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Carcass, sensory, and adipose tissue traits of Brangus steers fed casein-formaldehyde-protected starch and/or canola lipid¹

Department of Animal Science, Texas A&M University, College Station, 77843

	Abstract

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We predicted that providing rumen-protected starch to the small intestine would increase adiposity of intramuscular adipose tissue, and hence marbling scores. Eighteen 15-mo-old Brangus steers were assigned randomly to one of three dietary treatment groups: 1) cracked corn (Corn); 2) casein-formaldehyde-protected lipid (Canola Lipid); or 3) casein-formaldehyde-protected starch (Marble Plus). All diets were equally balanced for ME (2.91 Mcal/kg), CP (12.5%), and DM (89%). Ether extract was 3.7, 6.9, and 6.9% for the Corn, Canola Lipid, and Marble Plus diets, respectively, and the Marble Plus also contained 3.7% protected starch. Steers were fed the diets for 126 d before slaughter. Average daily feed intake (as-fed basis), ADG, and feed:gain ratio (P 0.23) did not differ among treatments. Carcasses across treatments did not differ (P = 0.26) in adjusted fat thickness, longissimus muscle area, hot carcass weight, dressing percentage, marbling scores, or USDA quality grade. Percentage of kidney, pelvic, and heart fat was higher (P < 0.01) and USDA yield grade tended (P = 0.08) to be higher, for carcasses from Canola Lipid- and Marble Plus-fed steers than for carcasses from Corn-fed steers. Of the descriptive meat sensory attributes, connective tissue amount (P = 0.06) and painty flavor (P = 0.12) tended to be greater in meat from Marble Plus steers than from Canola Lipid steers. Percentages of 18:2n-6 and 18:3n-3 were higher (P < 0.01), and 15:0, 16:0, and 17:0 were lower (P 0.07) in tissues from Canola Lipid- and Marble Plus-fed steers than in Corn-fed steers. Mean adipocyte volume was greater (P = 0.02) in i.m. adipose tissue and tended (P = 0.11) to be greater in s.c. adipose tissue of Canola Lipid steers (848 pL) vs. Corn steers (536 pL). Glucose incorporation into total lipids, glyceride-glycerol, and fatty acid fractions was highest (P < 0.01) in s.c. adipose tissue from steers fed Marble Plus but was unaffected (P 0.33) by diet in i.m. adipose tissue. Fatty acid synthetase activity tended (P = 0.08) to be higher in s.c. adipose tissue of Marble Plus steers, and NADP-malic dehydrogenase activity was higher (P = 0.03) in i.m. adipose tissue of Canola Lipid steers. We conclude that Marble Plus did not improve carcass quality, but also did not reduce beef sensory attributes. Any differences we observed in carcass characteristics, adipose tissue cellularity, or lipogenesis apparently were caused by the protected lipid rather than the protected starch.

Key Words: Adipose Tissue • Beef Cattle • Carcass Quality • Protected Fat • Starch

	Introduction

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According to the National Beef Quality Audit-1995 (NBQA-1995), insufficient marbling, along with excessive external, seam, and beef-trim fat were among the top 10 "quality" concerns in cattle (Smith, 1995). The Consensus Panel of the NBQA-1995 recommended the target quality grade mix to be 7% Prime, 21% Upper Choice, 34% Low Choice, and 38% Select (Savell, 1995). These targets reflect the demand for beef quality in both the U.S. and international markets. However, the chief complaint among international consumers regarding U.S. beef was excessive external fat (Morgan and Smith, 1995).

Previous research (Smith and Crouse, 1984; Miller et al., 1991) indicated that i.m. adipose tissue uses a higher proportion of glucose for fatty acid biosynthesis, whereas s.c. adipose tissue primarily utilizes acetate (Smith and Crouse, 1984). Between 61 to 76% of cracked or dry-rolled corn is digested in the rumen where it is broken down into volatile fatty acids (Theurer, 1986; Huntington, 1997). Postruminal starch digestion occurs mainly in the small intestine (Owens et al., 1986) and results in free glucose that is available for absorption by all adipose tissue depots. Feeding protected starch reduces ruminal digestion and increases glucose availability to the small intestine. This study proposed to determine the effectiveness of feeding casein-formaldehyde-protected starch in increasing marbling scores in beef cattle without increasing fat accumulation in other depots. The protected starch product to be fed contained protected Canola Lipid; therefore, the study also established the effect of protected lipid on adipose tissue growth and fatty acid composition. Taste panel evaluation of meat from animals fed casein–formaldehyde-protected supplements has not been reported, so an extensive taste panel evaluation of beef from these cattle was conducted.

	Materials and Methods

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Animals, Diets, and Feeding
This research was approved by the Texas A&M University IAACUC, AUP #2002-97. Eighteen Brangus steers of similar live weight were assigned randomly to one of three dietary treatment groups: 1) cracked corn (Corn); 2) casein-formaldehyde-protected lipid (Canola Lipid); or 3) casein-formaldehyde-protected starch (Marble Plus). Fatty acid analysis and composition of diets are shown in Table 1. All diets were balanced for ME, CP, and DM (NRC, 1996). The Canola Lipid and Marble Plus diets were designed to provide equal amounts of protected lipid (3.3%), and the Marble Plus diet also contained 3.7% protected starch (50% of the total starch provided by Marble Plus; manufacturer’s specifications). The protected lipid in the Canola Lipid and Marble Plus diets contained both canola and soybean lipids. Cattle were fed individually using electronic gate feeders, and ADFI (on as-fed basis) was determined. The steers grazed on natural pasture until assignment to treatment at 15 mo of age, and then the diets were fed for 126 d. Cattle were weighed at 28-d intervals at approximately 0800, before feeding.

Warner-Bratzler Shear Force Determination
Steaks designated for shear force evaluation were broiled on a Faberware Open-Hearth Broiler (Mt. Prospect, IL) according to AMSA (1995). Steaks were turned once at 40°C and brought to an internal temperature of 71°C. Internal temperature was measured using iron-constantan thermocouples connected to a multichannel strip chart recorder (Omega, Samford, CT). Once internal temperature was reached, steaks were allowed to cool to room temperature. Six 1.27-cm-diameter cores were taken from each steak parallel to the muscle fibers. Each core was sheared once perpendicular to the grain of the muscle fiber using a Warner-Bratzler shearing machine (GR Manufacturing Co., Manhattan, KS).

Sensory Evaluation
Sensory evaluation was conducted at the TexasA&M University Meat Products Evaluation Laboratory. Two steaks were cooked according to the methods described above. The steaks were trimmed free of fat and connective tissue and cut into 1-cm³ samples. A seven-person, trained (Cross et al., 1978) sensory panel was used to evaluate the second steak for juiciness, muscle fiber tenderness, overall tenderness, connective tissue amount, and overall flavor using an eight-point scale. The same panel evaluated the third steak for aromatics (cooked beef/brothy, cooked beef fat, serum/bloody, cowy/grainy, cardboard, painty, fishy, and liver/organy), mouthfeels (metallic and astringent), and basic tastes (sour, bitter, salty, and sweet) using the Spectrum universal intensity scale (Meilgaard et al., 1991).

Lipogenesis
Two-hour in vitro incubations were performed on fresh pieces of s.c. and i.m. adipose tissue dissected from the 5th to 8th thoracic rib portion removed from the carcass. Lipogenesis from glucose was measured in duplicate by placing the adipose tissue explants (80 to 120 mg) into 20-mL scintillation vials with 3 mL of incubation media containing KHB buffer (pH 7.4), 5 mM sodium acetate, 5 mM glucose, 10 mM HEPES, ± 0.1 mU/mL of insulin (Sigma Chemical Co., St. Louis, MO), and 0.5 µCi [U-¹⁴C]glucose (American Radiolabeled Chemicals, St. Louis, MO). The vials were gassed for 30 s with 95% O₂:5% CO₂, capped, and incubated in a shaking water bath at 37°C for 2 h. Reactions were terminated by the addition of 3 mL of 5% trichloroacetic acid. The samples were filtered and rinsed with KHB buffer and 0.154 M NaCl to remove unincorporated radioisotope and then were transferred to vials that contained 15 mL of CHCl₃:CH₃OH (2:1, vol/vol). Neutral lipids were extracted according to Folch et al. (1957), as modified by Smith (1983). Fatty acids synthesized from glucose were separated into the glyceride-fatty acids and glyceride-glycerol fractions by saponification (Hood et al., 1972).

Cellularity
Adipose tissue samples were sliced into 1-mm-thick sections and fixed with osmium tetroxide using the procedure described by Etherton et al. (1977), as modified by Prior (1983). The fixed cells were filtered through 250-, 62-, and 20-µm nylon mesh screens with 0.01% Triton X-100 in 0.154 M NaCl to prevent cell clumping. Cell fractions collected from the 62- and 20-µm mesh screens were used to determine mean and peak diameter, mean and peak volume, and adipocytes per gram of tissue with a Coulter Counter, model ZM, and Coulter Channelyzer 256 (Beckman Coulter, Miami, FL).

Peak diameter indicates the diameter of cell occurring most frequently, and peak volume indicates the volume of cell that contributes the most to total adipocyte volume. Biphasic adipocyte diameter distributions have two peak diameters. We typically observe one peak diameter at 20 to 30 µm and a second, larger peak diameter at approximately 100 µm in i.m. adipose tissue and 150 µm in s.c. adipose tissue.

Fatty Acid Composition
Total lipids were extracted from 100 mg of s.c. and i.m. fat samples using 20 mL of chloroform:methanol (2:1, vol/vol) after the method of Folch et al. (1957). The lipid extracts were saponified by the addition of 0.5 N KOH in methanol and methylated with boron trifluoride-methanol following a modification of the procedure of Morrison and Smith (1964). Fatty acid methyl esters (FAME) were analyzed using a flame ionization gas chromatograph (Chrompack, model 437A, Packard, Raritan, NJ) equipped with a 30 m x 0.53 mm capillary column (J&W DB-WAX #125-7032, Folsom, CA). The column ran isothermally at 185°C. The injector port and detector were maintained at 250°C. The FAME in 1 µL of hexane was injected onto the column using a microliter syringe. Flow rates were 13 mL/min for the carrier gas (helium), 20 mL/min for the hydrogen, and 250 mL/min for the breathing air. Chromatograms were recorded with a computing integrator (Spectraphysics model SP 4290, San Jose, CA). Identification of sample fatty acids was made by comparing the relative retention times of FAME peaks from samples with those of standards (reference standard GLC-68B, Nu-Chek Prep, Elysian, MN).

Enzyme Activity
The supernate from previously frozen tissue samples was rapidly thawed to 37°C immediately before measurement of enzyme activity. All enzyme assays were determined using spectrophotometric absorbance (Cary-300, Varian Instruments, Walnut Creek, CA) of solutions at 340 nm.

Glucose-6-phosphate and 6-phosphogluconate dehydrogenase activities were determined by the procedure of Bernt and Bergmeyer (1974). Each cuvette contained 1.2 mL of 0.4 M triethanolamine (pH 7.4), 0.1 mL of NADP⁺ (8 mg/mL), 1.55 mL of distilled deionized water, and 0.05 mL of homogenate. The baseline absorbance was determined and the reaction started with 0.1 mL of 6-phosphogluconate (50 mg/mL). After the absorbance had stabilized (3 to 5 cycles), 0.1 mL of glucose-6-phosphate (40 mg/mL) was added, and the absorbance measured for three to five cycles.

The activity of NADP⁺-malic dehydrogenase was assayed as described by Ochoa (1955). Cuvettes contained 1.65 mL of distilled deionized water, 1.2 mL of buffer (0.1 M Tris HCl, 0.1 M Tris base, and 4 mM MgCl₂; pH 7.3), 0.1 mL of NADP⁺ (8 mg/mL), and 0.1 mL of homogenate. The baseline absorbance was determined and the reaction initiated with 0.05 mL of malate (30 mg/mL).

Fatty acid synthetase was assayed by the procedures of Martin et al. (1961). Cuvettes contained 1.5 mL of distilled deionized water, 1.3 mL of buffer (0.1 M KH₂PO₄ and 0.1 mM EDTA; pH 6.8), 0.1 mL of NADPH (6.5 mg/mL), and 0.1 mL of homogenate. The baseline absorbance was established before initiating the reaction with 0.1 mL of a solution containing acetyl-CoA (4.41 mg/mL) and malonyl-CoA (2.61 mg/mL) in buffer.

Statistical Analysis
Data for production, carcass, taste panel, cellularity, and lipogenic enzyme activities were analyzed as a completely randomized design using the SuperAnova program (Abacus Concepts, Inc., Berkeley, CA), using hot carcass weight as a covariate. Data for in vitro incubations with radiolabeled glucose were analyzed as a 3 (Corn, Canola Lipid, and Marble Plus) x 2 (with or without insulin) factorial with hot carcass weight as a covariate. The model tested the main effects of treatment and insulin and their interaction. When treatment or interaction effects were significant (P < 0.05), means were separated by the Fisher’s Protected LSD method contained in the SuperAnova program.

	Results

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Production and Carcass Traits
We designed the experiment to avoid differences in ADG, ADFI, and thus feed:gain. However, there were numerical differences among treatments for these production traits, although none of the differences were statistically significant (P 0.23; Table 2). Live animal weights, measured periodically from d 0 to 126, were not different among treatments (P 0.34). Hot carcass weight, adjusted fat thickness, and longissimus muscle area were not different among treatments (P = 0.29 to 0.51; Table 3). Percentage of kidney, pelvic, and heart (KPH) fat was 28 to 52% higher (P < 0.01) and yield grades tended to be higher (P = 0.08) for carcasses from Canola Lipid-fed cattle than in carcasses from Corn- or Marble Plus-fed cattle. There were no differences in marbling score or quality grade among treatments (P 0.26).

View larger version (23K): [in this window] [in a new window]   Figure 1. Relative volume proportions of s.c. adipose tissue of steers fed cracked corn (Corn), Canola Lipid (CL), or Marble Plus (MP). Values are means (n = 6 per treatment group). Average SEM for each treatment group are attached to the symbols. *P < 0.03, CL vs. Corn.

View larger version (24K): [in this window] [in a new window]   Figure 2. Relative volume proportions of i.m. adipose tissue of steers fed cracked corn (Corn), Canola Lipid (CL), or Marble Plus (MP). Values are means (n = 6 per treatment group). Average SEM for each treatment group are attached to the symbols. *P < 0.05, CL vs. Corn.

Total lipid, glyceride-glycerol, and fatty acid synthesis from glucose were higher (P < 0.01) in s.c. adipose tissue of steers fed Marble Plus than in steers fed Corn or Canola Lipid (Table 9). There were no differences among treatments for glucose incorporation into lipid fractions in i.m. adipose tissue (P 0.33), but insulin increased glyceride–glycerol synthesis (P = 0.04) and decreased fatty acid synthesis (P = 0.01) in i.m. adipose tissue.

	Discussion

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Protein–formaldehyde-protected fat has been added to high-concentrate diets without negative effects on ADG or feed efficiency (McCartor et al., 1979; Haaland et al., 1981) and may provide more consistent results than unprotected fat (Ashes et al., 1993). The Canola Lipid and Marble Plus diets contained 3.2% more ether-extractable lipid than the Corn diet and almost 80% of the total lipid in the Canola Lipid and Marble Plus diets was protected from ruminal digestion (information provided by the manufacturer). The lack of statistical significance across treatments for ADFI and ADG (P = 0.40 and 0.43, respectively) suggests that we would not have seen an effect of protected lipid or starch on feed intake or growth rate even if we had used a greater number of animals. Consistent with our results, Krehbiel et al. (1995) reported that differences in the rumen digestibility of starch in fat-supplemented diets containing dry-rolled or high-moisture corn had little impact on ADG or feed efficiency in yearling steers.

We hypothesized that increasing starch availability to the small intestine would increase glucose uptake, resulting in increased lipid synthesis in i.m. adipocytes and higher marbling scores. Free glucose is used to support triacylglycerol synthesis for bovine adipose tissue depots by supplying glyceride-glycerol (Smith and Prior, 1982; Smith, 1983; Owens et al., 1986). However, i.m. adipose tissue uses a higher proportion of glucose for de novo fatty acid biosynthesis than s.c. adipose tissue (Smith and Crouse, 1984; Miller et al., 1991). We therefore attempted to provide more starch to the small intestine to increase marbling scores.

The addition of protected starch did not affect yield grade or improve marbling score or quality grade in diets containing cracked corn. This may have been the result of the lower rumen digestibility of cracked corn, which varies from 61 to 76% (Owens et al., 1986; Theurer, 1986; Huntington, 1997). Thus, the additional starch provided to the small intestine by the protected starch in the Marble Plus diet may not have been sufficiently greater than that provided by cracked corn to observe any effect on marbling development.

Feeding protected lipid has been shown to increase adjusted fat thickness and numerical yield grade and, in some cases, increase marbling score and quality grade to varying degrees (Garrett et al., 1976). The most consistent result from feeding protected lipid is an increase in percentage KPH fat. Although equal amounts of protected lipid were included in the Canola Lipid and Marble Plus diets to account for any effect on yield or quality grade factors by the protected lipid, yield grade and quality grade were numerically higher (P = 0.08 and 0.26, respectively) in the carcasses of the Canola Lipid-fed steers. The protected lipid in the Canola Lipid diet significantly increased percentage KPH above that found in carcasses from Marble Plus-fed steers. The lower percentage KPH fat (and numerically lower adjusted fat thickness) in carcasses of Marble Plus-fed steers may have been the result of lower acetate production in the rumen due to our experimental design. The Marble Plus-fed cattle ate less cracked corn than the Canola Lipid-fed cattle (43.5 vs. 54.0%), which would have reduced ruminal acetate production. However, the Corn-fed cattle ate the greatest amount of cracked corn (71.7%) but had less percentage KPH fat than the Canola Lipid-fed steers. Therefore, acetate production from the cracked corn component of the diets does not seem to explain differences in percentage KPH fat between the Marble Plus and Canola Lipid steers.

Fatty Acid Composition
We have demonstrated that unprotected supplemental fats have only minor effects on fatty acid composition in ruminants due to the biohydrogenation of unsaturated fatty acids by microorganisms in the rumen (e.g., St. John et al., 1987; Chang et al., 1992; Ekeren et al., 1992). Casein-formaldehyde encapsulation of lipids greatly reduces ruminal biohydrogenation of fatty acids, allowing absorption of unsaturated fatty acids in the small intestine and subsequent incorporation into the adipose tissues of ruminants (Oltjen and Dinius, 1975). Feeding protected lipid has been demonstrated to be an effective method to alter the fatty acid composition of ruminant adipose and muscle tissues (Cook et al., 1972; Hood and Thornton, 1976; Ashes et al., 1993). The amount of change in fatty acid composition can vary depending on the fatty acid composition of the protected lipid and the length of feeding. Garrett et al. (1976) found decreased concentrations of 16:0, 16:1n-7, and 18:1n-9 and increased concentrations of 18:2n-6 in the muscle and adipose tissue of cattle fed protein-encapsulated vegetable oil (70% whole sunflower seeds, 30% whole soybeans) for 115 d. Ashes et al. (1993) observed decreased concentrations of 14:0 and 16:0, and increased concentrations of 18:1n-9, 18:2n-6, and 18:3n-3 in cattle fed protected canola oil for 133 d. In contrast, feeding protected cottonseed oil (26% 16:0 and 60% 18:2n-6) for 150 d did not affect the concentration of 16:0, increased concentration of 18:0, decreased concentration of 18:1n-9, and increased the concentration of 18:2n-6 five- to eightfold in adipose tissue of cattle (Gulati et al., 1996). Gulati et al. (1996) attributed the lack of change in concentration of 16:0 to its higher proportion found in cottonseed oil. Both Gulati et al. (1996) and Yang et al. (1999) concluded that the changes in concentration of 18:0 and 18:1n-9 in the adipose tissues of cattle fed protected cottonseed oil were because of direct inhibition of stearoyl-CoA desaturase (SCD) by cyclopropenoid fatty acids and/or depressed SCD gene expression by 18:2n-6.

The protected lipid in our diets contained a mixture of canola and soybean oils; therefore, our results were similar to those reported by Ashes et al. (1993). We saw a twofold increase in 18:2n-6 concentration in both s.c. and i.m. adipose tissues of Canola Lipid- and Marble Plus-fed cattle relative to Corn-fed cattle. The lower concentrations of 18:2n-6 the s.c. and i.m. adipose tissues of Corn-fed cattle, even though the Corn diet contained 20% more 18:2n-6 than the Canola Lipid or Marble Plus diets, provided evidence for the effectiveness of the casein-formaldehyde protection in preventing ruminal biohydrogenation.

Concentrations of 14:0, 15:0, 16:0, and 17:0 were lower or tended to be lower in s.c. and i.m. adipose tissues from cattle fed protected lipids than in Corn-fed cattle. The lesser concentrations of 15:0 and 17:0 and their corresponding monounsaturates in s.c. adipose tissue is direct evidence of reduced ruminal propionic acid synthesis in those cattle fed the protected lipid supplements. There was no 15:0 or 17:0 in the corn diet; ruminal metabolism of the cracked corn would have provided sufficiently more propionic acid to cause greater concentrations of odd-chained fatty acids in adipose tissues of Corn-fed cattle than in adipose tissue of Marble Plus- or Canola Lipid-fed cattle.

The concentration of 18:1n-9 in s.c. and i.m. adipose tissues was similar to the concentration of 18:1n-9 in the Canola Lipid and Marble Plus diets, but exceeded that in the Corn diet by nearly 100%. The concentration of 18:1n-9 was similar to that typically observed in bovine adipose tissue (St. John et al., 1987; Smith et al., 1998). Oleic acid is the primary end product of fatty acid biosynthesis, and the activities of the fatty acid synthetase, fatty acid elongase, and SCD enzyme systems ensure that it does not fall below some minimal concentration in animal tissues. Only substantial inhibition of the SCD enzyme activity can measurably lower the adipose concentration of 18:1n-9 (Smith et al., 1998; Yang et al., 1999). The lack of effect of the protected lipid diets on the MUFA:SFA ratio indicates that these diets had little effect on SCD enzyme activity.

Taste Panel Evaluation
A primary concern we had in feeding the protected lipid was the potential development of off-flavors associated with either lipid oxidation resulting from increased levels of 18:2n-6 and 18:3n-3 in the meat, or from some other component in the protected lipid supplement. Oleic acid has been associated with increased flavor acceptance in beef, but 18:2n-6 has been associated with decreased juiciness and 18:3n-3 with decreased flavor acceptance (Dryden and Marchello, 1970; Westerling and Hedrick, 1979; Melton et al., 1982). To our knowledge, this is the first report of the taste panel evaluation of meat from animals fed casein-formaldehyde-protected feedstuffs. We observed a greater connective tissue amount in beef from Marble Plus cattle than in beef from Canola Lipid cattle, but this difference was small (6.9 vs. 6.4, where 1 = none and 8 = abundant connective tissue amount).

Overall flavor intensity was not different among steaks from cattle fed the Corn, Canola Lipid, or Marble Plus diets, although there was a slight tendency (P = 0.12) for the painty aromatic to be elevated in meat from the Marble Plus animals. Because the painty aromatic was not observed in any samples from the Canola Lipid cattle but was detected in samples from the Corn-fed cattle, it is not likely that it was caused by the protected lipid/starch supplement.

Adiposity
We typically measure adipose tissue cellularity because it is more sensitive to dietary manipulations than marbling score or adjusted fat thickness. Subcutaneous adipose tissue contained fewer cells per gram of tissue and larger mean diameters and volumes than i.m. adipose tissue, which is consistent with previous reports (Hood and Allen, 1973; Smith and Crouse, 1984; May et al., 1995). Consistent with our goals, Marble Plus significantly increased peak diameters of i.m. adipocytes. Marble Plus had no effect in s.c. adipose tissue, and for both adipose tissue depots, Canola Lipid was more effective than Marble Plus in increasing adipocyte diameter. Thus, contrary to our expectations, the protected lipid, rather than the protected starch, was more effective in increasing adiposity of i.m. and s.c. adipose tissues.

We cannot explain why the nearly 60% greater peak volume in i.m. adipose tissue of Canola Lipid-fed steers relative to Corn-fed did not result in greater marbling scores for the Canola Lipid steers. The Canola Lipid diet more than doubled the PUFA:SFA ratio in i.m. adipose tissue compared with the Corn diet. This would have reduced the melting point of i.m. adipose tissue lipids of steers fed Canola Lipid, and may have reduced the visual appearance of marbling during grading.

Lipogenesis
The lipogenic enzyme activities were similar to those reported in previous studies (Smith and Crouse, 1984; Miller et al., 1991; May et al., 1995). The tendency (P = 0.08) for greater fatty acid synthetase activity in s.c. adipose tissue of Marble Plus-fed steers (compared with Corn- or Canola Lipid-fed steers) was consistent with the significantly greater rate of de novo fatty acid biosynthesis observed in s.c. adipose tissue for this treatment group.

The larger mean volumes of s.c. and i.m. adipocytes in Canola Lipid-fed steers apparently were not the result of elevated rates of de novo fatty acid biosynthesis because lipogenesis was unaffected by Canola Lipid (relative to the Corn diet). Both i.m. and s.c. adipose tissues actively esterify exogenous fatty acids (Lin et al., 1992); the data suggest that esterification of dietary lipids was responsible for the larger adipocytes observed in adipose tissues of steers fed Canola Lipid.

In agreement with previous reports (Prior and Jacobson, 1979; Smith et al., 1983; Miller et al., 1989), glucose incorporation in short-term incubations of s.c. adipose tissues was not affected by insulin. However, insulin stimulated glucose incorporation into glyceride-glycerol, and paradoxically inhibited glucose incorporation into fatty acids, although both effects were small. Chilliard and Faulconnier (1995) reported insulin stimulation of lipogenesis from glucose in 24-h incubations of perirenal adipose tissues from Holstein cows, and we previously observed that insulin stimulated lipogenesis in 48-h explant cultures of s.c. adipose tissues of Angus steers, but not Santa Gertrudis steers, and proposed that the effects of insulin were breed-dependent (Miller et al., 1991). The stimulation by insulin we observed suggests that i.m. adipose tissue of these cattle was more sensitive to insulin than s.c. adipose tissue.

Summary
It is apparent that any effects of the protected lipids on adiposity were due to the protected lipid rather than the protected starch. Both supplements provided the same amount of protected lipid to the diet and changed fatty acid composition of adipose tissues similarly, but clearly there were different responses in adiposity to Marble Plus and Canola Lipid. The cellularity and carcass data suggest greater absorption of the lipid from Canola Lipid than from Marble Plus, but this was not supported by our results for ADG or ADFI. At this point, we do not know why Marble Plus did not cause the same metabolic and carcass responses observed with Canola Lipid.

	Implications

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The inclusion of Marble Plus, a source of casein-formaldehyde-protected starch, in the diet necessitated a concomitant reduction in dietary corn to equalize metabolizable energy intake. Therefore, there may be no effective means of increasing the concentration of starch in the small intestine in countries such as the United States that use corn as a primary feed ingredient. Because of this, it will be difficult to test the hypothesis that increasing blood glucose by dietary means will stimulate intramuscular adiposity.

	Footnotes

 
¹ The authors thank Rumentek Industries Pty Ltd., Sydney, Australia for provision of the Marble Plus and Canola Lipid. Published as a technical article, Texas Agric. Exp. Stn.

² Correspondence: 2471 TAMU (phone: 979-845-3939; fax: 979-458-2702; E-mail: sbsmith{at}tamu.edu).

Received for publication November 8, 2002. Accepted for publication June 12, 2003.

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