J. Anim Sci. 2008. 86:640-650. doi:10.2527/jas.2006-812
© 2008 American Society of Animal Science
Effects of supplemental fat source on nutrient digestion and ruminal fermentation in steers1
S. P. Montgomery*,
J. S. Drouillard*,2,
T. G. Nagaraja
,
E. C. Titgemeyer* and
J. J. Sindt*
* Department of Animal Sciences and Industry, and and
Department of Diagnostic Medicine/Pathobiology, Kansas State University, Manhattan 66506-1600
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Abstract
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Five Holstein steers (235 kg of BW) fitted with ruminal, duodenal, and ileal cannulas were used in a 5 x 5 Latin square design experiment to determine the effects of supplemental fat source on site and extent of nutrient digestion and ruminal fermentation. Treatments were diets based on steam-flaked corn containing no supplemental fat (control) or 4% (DM basis) supplemental fat as tallow, dried full-fat corn germ (corn germ), corn oil, or flax oil. Fat supplementation decreased (P < 0.08) ruminal starch digestion but increased (P < 0.03) small intestinal starch digestion as a percentage of intake. Feeding corn germ decreased (P < 0.09) ruminal starch digestion and increased (P < 0.03) large intestinal starch digestion compared with steers fed corn oil. Large intestinal starch digestion was less (P < 0.04), and ruminal NDF digestion was greater (P < 0.09) for steers fed tallow compared with steers fed other fat sources. Small intestinal (P < 0.08) and total tract NDF digestibilities were greater (P < 0.02) for steers fed corn germ than for those fed corn oil. Feeding tallow increased total ruminal VFA (P < 0.03) and NH3 (P < 0.07) concentrations compared with steers fed the other fat sources. Feeding corn germ led to a greater (P < 0.02) rate of ruminal liquid outflow compared with corn oil. A diet x hour interaction (P < 0.04) occurred for ruminal pH, with steers fed corn oil having the greatest ruminal pH 18 h after feeding, without differences at other time points. Fat supplementation increased (P < 0.09) ruminal concentrations of Fusobacterium necrophorum. Duodenal flow of C18:3n-3 was greater (P < 0.01) for steers fed flax oil compared with those fed corn oil. Feeding corn germ led to less (P < 0.01) ruminal biohydrogenation of fatty acids compared with corn oil. Steers fed tallow had greater small intestinal digestibility of C14:0 (P < 0.02) and C16:1 (P < 0.04) than steers fed the other fat sources. Fat supplementation decreased (P < 0.06) small intestinal digestibility of C18:0. Feeding corn germ decreased (P < 0.10) small intestinal digestibility of C18:1 compared with corn oil. It appears that source of supplemental fat can affect the site and extent of fatty acid and nutrient digestion in steers fed diets based on steam-flaked corn.
Key Words: steer • finishing diet • fatty acid • nutrient • digestibility
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INTRODUCTION
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Feeding supplemental fat improves G:F of finishing cattle (Brandt et al., 1992
; Zinn and Shen, 1996; Ramirez and Zinn, 2000). Although tallow is a supplemental fat source commonly fed to finishing cattle, the use of other supplemental fat sources in finishing cattle diets has been explored. Montgomery et al. (2005) reported that growth performance of finishing cattle fed supplemental fat as tallow or as dried full-fat corn germ (corn germ) was similar. Montgomery et al. (2005) also reported fewer abscessed livers in cattle fed finishing diets containing corn germ, suggesting a possible inhibitory effect of corn oil on the growth of Fusobacterium necrophorum, a ruminal bacterium recognized as the primary etiologic agent of liver abscesses (Nagaraja and Chengappa, 1998). LaBrune et al. (2002b) reported that growth performance of steers fed finishing diets containing supplemental fat as tallow or ground flaxseed was not different. LaBrune et al. (2002a) also reported that feeding supplemental fat as ground flaxseed increased the n-3 fatty acid concentration in meat of cattle, indicating that some n-3 fatty acids from flax oil escaped ruminal biohydrogenation.
Our objectives were to evaluate the effects of supplemental fat sources consisting of tallow, corn germ, corn oil, or flax oil on site and extent of nutrient digestion, ruminal fermentation, biohydrogenation of fatty acids, and ruminal concentrations of F. necrophorum in steers fed diets based on steam-flaked corn.
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MATERIALS AND METHODS
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The following experiment was approved by the Kansas State University Animal Care and Use Committee.
Five Holstein steers (235 ± 24 kg of BW) fitted with ruminal, duodenal (double L; 6 cm caudal to the pyloric sphincter) and ileal (double L; 10 cm cranial to the ileocecal junction) cannulas were used in a 5 x 5 Latin square design experiment. Treatments were finishing diets based on steam-flaked corn (Table 1) containing no supplemental fat (control) or 4% supplemental fat as tallow, corn germ, corn oil, or flax oil. Corn was steam-conditioned for 45 min using a 2.7-m3 steam chest, after which it was subsequently rolled to a density of 335 g/L (i.e., 26 lb per bushel). Five consecutive 15-d periods were used, each consisting of 10 d for adaptation, 4 d for digesta collection, and 1 d for sampling ruminal fluid for measurement of ruminal pH, VFA, and liquid dilution rate. Steers were housed in a tie-stall barn equipped with individual feed bunks, rubber-matted floors, and automatic water fountains. Steers were individually weighed at the beginning of the experiment. Diets were fed at 0800 throughout the experiment and were offered for ad libitum consumption. Feed refusals were collected before feeding on d 11 through 14, weighed, and subsamples were obtained and frozen at –20°C. Representative diet samples were obtained before feeding and on d 10 through 13 and frozen at –20°C.
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Table 1. Composition of the experimental diets (% of DM) containing no supplemental fat (control) or supplemental fat as tallow, dried full-fat corn germ (corn germ), corn oil, or flax oil
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Duodenal and ileal flows and fecal output were estimated by feeding 10 g/d of chromic oxide, hand-mixed into the diet of each steer immediately before feeding on d 4 through 14. On d 11 through 14, duodenal (
300 mL) and ileal (200 mL) chyme and fecal samples (300 g wet basis) obtained from the rectum of steers were collected 3 times daily and immediately frozen at –20°C. Duodenal, ileal, and fecal samples were collected every 8 h, with the sampling time advanced by 2 h each day, so that samples were obtained at 2-h intervals after feeding in a 24-h period during the 4-d collection period. Feed refusals and duodenal, ileal, and fecal samples for each steer in each period were thawed and composited. Feed refusals and fecal samples were dried at 55°C, and duodenal and ileal samples were lyophilized. Feed refusals, duodenal, ileal, and fecal samples were ground in a Wiley mill (Thomas Scientific, Swedesboro, NJ) to pass a 1-mm screen. Chromium concentrations of feed refusals, duodenal, ileal, and fecal samples were determined by atomic absorption spectrophotometry with an acetylene/air flame (Perkin Elmer 3110, Norwalk, CT) according to the methods of Williams et al. (1962). Diet samples were composited by treatment within period, dried at 55°C, and ground to pass through a 1-mm screen. Dietary, feed refusal, fecal, and digesta samples were dried at 105°C in a forced-air oven to determine DM and were ashed at 450°C for 8 h to determine OM. Determination of NDF was conducted using an AN-KOM fiber analyzer (ANKOM Technology Corp., Fairport, NY). Starch content was determined according to the methods of Herrera-Saldana and Huber (1989) using a Technicon Autoanalyzer III (Bran and Luebbe, Elmsford, NY) to measure free glucose (Gochman and Schmitz, 1972). Nitrogen was determined using a LECO FP-2000 N analyzer (LECO Corp., St. Joseph, MI). Methylation of fatty acids was conducted according to Sukhija and Palmquist (1988), and methyl esters of fatty acids were analyzed using a GC-17A gas chromatograph (Shimadzu Corp., Kyoto, Japan) with methyl-tridecanoate (T0627; Sigma-Aldrich, St. Louis, MO) as an internal standard.
On d 11 through 14 of each period, approximately 500 g of whole ruminal contents was collected from each steer once daily and immediately blended using a Waring blender (Waring Products Division, New Hartford, CT) to dislodge particle-associated bacteria. Approximately 500 mL of saline solution was added to aid in blending. Blended ruminal contents were strained through 8 layers of cheesecloth, and the strained ruminal fluid was frozen at –20°C. Collection time was advanced 6 h each day so that a sample was obtained every 6 h in a 24-h period during the 4-d collection period. Upon thawing of ruminal fluid, bacterial cells were isolated using differential centrifugation (Cecava et al., 1990). Isolated bacterial cells were lyophilized and analyzed for N as described previously, and for cytosine according to the methods described by Milton et al. (1996).
On d 11 through 13 of each period before feeding, whole ruminal contents (approximately 500 g) of each steer were collected and strained through 4 layers of cheesecloth. The strained rumen fluid was immediately transported to the laboratory and was used to determine the concentration of F. necrophorum. The enumeration procedure was according to the most probable number technique using a selective lactate medium (Tan et al., 1994).
On d 15 of each period, whole ruminal contents (approximately 500 g) from each steer were collected before feeding, after which a 200-mL solution containing 3 g of CoEDTA (providing 400 mg of Co) was immediately pulse-dosed through the ruminal cannula. Whole ruminal contents were subsequently sampled at 2, 4, 6, 8, 12, 18, and 24 h after dosing. Whole ruminal contents were strained through 4 layers of cheesecloth. A portable pH meter was used to measure pH of strained ruminal fluid at time of sampling, after which 8 mL of strained ruminal fluid was added to 2 mL of 25% (wt/ vol) m-phosphoric acid and frozen at –20°C. Approximately 20 mL of strained ruminal fluid was frozen at –20°C immediately after collection.
Acidified ruminal fluid samples were thawed and centrifuged at 30,000 x g for 20 min, and a portion of the supernatant fluid was analyzed for VFA concentrations using GLC (Hewlett-Packard 5890A, Palo Alto, CA; 183 x 0.635-cm column with GP 10% SP-1200/1% H3PO4 on 80/100 Chromosorb W AW; Supelco, Bellefonte, PA), with N2 as the carrier gas, a flow rate of 80 mL/min, and a column temperature of 130°C. A portion of the supernatant fluid was analyzed for NH3 concentration using a Technicon Autoanalyzer III (Broderick and Kang, 1980). Nonacidified ruminal fluid samples were thawed and centrifuged at 30,000 x g for 20 min, and the supernatant fluid was analyzed for Co concentration using atomic absorption spectrophotometry.
Calculations and Statistical Analyses
Chromic oxide was used as a digestion marker to estimate digestibilities at each of the regions within the digestion tract. Duodenal, ileal, and fecal output for each steer within each period were calculated as the amount of chromic oxide consumed (g/d) divided by the chromic oxide concentration in the duodenal and ileal chyme or feces (g/g of DM). Nutrient digestibilities were calculated by the following formula: [1 – (output of nutrient/intake of nutrient)] x 100. Amount of duodenal N of microbial origin was determined by dividing duodenal cytosine flow by the measured microbial cytosine:N ratio. Duodenal N of feed origin was determined by subtracting microbial N from total duodenal N. Organic matter truly fermented in the rumen was determined by subtracting microbial OM from total OM reaching the duodenum.
Liquid dilution rates were determined using the REG procedure (SAS Inst. Inc., Cary, NC) to regress the natural logarithms of Co concentration against time (Grovum and Williams, 1973). The resulting slopes represented the liquid dilution rates. Ruminal fluid volume was calculated by dividing the amount of Co dosed by the calculated (antilog of the intercept) ruminal fluid Co at 0 h. Ruminal liquid turnover time was calculated as the inverse of the ruminal liquid dilution rate, and ruminal liquid flow was calculated by multiplying ruminal liquid volume by ruminal liquid dilution rate. Percentage of ruminal biohydrogenation of dietary 18C unsaturated fatty acids was calculated using the equation of Wu et al. (1991): percentage of biohydrogenation = 100 – 100[(total duodenal digesta unsaturated 18C/total 18C in duodenal digesta)/(total unsaturated 18C intake/total 18C intake)].
Digestibility, ruminal liquid dilution rate, ruminal liquid volume, and log-transformed F. necrophorum data were analyzed as a 5 x 5 Latin square using the MIXED procedure of SAS. Model fixed effect consisted of diet, and random effects consisted of steer and period. Ruminal pH, VFA, and NH3 data were analyzed as a 5 x 5 Latin square with repeated measures over time; the model included fixed effects of diet, time, and the interaction between the 2. Random effects consisted of steer and period. The covariance structure was compound symmetry. Orthogonal contrasts were used to compare control with the mean of diets containing supplemental fat; tallow with corn germ, corn oil, and flax oil; corn germ with corn oil; and corn oil with flax oil. Because preplanned contrasts were used, comparisons were not protected by an overall F-test. During period 4 of the experiment, 1 steer consumed less than 1% of BW of the corn oil diet daily, and therefore all data for this steer were deleted.
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RESULTS AND DISCUSSION
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Nutrient Intake and Digestibility
Effects of fat supplementation on nutrient intake and site and extent of OM, NDF, starch, and N digestion are presented in Table 2. Fat supplementation did not affect (P < 0.94) intake of DM, OM, NDF, starch, or N. However, intake of NDF was greater (P < 0.01) for steers fed corn germ compared with those fed corn oil, because the addition of corn germ increased the concentration of NDF in the diet. Effects of fat supplementation on feed intake have been inconsistent, with fat supplementation reported to decrease (Krehbiel et al., 1995; Zinn and Shen, 1996; Ramirez and Zinn, 2000) or not affect (Zinn, 1989; Brandt and Anderson, 1990; Brandt et al., 1992) feed intake in cattle fed high-grain finishing diets.
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Table 2. Effects of feeding diets containing no supplemental fat (control) or supplemental fat as tallow, dried full-fat corn germ (corn germ), corn oil, or flax oil on site and extent of nutrient digestion in steers
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Fat supplementation did not affect (P < 0.80) duodenal flow of OM, NDF, total N, microbial N, or feed N. Fat supplementation increased (P < 0.05) starch flow to the duodenum, which was greater (P < 0.09) for steers fed corn germ than for steers fed corn oil. Greater duodenal starch flow in steers fed diets containing supplemental fat was the result of fat supplementation decreasing (P < 0.08) apparent ruminal starch digestion, which was less (P < 0.09) for steers fed corn germ than for steers fed corn oil. Although fat supplementation frequently has no effect on ruminal starch digestion in cattle consuming finishing diets based on steam-flaked grains (Zinn, 1992, 1994; Zinn and Plascencia, 1996), the decrease in apparent ruminal starch digestion in steers fed diets containing supplemental fat in our experiment is in agreement with data of Zinn et al. (2000), who reported decreased ruminal starch digestion in steers fed 6% supplemental fat compared with those fed 2% supplemental fat. Fat supplementation did not affect (P < 0.79) apparent ruminal digestion of OM, NDF, feed N, or microbial efficiency. Tallow supplementation surprisingly increased ruminal NDF digestion (P < 0.09) compared with steers fed the other fat sources. Lower ruminal OM digestion in steers fed grain-based diets containing supplemental fat has been previously reported (Zinn and Shen, 1996; Zinn et al., 2000; Plascencia et al., 2003) and was attributed to decreased ruminal digestibility of fiber or the decreased ruminal digestibility of fat itself. However, fat supplementation had no effect on ruminal OM digestion in some experiments (Plascencia et al., 1999). Although it has been demonstrated that less than 10% added fat can decrease ruminal digestibility of structural carbohydrates by as much as 50% or more (Ikwuegbu and Sutton, 1982; Jenkins and Palmquist, 1984), effects of fat supplementation on ruminal fiber digestion in cattle fed high-grain finishing diets is inconsistent, with fat supplementation either decreasing (Zinn and Shen, 1996; Zinn et al., 2000; Plascencia et al., 2003) or not affecting ruminal fiber digestion (Zinn, 1988; Plascencia et al., 1999). According to Plascencia et al. (1999), the inconsistency of reports of supplemental fat effects on ruminal fiber digestion might be related to ruminal pH. Cellulolytic bacteria are pH sensitive, and their growth is inhibited when ruminal pH is below 6.0 (Russell and Wilson, 1996). In our experiment, ruminal pH (Figure 1) of steers remained less than 6.0 for extended periods of time, which might have masked any negative effects of fat supplementation on apparent ruminal fiber digestion. Failure of fat supplementation to affect ruminal feed N digestion was consistent with Zinn and Shen (1996), Plascencia et al. (1999), and Plascencia et al. (2003). Failure of fat supplementation to increase ruminal microbial efficiency was in agreement with Plascencia et al. (1999), but it was inconsistent with Zinn (1988), Zinn (1994), and Zinn and Shen (1996). Increases in the efficiency of microbial protein synthesis in the rumen in response to fat supplementation are believed to be a result of either decreased ruminal protozoa numbers, and therefore decreased predation of bacteria and decreased bacterial N recycling (Ikwuegbu and Sutton, 1982; Jenkins and Palmquist, 1984), or decreased ruminal OM digestion (Doreau and Ferlay, 1995). Ruminal OM digestion was not affected by fat supplementation in our experiment, which might have contributed to the failure of fat supplementation to affect ruminal microbial protein synthesis.
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Figure 1. Ruminal pH in steers fed diets containing no supplemental fat (control) or supplemental fat as tallow, dried full-fat corn germ (corn germ), corn oil, or flax oil. Values are means ± SEM; n = 5 for all treatments except corn oil, for which n = 4. A diet x hour interaction (P < 0.04) was observed for ruminal pH. Steers fed corn oil had greater ruminal pH 18 h after feeding but not at the other time points, compared with steers fed control, tallow, corn germ, or flax oil. Main effects of fat source on ruminal pH: tallow vs. the average of corn germ, corn oil, and flax oil (P < 0.05) and corn germ vs. corn oil (P < 0.02).
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Fat supplementation did not affect (P < 0.97) apparent small intestinal digestibility of OM, NDF, or N but increased (P < 0.03) apparent small intestinal starch digestion as a percentage of intake. Steers fed corn germ had greater (P < 0.03) starch digestibility as a percentage of intake in the large intestine than did steers fed corn oil. In addition, steers fed tallow had decreased (P < 0.04) large intestinal starch digestion than those fed other fat sources. Combined, these data suggest that providing supplemental fat to steers consuming finishing diets based on steam-flaked corn can shift the site of starch digestion so that less starch is digested ruminally and more starch is digested in the small intestine. Steers fed diets containing corn germ had greater (P < 0.02) apparent total tract digestibility of NDF than did steers fed corn oil and might be a result of corn germ either containing more NDF with greater potential fermentability than that in the corn oil diet, or the free oil found in corn oil inhibited NDF digestion to a greater extent.
Ruminal Characteristics
A diet x hour interaction (P < 0.04) occurred for ruminal pH (Figure 1
), with steers fed corn oil having the greatest ruminal pH at 18 h after feeding but not at other time points. The basis for such an effect on ruminal pH is not certain. In general, ruminal pH of cattle fed high-grain finishing diets is not affected (Zinn, 1989; Bock et al., 1991; Plascencia et al., 1999) or slightly decreased (Krehbiel et al., 1995) by fat supplementation. The main effects of fat source on ruminal pH were that ruminal pH was lower (P < 0.05) in steers fed tallow compared with steers fed corn germ, corn oil, or flax oil and was lower (P < 0.02) for steers fed corn germ compared with steers fed corn oil.
Effects of fat supplementation on ruminal VFA, NH3, and liquid kinetics are presented in Table 3. Steers fed tallow had greater (P < 0.03) ruminal concentrations of total VFA compared with steers fed the other fat sources, and steers fed corn oil had greater (P < 0.07) proportions of acetate, isobutyrate, and isovalerate. The effects of fat supplementation on ruminal VFA proportions are mixed; supplemental fat has been shown to either have no effect on ruminal VFA proportions (Bock et al., 1991; Zinn and Shen, 1996) or to decrease the proportion of ruminal acetate and increase the proportion of ruminal propionate (Boggs et al., 1987; Zinn, 1988). Fat supplementation did not affect (P = 0.40) ruminal NH3 concentrations. However, steers fed tal-low had greater (P < 0.07) ruminal concentrations of NH3 compared with steers fed the other fat sources. Although supplemental fat has been reported to decrease ruminal NH3 concentrations (Ikwuegbu and Sutton, 1982), an extensive review of the literature by Doreau and Ferlay (1995) found little effect of supplemental fat on ruminal N metabolism. Fat supplementation did not affect (P < 0.93) ruminal liquid volume, ruminal liquid dilution rate, ruminal liquid turnover time, or ruminal liquid outflow. However, steers fed corn germ had a greater (P < 0.02) rate of ruminal liquid outflow compared with steers fed corn oil. Although fat supplementation has been reported to increase both ruminal liquid (Bock et al., 1991) and solid digesta (Czerkawski et al., 1975; Boggs et al., 1987) passage rate in some experiments, it appears that providing supplemental fat has little effect on ruminal passage rate (Doreau and Ferlay, 1995).
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Table 3. Effects of feeding diets containing no supplemental fat (control) or supplemental fat as tallow, dried full-fat corn germ (corn germ), corn oil, or flax oil on ruminal VFA, NH3, and liquid kinetics in steers
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Ruminal F. necrophorum
Effect of supplemental fat source on ruminal concentration of F. necrophorum, the primary etiological agent of liver abscesses (Nagaraja and Chengappa, 1998), is presented in Figure 2. Fat supplementation increased (P < 0.09) ruminal concentrations of F. necrophorum. Montgomery et al. (2005) reported that corn germ decreased the incidence of liver abscesses in finishing cattle in 2 experiments. Liver abscesses are believed to result primarily from acidosis-induced rumenitis, which allows F. necrophorum to enter the portal circulation and infect the liver (Nagaraja and Chengappa, 1998). Because corn germ replaced dry-rolled or steam-flaked corn and also decreased DMI, Montgomery et al. (2005) theorized that lesser incidence of liver abscesses in cattle fed corn germ might have resulted from decreased starch intake or altered feed intake patterns, which might have decreased the incidence of acidosis and subsequent rumenitis. Montgomery et al. (2005) also suggested that fatty acids in corn germ might have suppressed the growth of F. necrophorum. However, results of this experiment refute the hypothesis that growth of F. necrophorum is suppressed in cattle fed corn germ. Long chain fatty acids can suppress the growth of ruminal bacteria and protozoa (Henderson, 1973; Maczulak et al., 1981; Jenkins and Palmquist, 1993). Furthermore, unsaturated fatty acids are considered to be more toxic to gram-positive bacteria than gram-negative bacteria (Hartfoot and Hazlewood, 1997). Because F. necrophorum is gram-negative, perhaps supplemental fat decreased growth of gram-positive bacteria, as well as decreased predation of F. necrophorum by protozoa, thereby increasing numbers of F. necrophorum in our experiment. Unsaturated fatty acids have been shown to increase growth of Fusobacterium sp. isolated from cecal contents of rats (Morotomi et al., 1976), indicating that unsaturated fatty acids may serve as growth factors for F. necrophorum.
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Figure 2. Ruminal concentration of Fusobacterium necrophorum [log most probable number (MPN)/g] of steers fed diets containing no supplemental fat (control) or supplemental fat as tallow, dried full-fat corn germ (corn germ), corn oil, or flax oil. Values are means ± SEM, n = 5 for all treatments except corn oil, for which n = 4. Control vs. the average of diets containing supplemental fat (P < 0.09).
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Fatty Acid Availability
Effects of supplemental fat source on intake and intestinal flow of fatty acids are presented in Table 4. Fat supplementation increased (P < 0.01) intake of total fatty acids compared with control. Steers fed tallow had greater (P < 0.01) intakes of C12:0, C14:0, C15:0, C16:0, C16:1, C17:0, and C18:0 compared with steers fed the other fat sources. Steers fed tallow had decreased (P < 0.01) intakes of C18:2 compared with steers fed the other fat sources, and steers fed diets containing corn oil had greater (P < 0.01) intakes of C18:2 compared with those fed flax oil. Because of the increased concentration of C18:3n-3 in diets containing flax oil, steers fed tallow had decreased (P < 0.01) intakes of C18:3n-3 compared with steers fed the other fat sources.
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Table 4. Effects of feeding diets containing no supplemental fat (control) or supplemental fat as tallow, dried full-fat corn germ (corn germ), corn oil, or flax oil on intake and flow of fatty acids in steers
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Fat supplementation increased (P < 0.06) duodenal flows of C14:0, C16:0, C17:0, C18:0, C18:1, C18:2, C18:3n-3, and total fatty acids. Steers fed tallow had greater (P < 0.07) duodenal flows of C14:0, C15:0, C16:0, C16:1, and C17:0 compared with those fed the other fat sources. Steers fed tallow had lower (P < 0.07) duodenal flows of C18:2 and C18:3n-3 compared with those fed the other fat sources. Feeding steers corn germ increased (P < 0.01) duodenal flow of C18:1 and C18:2 compared with those fed corn oil, suggesting a protective effect for unsaturated fatty acid contained in corn germ. Steers fed flax oil had increased (P < 0.10) duodenal flow of C18:1 and C18:3n-3 compared with those fed corn oil, indicating that providing flax oil as a source of supplemental fat can increase duodenal flow of C18:3n-3 in steers fed finishing diets based on steam-flaked corn. Increased duodenal flow of C18:3n-3 in steers fed flax oil in our experiment agrees with reports of increased C18:3n-3 concentrations in the meat of cattle fed flaxseed (Kronberg et al., 2006; Mach et al., 2006; Maddock et al., 2006), indicating that some C18:3n-3 reaches the duodenum to be subsequently absorbed in the small intestine.
Ruminal biohydrogenation was lower (P < 0.01) for steers fed corn germ compared with steers fed corn oil and was lower (P < 0.03) for steers fed flax oil compared with those fed corn oil. Biohydrogenation of unsaturated FFA begins with an isomerization reaction. In the case of linoleic acid (C18:2, cis-9, cis-12), a cis-9, cis-12 diene double bond configuration is present, and the isomerization reaction converts the cis-12 double bond to a trans-11 isomer (Kepler et al., 1970). In order for the initial step of biohydrogenation to be successful, the isomerase requires a free carboxyl group on the unsaturated FFA, thereby making lipolysis a prerequisite for esterfied fatty acids. Physical protection of the oil in corn germ from microbial activity would be a likely explanation for the lower biohydrogenation of corn germ than of corn oil.
Fat supplementation increased (P < 0.04) ileal flow of C14:0, C16:0, C17:0, C18:0, C18:1, and total fatty acids compared with steers fed the control diet. Ileal flow of C14:0 and C18:0 was greater (P < 0.08) for steers fed corn oil compared with those fed corn germ. Feeding steers tallow increased (P < 0.02) ileal flow of C16:0, C16:1, and C17:0 compared with those fed the other fat sources. Steers fed corn germ had greater (P < 0.01) ileal flows of C18:1 and C18:2 compared with steers fed corn oil, and steers fed corn germ, corn oil, or flax oil had greater (P < 0.07) ileal flow of C18:2 than those fed tallow.
Fat supplementation increased (P < 0.02) fecal excretion of C16:0, C18:0, C18:1, and total fatty acids in steers. Feeding steers tallow increased (P < 0.07) fecal excretion of C16:0, C16:1, and C17:0 compared with those fed the other fat sources, and fecal excretion of C16:0 was greater (P < 0.03) for steers fed corn oil compared with steers fed flax oil. Excretion of C18:0 in the feces was lower (P < 0.02) for steers fed corn germ compared with those fed corn oil and was higher (P < 0.10) for steers fed corn oil compared with steers fed flax oil. Overall, differences in fecal excretions of C18:0 appear to be related to C18:0 flow to the ileum, rather than biohydrogenation of PUFA in the large intestine. Feeding steers corn germ increased (P < 0.02) fecal excretion of C18:1 and C18:2 compared with those fed corn oil. Fecal excretion of total fatty acids was greater (P < 0.03) for steers fed corn oil compared with those fed corn germ.
Effects of supplemental fat source on small intestinal fatty acid availability are shown in Table 5. Steers fed tallow had greater (P < 0.04) apparent small intestinal availability of C14:0 and C16:1 compared with those fed the other fat sources. Fat supplementation decreased (P < 0.06) apparent small intestinal digestion of C18:0 in steers, which was in agreement with Bock et al. (1991), Elliot et al., (1999), and Plascencia et al. (2003). Apparent small intestinal digestion of C18:1 was greater (P < 0.10) for steers fed corn oil compared with those fed corn germ. Zinn (1994) summarized 4 experiments in which cattle were fed supplemental fat and reported that 95% of the variation in intestinal digestibility of fat can be explained by dietary fat intake; as fat intake increases, intestinal fat digestibility decreases. In contrast to the observations of Zinn (1994) that small intestinal digestibility of fat decreases with increased fat intake, Doreau and Ferlay (1994) summarized 13 experiments and reported no relationship between small intestinal availability of fatty acids and level of fatty acid intake.
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Table 5. Effects of feeding diets containing no supplemental fat (control) or supplemental fat as tallow, dried full-fat corn germ (corn germ), corn oil, or flax oil on small intestinal fatty acid availability in steers
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Providing supplemental fat to finishing cattle fed steam-flaked corn-based diets might decrease ruminal starch digestion, thereby increasing amounts of starch digested in the small intestine. However, total tract starch digestion appears to be unaffected by fat supplementation. Ruminal biohydrogenation of fatty acids from dried full-fat corn germ is less than that of corn oil, which allows more PUFA to reach the small intestine in cattle fed diets based on steam-flaked corn. Supplemental fat as flax oil increases duodenal flow of C18:3n-3, which might help increase carcass concentration of C18:3n-3. Ruminal concentrations of F. necrophorum are increased with fat supplementation in steers fed diets based on steam-flaked corn. However, the effect that this might have on the incidence of abscessed livers is not known.
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Footnotes
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1 Article no. 06-254-J from the Kansas Agricultural Experiment Station. 
2 Corresponding author: jdrouill{at}ksu.edu
Received for publication December 12, 2006.
Accepted for publication December 17, 2007.
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LITERATURE CITED
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Bock, B. J., D. L. Harmon, R. T. Brandt Jr., and J. E. Schneider. 1991. Fat source and calcium level: Effects on finishing steer performance, digestion, and metabolism. J. Anim. Sci. 69:2211–2224.[Abstract]
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Abstract
INTRODUCTION
