietary treatment. Beginning at 0400 on d 15 of each sampling period, duodenal (200 mL) and fecal (50 mL) samples were taken every 4 h. On d 16, collection times were advanced 2 h so that samples were collected to represent every 2 h in a 24-h period. Fecal samples were dried in a 55°C forced-air oven, ground (Wiley mill, 1-mm screen, Thomas Hill and Sons, Philadelphia, PA), and composited (equal dry weight basis) within heifer for each period. Duodenal digesta samples were composited (equal volume) within heifer for each period. Duodenal digesta samples were then lyophilized (Genesis SQ 25 Super ES freeze dryer, The VirTis Co., Gardiner, NY) and ground (Wiley mill, 1-mm screen, Thomas Hill and Sons).
Immediately before the 0600 feeding on d 17 of each period (0 h), approximately 200 mL of whole ruminal contents was collected from the center of the ruminal mat (dorsal to the cranial pillar). Ruminal samples were then collected at 3, 6, 9, 12, 15, 18, and 21 h. Ruminal pH was immediately measured on whole ruminal contents using a combination electrode (Orion Research Inc., Boston, MA), and 10 mL of ruminal fluid was strained through eight layers of cheesecloth. The resulting ruminal fluid was acidified with 0.1 mL of 7.2 N H2SO4 and immediately frozen. The remaining 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 1.0 min to dislodge particulate-associated bacteria. The homogenized sample was then strained through eight layers of cheesecloth for subsequent bacterial isolation by differential centrifugation (Merchen and Satter, 1983). The resulting bacterial isolate was lyophilized (Genesis SQ 25 Super ES freeze dryer, The VirTis Co.) and ground with a mortar and pestle for subsequent laboratory analyses.
Laboratory Analyses
Feed, duodenal, microbial, and fecal samples were analyzed for DM and ash (AOAC, 1990). Nitrogen content of feed, microbial, duodenal, and fecal samples was determined using a Leco FP-528 N analyzer (Leco Corp., Henderson, NV). Neutral detergent fiber content of feed, feces, and duodenal digesta was determined using an Ankom 200 fiber analyzer (Ankom Technology, Fairport, NY). Chromium concentration in duodenal digesta and feces was determined (Hill and Anderson, 1958) by atomic absorption spectrophotometry (model 210 VGP AASpectr., Buck Scientific, E. Norwalk, CT) with an air-plus-acetylene flame.
Acidified ruminal fluid samples were centrifuged at 10,000 x g for 10 min, and a 2.5-mL aliquot of the resulting supernatant was added to 0.5 mL of 25% metaphosphoric acid containing 2 g/L of 2-ethyl-butyric acid (Goetsch and Galyean, 1983). These samples were then centrifuged for 10 min at 10,000 x g, and the supernatant fluid was analyzed for concentrations of VFA using a Hewlett-Packard 5890 GLC (Hewlett-Packard, Avondale, PA) equipped with a 15 m x 0.533 mm (i.d.) column (Nukol, Supelco, Bellefonte, PA) with a ramp temperature of 110 to 150°C at 8°C/min. Helium was used as the carrier gas with a column flow rate of 20 mL/min. Injector and detector temperatures were 250°C. Ruminal NH3 concentration was determined by the phenol- hypochlorite procedure (Broderick and Kang, 1980).
Feed, duodenal digesta, and fecal samples were subjected to direct transesterification by incubating samples in a 16-mm x 125-mm screw-cap tubes with 2.0 mL of HCl in methanol (1.09 M HCl; 9.12 mL of HCl/100 mL of methanol) and 2 mL of methanol including 1 mg of tridecanoic acid methyl ester (Sigma, St. Louis, MO), as an internal standard, at 85°C for 1 h. Fatty acid methyl esters were extracted in 2.0 mL of hexane and transferred to GC vials containing a 1-mm bed of anhydrous sodium sulfate. Weight percentages of CLA can be underestimated by using an acid catalyst to prepare fatty acid methylesters (Kramer et al., 1997). However, fatty acids in duodenal contents of ruminants are predominately in the nonesterified form because of the extensive fatty acid hydrolysis occurring in the rumen. Alkaline catalysts do not react with nonesterified fatty acids (Hammond, 1993); thus, use of acid catalyst is appropriately required. Moreover, treatment effects remained intact when acid and alkaline catalysts were compared (Murrieta et al., 2003). Separation of fatty acid methyl esters was achieved by GLC (model 5890 series II, Hewlett-Packard) with a 100-m capillary column (SP-2560, Supelco) and He as a carrier gas at 0.5 mL/min. Oven temperature was maintained at 175°C for 40 min and then ramped to 240°C at 10°C/min to 240°C. Injector and detector temperature was 250°C. Identification of peaks was accomplished using purified standards (Nu-Chek Prep, Elysian, MN; Matreya, Pleasant Gap, PA).
Calculations and Statistical Analyses
Nutrient flows and digestibilities were estimated as previously described by Kucuk et al. (2001) and Scholljegerdes et al. (2004). Biohydrogenation of unsaturated fatty acids (C18) was calculated using the equation of Tice et al. (1994). Additionally, the following equations were used to evaluate the percentages of biohydrogenation intermediates at the duodenum:
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where D = duodenal flow of the respective fatty acid, and I = intake of the respective fatty acid.
Single degree of freedom orthogonal contrasts were used to compare effects of Control vs. supplemental safflower seeds (Lineolate and Oleate), as well as Linoleate vs. Oleate. All data were analyzed using the Mixed procedure of SAS (SAS Inst., Inc., Cary, NC). The model included animal as the random variable. Additionally, the model used for analysis of time course data included effects of animal, period, treatment, time, and treatment x time. Animal x period x treatment was used to specify variation among animals using the Random statement. Autoregression order one was determined to be the most desirable covariance structure according to the Akaike’s information criterion. No treatment x sampling hour interactions (P = 0.11 to 0.99) were detected for time course data; therefore, only the main effects were reported.
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Results and Discussion |
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TopAbstractIntroductionMaterials and MethodsResults and DiscussionImplicationsLiterature Cited |
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Total OM intake differed (P < 0.001) across all treatments (data not shown) due to the level at which supplement was fed to equalize intake of TDN (Table 2). Decreased duodenal OM flow (P = 0.02) for Control reflected the improved (P = 0.01) true ruminal OM disappearance observed for this treatment because microbial OM flow to the duodenum was not affected (P = 0.19 to 0.21) by treatment. Whitney et al. (2000) demonstrated that as dietary soybean oil level increased from 3% to 6%, IVDMD decreased linearly (P < 0.001) compared with a corn–soybean meal-based control, which was similar to the Control supplement fed in the present experiment. Expressing duodenal OM flow on a fatty acid free basis did not account (P = 0.40) for the increased duodenal OM flow (data not shown) for the safflower seed supplements, indicating that the OM of the corn-based Control supplement was more digestible than the supplements containing safflower seeds. Post-ruminal OM disappearance (percentage of intake) did not differ (P = 0.72 to 0.95) across treatments. Cattle fed Control had lower (P < 0.001) fecal OM output, reflecting the greater (P < 0.001) total-tract OM disappearance for this treatment. No differences (P = 0.40 to 0.68) were noted between cattle fed Linoleate and Oleate for fecal OM output or total-tract OM disappearance. In contrast to our results, supplementing lower levels of soybean oil to beef heifers (0.03% of BW) grazing bromegrass pasture did not negatively influence ruminal, postruminal, or total-tract OM digestibility (Brokaw et al., 2001). Aldrich et al. (1997) reported no differences in ruminal and postruminal OM disappearance, but observed greater digestion of OM in the total tract for nonfat-supplemented steers compared with steers supplemented with whole canola seeds. When supplemental soybean oil was given via ruminal or duodenal infusion to beef heifers consuming grass hay (Krysl et al., 1991), true ruminal OM digestibility was not influenced; however, total-tract OM digestibility was lower for the ruminally infused treatments compared with the control and duodenally infused treatments. However, relatively low ruminal OM digestibilities coupled with a relatively large standard error may have precluded detection of a significant difference between treatments in the study by Krysl et al. (1991). Our results agree with those of Elliot et al. (1997), who demonstrated a decrease in ruminal OM digestibility and no difference in postruminal OM digestibility when tallow was supplemented to beef steers consuming a 60% forage diet; however, as saturation and esterification increased, there was a linear decrease in total tract OM digestibility. Thus, decreased total-tract OM disappearance for cattle receiving supplemental lipids may be attributed to decreased ruminal OM disappearance.
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Despite supplements initially formulated to provide similar quantities of dietary N, N intake by cattle fed the high-linoleate safflower seeds was greater (P < 0.001) because the N content of the seeds throughout the trial were higher than originally estimated from pretrial laboratory analysis (Table 3). Nonetheless, total, microbial, and nonmicrobial N flow to the duodenum did not differ (P = 0.27 to 1.0) across treatments. These results are similar to those reported by Brokaw et al. (2001). True ruminal N degraded (g/d) tended (P = 0.12) to be greater for Linoleate and Oleate compared with Control, and tended (P = 0.13) to be greater for Linoleate than for Oleate. Using the average ruminal N degradability value for bromegrass hay reported by Scholljegerdes et al. (2004) in addition to average ruminally degraded protein values of the NRC (2001) for cracked corn, soybean meal, and safflower seed meal (assuming ruminally degraded protein for whole seeds is equal to the seed meal), we estimated that ruminally degraded N would have ranked Linoleate (+ 16 g/d) >Oleate (+ 5 g/d) >Control. A similar ranking (Linoleate = 46 g, Oleate = 12 g, and Control = –80 g) was noted for postexperimental evaluations using the NRC (1996), with microbial efficiency adjusted to 10% of TDN (Lardy et al., 2004). Although the Control diet seemed to be deficient in ruminally degraded protein, estimates of N recycling according to NRC (1985) equations indicated that N recycled to the rumen should have been more than sufficient (approximately 65 g) to maintain proper ruminal N status. Postruminal N disappearance did not differ (P = 0.22 to 0.34) across treatments; however, total-tract N disappearance was greater (P < 0.001) for fat-supplemented heifers, as well as between fat supplements with Linoleate being greater (P = 0.04) than Oleate. Brokaw et al. (2001) reported that ruminal and total-tract N disappearance did not differ, but lower-tract N disappearance (percentage of intake) decreased when grazing beef heifers were given cracked corn or soybean oil. Microbial efficiency did not differ (P = 0.15 to 0.92) among treatments, which agrees with other research (Murphy et al., 1987; Kalscheur et al., 1997; Brokaw et al., 2001) on the influence of supplemental fat on site and extent of digestion. The numerical increase in microbial efficiency for cattle fed supplemental fat in our experiment was also similar in magnitude to the nonsignificant increase previously reported by Krysl et al. (1991).
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Neutral detergent fiber intake was greatest (P < 0.001) by cattle fed safflower seeds (Table 4) as a result of differences in the NDF content of the supplements, or specifically, the seed coat on the safflower seeds. Whitney et al. (2000) noted that 6% added soybean oil had deleterious effects on digestibility of a forage-based diet, whereas 3% dietary soybean oil may have been optimal for maintaining digestion. Our laboratory also has reported that feeding soybean oil at 1.74% of the total diet did not affect NDF digestibility by heifers grazing bromegrass pastures (Brokaw et al., 2001). However, in the present experiment, ruminal NDF disappearance was decreased (P = 0.02) with the provision of cracked safflower seeds, and no difference (P = 0.78) was noted between safflower seed types. Murphy et al. (1987) indicated that decreased ruminal availability of the lipid in supplemental oilseeds could be an effective way to circumvent the negative effects fat exert on fiber digestion. However, if oil seeds are cracked, thereby increasing ruminal availability (Lammoglia et al., 1999), negative effects on ruminal NDF digestibility are more likely if fed above critical levels (Aldrich et al., 1997). In addition to providing supplemental oilseeds at a level that was slightly greater than normally assumed not to negatively influence ruminal fermentation (Jenkins, 1993), the combination of cracking the oilseeds to increase ruminal availability (Lammoglia et al., 1999) and the relatively indigestible oilseed hull (Aldrich et al., 1997) may have had additive negative effects on ruminal NDF digestibility. No differences (P = 0.55 to 0.91) were noted among treatments for postruminal NDF disappearance. Likewise, Brokaw et al. (2001) reported that the concentration of supplemental soybean oil to beef heifers grazing bromegrass pasture did not influence postruminal NDF digestibility (percentage of intake). Due to the greater ruminal NDF disappearance for Control without an effect on postruminal NDF disappearance, total-tract NDF disappearance was greater (P = 0.003) for Control than for the safflower seed treatments.
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Ruminal pH was greater (P = 0.05) for Linoleate than for Oleate, but pH was not different (P = 0.79) between Control and safflower-supplemented animals (Table 5). Nonetheless, pH values for all treatments were within the range considered acceptable for fiber digestion (Ørskov and Ryle, 1990). Ruminal ammonia concentrations did not differ (P = 0.23 to 0.32) across all treatments ranging from 4.1 to 5.5 mM, and were above the range (0.60 to 1.59 mM NH3) considered by Satter and Slyter (1974) to be sufficient for microbial N production.
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). Also, ruminal fermentation of corn can result in greater ruminal molar proportions of butyrate (Hess et al., 1996). Ruminal molar proportions for all of the VFA reported in Table 5
did not differ between Linoleate and Oleate safflower seed treatments. This was expected because fatty acid composition does not influence VFA production (Palmquist, 1991). Ruminal molar proportions of propionate were greater (P = 0.002) for cattle fed safflower seeds than Control. Krysl et al. (1991) attributed greater ruminal molar proportion of propionate in beef heifers infused with soybean oil in the rumen to decreased fiber digestion; however, Chalupa et al. (1986) suggested that increased molar proportions of propionate may be attributed to bacterial metabolism of glycerol. Supplemental lipids have also decreased methanogenesis (Jounay, 1994), which will increase proportions of propionate (Van Soest, 1994).
The acetate:propionate ratio reflected the observed difference in ruminal NDF disappearance, in that the ratio tended (P = 0.06) to be greatest for Control compared with Linoleate and Oleate. A decrease in the acetate:propionate ratio with supplemental fat has been noted by several researchers (Jenkins et al., 1989; Palmquist, 1991; Whitney et al., 2000). Ruminal molar proportions of isobutyrate and valerate were not different (P = 0.39 to 0.96) among all treatments. The trend for increased (P = 0.11) ruminal molar proportions of isovalerate for the Control treatment likely had little biological importance (Gunter et al., 1990).
Fatty Acid Intake and Disappearance
Fatty acid intake was greater (P < 0.001) by heifers fed safflower seeds, and heifers fed Oleate consumed 4.9 g/d more (P < 0.001) fatty acid than heifers fed Linoleate (Table 6). Intake of 18:1 was greatest (P < 0.001) for Oleate, and intake of 18:2 was greatest (P < 0.001) for Linoleate, reflecting differences in fatty acid composition of the safflower seeds (Table 1). The primary source of dietary 18:3 in the diets used for this experiment originated from the forage; however, there were minor differences in 18:3 content between the three supplements. Forage intake was held constant across treatment, and the minor differences noted for duodenal 18:3 caused the treatments to differ (P < 0.001), but this difference was small enough that the biological significance (46.0 to 46.7 g/d) was questionable.
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). Accumulation of unsaturated fatty acids in the rumen can be toxic to ruminal bacteria. Consequently, ruminal bacteria hydrogenate double bonds of unsaturated fatty acids in an effort to alleviate toxicity (Harfoot and Hazelwood, 1988). Thus, ruminal biohydrogenation of dietary unsaturated C18 fatty acids expressed as a percentage of intake was greater (P = 0.007) in heifers fed safflower seeds (Table 7). Biohydrogenation of dietary 18:3 tended (P = 0.07) to differ between Control and lipid-supplemented heifers, but did not differ (P = 0.68) between the Linoleate and Oleate treatments. Additionally, ruminal biohydrogenation of dietary unsaturated C18 fatty acids was higher (P < 0.001) for Linoleate than Oleate. Ruminal biohydrogenation of dietary 18:1 was greater (P < 0.001) for diets with safflower seed than Control, and Oleate was greater (P = 0.02) than Linoleate. Kalscheur et al. (1997) also noted increased biohydrogenation of dietary 18:1 in dairy cattle fed either high-oleate or high-linoleate sunflower oil. Biohydrogenation of 18:2 is initiated by the isomerization of the cis-12 double bond, which produces cis-9,trans-11 CLA (Harfoot and Hazelwood, 1988). The cis-9 double bond of the conjugated diene is then hydrogenated resulting in formation of 18:1trans-11. Complete biohydrogenation of dietary unsaturated C18 fatty acid results in ruminal accumulation of 18:0; however, Harfoot et al. (1973) reported that increased ruminal concentrations of 18:2 increased the extent of biohydrogenation of 18:2. Furthermore, Bauchart et al. (1990) showed that as linoleic acid intake increased (4.1 to 46.6 g/d) biohydrogenation of 18:2 concomitantly increased (55 to 91%, respectively). Kalscheur et al. (1997) also showed an increase in biohydrogenation of 18:2 when dairy cows where fed high-linoleate sun-flower oil. Harfoot et al. (1973) stated that ruminal levels of 18:2 is the predominant factor affecting biohydrogenation of 18:2, and as intake of 18:2 increases, the accumulation of biohydrogenation intermediates increase with time. In our study, the proportion of biohydrogenation intermediates reaching the small intestine was greater (P 0.002) for Linoleate than Oleate. Our results are comparable to those of Gould (2000), who noted more extensive ruminal biohydrogenation and a concomitant increase in production of biohydrogenation intermediates in lambs fed high-forage diets with 6% supplemental soybean oil.
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suggested that 18:1 should not be a source of 18:1trans-11; however, when these researchers evaluated 18:1trans-11 flow to the small intestine in dairy cattle fed high-linoleate or high-oleate sunflower oil, they observed similar duodenal flow of 18:1trans-11 for both treatments. Kalscheur et al. (1997) attributed this result to sufficient amounts of PUFA in the high-oleate sunflower oil diet to provide adequate precursors for the production of 18:1trans-11. In the current study, this was not the case because duodenal flow of 18:1trans-11 with Linoleate was twice that with Oleate supplements. Kemp et al. (1975) indicated that a minor cellulolytic bacterial species, Fusocillus spp., can isomerize 18:1cis-9 to 18:1trans-11. There may have been sufficient Fusocillus spp. present in the rumen of cattle fed Oleate to promote the accumulation of significant quantities of 18:1trans-11. Increased duodenal flow of 18:1trans-11 for both safflower treatments is noteworthy because 18:1trans-11 is the predominant precursor for 18:2cis-9,trans-11 production within animal tissues, including the mammary gland (Bauman et al., 2000). Duodenal flow of 18:2cis-9,trans-11 tended (P = 0.06) to be higher in heifers supplemented with oilseeds compared with Control. In contrast, heifers fed Control had greater (P = 0.03) 18:2trans-10,cis-12 flowing to the duodenum than did heifers fed the oil-seeds. Greater duodenal flow of 18:2trans-10,cis-12 for Control may be related to this CLA isomer, constituting a greater proportion of the CLA isomers when cereal grains are included in the diet (Kucuk et al., 2001). Compared with Control, total saturated and unsaturated fatty acid flow to the duodenum was greater (P < 0.001) in heifers fed safflower seeds; duodenal flow of these fatty acids did not differ (P = 0.44) between Linoleate and Oleate (Table 8). Kalscheur et al. (1997) noted that duodenal fatty acid flow in dairy cattle increased equally whether diets were supplemented with high-linoleate or high-oleate sunflower oil. Greater flow of fatty acid to the duodenum in cattle fed oilseeds was expected because negligible amounts of dietary long-chain fatty acid disappear from the rumen (Jenkins, 1993).
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). Postruminal disappearance of monounsaturated fatty acids and PUFA did not differ (P = 0.60 to 0.79) between Control and heifers fed safflower seeds, and the Linoleate treatment had greater postruminal disappearance of mono-unsaturated fatty acids and PUFA (P 0.001) than the Oleate treatment. Kalscheur et al. (1997) noted that when lactating dairy cows were fed either high-linoleate or high-oleate sunflower oil, postruminal digestibility of individual and total fatty acids did not differ. Postruminal 18:1trans-11 disappearance was not different (P = 0.48 to P = 0.54) across all treatments and averaged 92 ± 1.0%, indicating that 18:1trans-11 is readily digested. Efforts to increase the flow of 18:1trans-11 could be a useful way to increase 18:2cis-9,trans-11 content within ruminant tissues because 18:1trans-11 is the predominant precursor for 18:2cis-9,trans-11 production within adipose and mammary tissues (Bauman et al., 2000). Although postruminal disappearance of 18:1trans-11 did not differ across dietary treatments, heifers fed high-linoleate safflower seeds would have had the greatest quantity of 18:1trans-11 available for metabolism because intestinal supply of this fatty acid was greatest in these animals. Postruminal disappearance of total fatty acid was not influenced (P = 0.98) by supplementing safflower seeds but was greater (P = 0.05) for Linoleate than for Oleate. This may have been due to a greater proportion of the total fatty acids presented to the small intestine being comprised of PUFA for the Linoleate treatment compared with the Oleate treatment. Andrews and Lewis (1970) indicated that unsaturated fatty acids are more digestible than saturated fatty acids.
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, who observed decreased postruminal digestibility of saturated fatty acid and total fatty acid in dairy cows fed 60% forage diets with 6% fat from an animal-vegetable blend. Andrews and Lewis (1970) suggested that postruminal digestibility of unsaturated fatty acids is greater than saturated fatty acids because unsaturated fatty acids are more hydrophilic and more easily form micelles, and therefore are more readily absorbed. Previous research conducted in our laboratory (Gould, 2000; Kucuk et al., 2001) indicated that the CLA isomers are between 90 and 100% digestible in the small intestine of wethers fed a forage-based diet and supplemental soybean oil. Caution must be exercised, however, when interpreting results of apparent postruminal fatty acid disappearance because of the contribution of endogenous fatty acids (Kucuk et al., 2001).
Overall, supplementing beef heifers with high-fat safflower seeds can boost the quantity of saturated and unsaturated fatty acids that reach the small intestine. However, with regard to overall energy available at the small intestine, increased postruminal disappearance of fat did not compensate for the differences in intake and ruminal OM and NDF disappearance between treatments. We estimated dietary TDN as the sum of digestible fatty acid free OM and digestible fatty acids times 2.25. Our estimates of total TDN intake using results from this experiment indicated that the Control diet was greater (P < 0.001) than the diets supplemented with fat, and Linoleate tended (P = 0.12) to be greater than Oleate (7,923, 7,199, and 6,846 g/d, respectively). Preexperiment estimates of total TDN intake were 6,858, 6,570, and 6,581 g/d for Control, Linoleate, and Oleate, respectively. Comparisons of preexperiment dietary TDN to our estimates using the experimental results revealed that the deviation in expected vs. observed dietary TDN was within the normal range reported in the literature (Moore et al., 1999). Moreover, we anticipated TDN intake to be slightly greater for Control than for the oilseed diets; however, the TDN value of our diets containing safflower seeds may have been decreased because of the unexpected decrease in ruminal fiber and OM disappearance, which resulted in decreased disappearance of OM from the total tract for these treatments. Alternatively, the greater than expected magnitude of difference in TDN intake between the Control and the safflower seed diets could be due to positive associative effects for the Control treatment. Positive associative effects have been noted in many studies where researchers have fed grain-based supplements at less than 0.3% of BW (Gunter, 1996; Caton and Dhuyvetter, 1997). Corn was fed to the Control heifers at 0.28% of BW in the current experiment. Decreased postruminal disappearance of fatty acids for Oleate also resulted in a decrease in estimated total TDN intake compared with the Linoleate treatment. Thus, it may be erroneous to ascribe the same TDN value to all varieties of safflower seeds.
As humans become more cognizant of health issues and demand meat that is healthier, the development of ruminant-derived food products as functional foods will be an important goal for the livestock industry. Additionally, certain fatty acids have been shown to modulate various immune and inflammatory responses (Hwang, 2000), as well as contribute to improved reproductive success (Staples et al., 1998). Therefore, it is becoming more important to understand ruminal metabolism of dietary fatty acids, which will allow nutritionists to manipulate the diet to optimize intestinal supply of specific fatty acids that may have beneficial attributes. Heifers consuming safflower seeds high in linoleic acid compared with safflower seeds high in oleic acid had greater ruminal biohydrogenation of 18C fatty acids and, therefore, had greater duodenal flow of 18:1trans-11. We also observed an increase in duodenal flow of the 18:2cis-9,trans-11 isomer in cattle fed safflower seeds; however, duodenal flow of 18:2trans-10,cis-12, the isomer implicated to decrease fat deposition (Park et al., 1999), was not detected in heifers fed Oleate. In an experiment conducted by Bolte et al. (2002), lambs fed high-linoleate safflower seeds had higher muscle and adipose tissue levels of 18:1trans-11 and 18:2cis-9,trans-11 compared with lambs fed high-oleate safflower seeds or a low-fat control. The CLA status of the consumer is increased because 18:1trans-11 can be converted to 18:2cis-9,trans-11 by 
9 desaturase (Turpeinen et al., 2002). Therefore, consumers of beef produced from cattle fed safflower seeds in diets comparable to those reported herein would be expected to have increased CLA status. Results of the current study are also suggestive of increased 18:1trans-11 and 18:2cis-9,trans-11 available to the calf from the milk of lactating beef cow. Dairy cows fed high-oleate or high-linoleate sunflower seeds had increased milk output of both of these fatty acids (Kalscheur et al., 1997). The beneficial effects of CLA observed in laboratory animals (McGuire and McGuire, 2000) may occur in beef calves as well. Thus, determining the effects of oilseed supplementation on fatty acid output via the mammary gland of beef cows and subsequent fatty acid status of the nursing calf would be of great importance.
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2 Correspondence: P.O. Box 3684 (phone: 307-766-5173; fax: 307-766-2355; e-mail: brethess{at}uwyo.edu).
Received for publication April 15, 2004. Accepted for publication August 19, 2004.
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