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J. Anim Sci. 2007. 85:1731-1740. doi:10.2527/jas.2006-732
© 2007 American Society of Animal Science
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ANIMAL PRODUCTS

Corn oil supplementation to steers grazing endophyte-free tall fescue. II. Effects on longissimus muscle and subcutaneous adipose fatty acid composition and stearoyl-CoA desaturase activity and expression

E. Pavan* and S. K. Duckett{dagger},1

* Department of Animal and Dairy Science, University of Georgia, Athens 30602 and and {dagger} Department of Animal and Veterinary Science, Clemson University, Clemson, SC 29634-0311


   Abstract
Top
 Abstract
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 LITERATURE CITED
 
Eighteen steers were used to evaluate the effect of supplemental corn oil level to steers grazing endophyte-free tall fescue on fatty acid composition of LM, stearoyl CoA desaturase (SCD) activity and expression as well as cellularity in s.c. adipose. Corn oil was supplemented (g/kg of BW) at 0 (none), 0.75 (medium), and 1.5 (high). Cottonseed hulls were used as a carrier for the corn oil and were supplemented according to pasture availability (0.7 to 1% of BW). Steers were finished on a rotationally grazed, tall fescue pasture for 116 d. Fatty acid composition of LM, s.c. adipose, and diet was determined by GLC. Total linoleic acid intake increased linearly (P < 0.01) with corn oil supplementation (90.7, 265.1, and 406.7 g in none, medium, and high, respectively). Oil supplementation linearly reduced (P < 0.05) myristic, palmitic, and linolenic acid percentage in LM and s.c. adipose. Vaccenic acid (C18:1 t11; VA) percentage was 46 and 32% greater (linear, P = 0.02; quadratic, P = 0.01) for medium and high, respectively, than none, regardless of tissue. Effect of oil supplementation on CLA cis-9, trans-11 was affected by type of adipose tissue (P < 0.01). In the LM, CLA cis-9, trans-11 isomer was 25% greater for medium than for none and intermediate for high, whereas CLA cis-9, trans-11 CLA isomer was 48 and 33% greater in s.c. adipose tissue for medium and high than for none, respectively. Corn oil linearly increased (P ≤ 0.01) trans-10 octadecenoic acid and CLA trans-10, cis-12; however, values were low (<0.35 and <0.035% of total fatty acids, respectively). Oil supplementation did not change (P > 0.05) the percentage of total SFA, MUFA, or PUFA but linearly increased (P = 0.03) n-6:n-3 ratio from 2.4 to 2.9 in none and high, respectively. Among tissues, total SFA and MUFA were greater in s.c. adipose than LM, whereas total PUFA, n-6, and n-3 fatty acids and the n-6:n-3 ratio were lower. Trans-10 octadecenoic acid, VA, and CLA trans-10, cis-12 were greater (P < 0.01) in s.c. adipose than in LM. Oil supplementation did not alter (P > 0.05) stearoyl CoA desaturase activity or mRNA expression. Corn oil supplementation to grazing steers reduced the percentages of highly atherogenic fatty acids (myristic and palmitic acids) and increased the percentages of antiatherogenic and anticarcinogenic fatty acids (VA and cis-9, trans-11 CLA).

Key Words: beef • conjugated linoleic acid • fatty acid • forage


   INTRODUCTION
Top
 Abstract
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 LITERATURE CITED
 
Increasing attention has been placed on enhancing the content of CLA, the cis-9, trans-11 isomer, in beef as a result of its anticarcinogenic and antiatherogenic effects (Scollan et al., 2006Go). Milk and beef represent the major sources of CLA in the human diet (Ritzenthaler et al., 2001Go). Research in lactating dairy cows (Griinari et al., 2000Go; Kay et al., 2004Go; Mosley et al., 2006Go) has shown that over 85% of CLA, cis-9, trans-11 isomer, results from desaturation of trans-11 vaccenic acid (VA) via the stearoyl-CoA desaturase (SCD) enzyme present in mammalian adipose tissues (Ntambi, 1995Go). In beef, VA is present in adipose tissues at levels 1.4 times higher than the CLA cis-9, trans-11 isomer (Gillis et al., 2004Go) due to greater duodenal outflow of VA during ruminal biohydrogenation (Duckett et al., 2002Go; Sackmann et al., 2003Go). Because the majority of CLA in beef fat originates from VA, enhancing the proportion of VA and CLA in beef products is potentially of importance for human health.

Research directed at enhancing the CLA proportion of beef fat has utilized supplementation of plant oils or oilseeds to increase linoleic acid content of high-concentrate finishing cattle diets; however, only minor increases in CLA have been reported (Beaulieu et al., 2002Go; Madron et al., 2002Go; Gillis et al., 2004Go). In grazing cattle, higher levels of CLA cis-9, trans-11 and VA proportions in milk or beef fats have been reported (Dhiman et al., 1999Go; Scollan et al., 2001Go; Realini et al., 2004Go). Sackmann et al. (2003)Go reported a linear increase in duodenal outflow of VA and a linear decrease in trans-10 octadecenoic acid when dietary forage level increased from 12 to 36% in finishing cattle diets. The proportion of CLA in the i.m. fat of heifers fed a silage-based diet increased linearly when the fat source in the concentrate was gradually changed from lard to a linoleic-rich sunflower oil (Noci et al., 2005bGo). However, there is limited information on the effect of increasing fat supplementation, in particular plant oils, to grazing beef cattle and its effect on VA, CLA, and SCD.

Thus, our objective was to determine the effect of supplemental corn oil level to steers grazing endophyte-free tall fescue on tissue fatty acid (FA) composition, adipose cellularity, and SCD activity and expression.


   MATERIALS AND METHODS
Top
 Abstract
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 LITERATURE CITED
 
Animals, Management, Pasture, and Treatments
The experiment was approved by the University of Georgia Animal Care and Use Committee.

Eighteen Angus steers (438 ± 4 kg) were finished on a rotationally grazed, 21-ha, endophyte-free tall fescue pasture (Festuca arundinacea) for 116 d. Three dietary treatments were defined by the level of corn oil supplementation (g/kg of BW): 0 (none), 0.75 (medium), or 1.5 (high). Pelleted cottonseed hulls were used as a carrier for the oil supplement and were fed at equal amounts to all steers regardless of treatment throughout the experiment. Levels of cottonseed hulls fed to steers were adjusted across treatments during the experiment according to pasture availability (0.7 to 1% of BW). Forage and supplement samples were collected during the supplementation period and processed as described by Pavan et al. (2007)Go. Additional information regarding in vivo digestibility, performance, and carcass quality are available in Pavan et al. (2007)Go.

Sample Collection
Animals were transported (45 km) to the University of Georgia Meat Science and Technology Center in Athens and slaughtered after an overnight feed withdrawal. Within 30 min of exsanguination, a sample of s.c. adipose from the tail-head region was removed from each carcass and rinsed with sterile saline solution. Approximately, 10 g of s.c. adipose was immediately frozen in liquid nitrogen and stored at –80°C for subsequent RNA extraction. Samples of s.c. adipose (10 g) were obtained for determination of SCD enzyme activity according to Smith et al. (2002)Go. The remaining s.c. adipose tissue was stored frozen at –80°C for adipose cellularity. During evisceration, samples of digesta from the abomasum (approximately 250 mL) were obtained to estimate the proportion of CLA coming from endogenous desaturation of VA (Daniel et al., 2004Go). The conversion of VA to CLA was calculated based on the ratios of CLA to VA in abomasum compared with s.c. adipose or LM. Abomasal samples were immediately stored at –20°C, freeze-dried, and ground through a Wiley mill (Wiley mill model 4; Thomas Scientific, Swedesboro, NJ) equipped with a 1-mm screen for subsequent FA analyses. Pre- and postgrazing pasture samples from each paddock were collected and lyophilized. Lyophilized forage samples were ground through the Wiley mill equipped with a 1-mm screen and stored at –20°C for FA analyses.

At 24 h postmortem, samples of s.c. adipose tissue and a 2.54-cm-thick LM steak were removed from the left half of each carcass at the 13th rib region. All external fat and connective tissue were removed from the LM. The s.c. adipose and LM samples from each carcass were stored at –20°C; before analyses, the samples were pulverized in liquid nitrogen for subsequent proximate and FA analyses.

Proximate Analyses
Duplicate samples of LM were analyzed for nitrogen content by the combustion method using a Leco FP-2000 N analyzer (Leco Corp., St. Joseph, MI) and multiplied by 6.25 to determine CP content. Moisture content was determined by weight loss after drying at 100°C for 24 h. Total ash content was determined by ashing at 600°C for 8 h (AOAC, 1990Go). Total lipids were extracted in duplicate from LM and s.c. adipose samples according to the procedures of Folch et al. (1957)Go.

Fatty Acid Composition
Subcutaneous and LM lipid extracts containing approximately 4 mg of total lipids were transmethylated according to the method of Park and Goins (1994)Go. Fatty acid methyl esters (FAME) were analyzed using a HP6850 (Hewlett-Packard, San Fernando, CA) gas chromatograph equipped with a HP7673A (Hewlett-Packard) automatic sampler. Separations were accomplished using a 100-m SP2560 (Supelco, Bellefonte, PA) capillary column (0.25-mm i.d. and 0.20-µm film thickness). Column oven temperature increased from 150 to 160°C at 1°C per min, from 160 to 167°C at 0.2°C per min, from 167 to 225°C at 1.5°C per min, and then held at 225°C for 16 min. The injector and detector were maintained at 250°C. Sample injection volume was 1 µL. Hydrogen was the carrier gas at a flow rate of 1 mL per min. Individual FA were identified by comparison of retention times with standards (Sigma, St. Louis, MO; Supelco; Matreya, Pleasant Gap, PA). The FA were quantified by incorporating an internal standard, methyl heptacosanoic (C27:0) acid, into each sample during methylation and expressed as a percentage of total FA. Forage, supplements, and abomasal FAME were obtained by direct transmethylation of lyophilized samples according to Park and Goins (1994)Go and analyzed as s.c. adipose and LM. Cholesterol content of LM was determined according to Du and Ahn (2002)Go and quantified by incorporating an internal standard, stigmasterol, into each sample.

Stearoyl-CoA Desaturase Activity and Gene Expression
Samples of s.c. adipose tissue (5 g) collected at slaughter were immediately processed in duplicate for SCD enzyme activity according to Smith et al. (2002)Go. Enzyme activity was determined according to St. John et al. (1991)Go with the following modifications. Unlabeled stearic acid was used in the assay; extracts were trans-methylated (Park and Goins, 1994Go) to quantify the amount of stearic and oleic acids by GLC as described above; and blanks were also included in the assay to determine purity of stearic acid. The activity values were calculated as a ratio among oleic acid and the remaining stearic acid.

Total RNA was extracted from the s.c. adipose samples according the TRIzol procedure (Invitrogen, Carlsbad, CA), which included an initial centrifugation step to remove lipids from the extract according to the manufacturer’s directions. Total RNA was quantified using Quant-iT RNA assay kit (Invitrogen). Five micrograms of RNA was separated in an agarose gel (Pellé and Murphy, 1993Go) and transferred to nylon membranes by downward capillary transfer (Turboblotter, Schleicher & Schuel Inc., Keene, NH). Northern blots were performed by hybridization with 32P-labeled SCD probes (GenBank accession No. AF188710) and exposed to film for 38 h. Gene expression was quantified by densitometry of the SCD bands and was normalized to the 18S ribosomal RNA bands.

Adipose Cellularity
Adipose cellularity was determined in duplicate by osmium tetroxide fixation according to Mersmann and MacNeil (1986)Go. Adipocyte number and size distribution in the range of 20 to 240 µm were measured electronically using a Coulter Counter (Coulter Electronics, Hialeah, FL). Only counts in the 30 µm and above were included in calculations of cell number, diameter, and volume (Lee et al., 1994Go). Peak cell diameter specifies the diameter of the cell occurring most frequently, and peak cell volume specifies the volume of the cell that contributes the most to total adipocyte volume.

Statistical Analysis
Data were analyzed by ANOVA as a completely randomized design with 3 corn oil levels and with individual animal serving as the experimental unit (n = 6), with the exception of the FA data. Intake data from 1 steer in medium was removed due to an apparent malfunction of the controlled release marker used to estimate forage DMI (Pavan et al., 2007Go). Tissue FA profile data were analyzed by ANOVA according to a split-plot design with the 3 corn oil levels as the main plots and the 2 adipose tissues as the subplot treatments. The effect of corn oil level was tested using the variance between animals within corn oil level (n = 6) as the error term. Effect of adipose tissue and the corn oil level x adipose tissue interaction was tested against the residual error. The MIXED procedure (SAS Inst. Inc., Cary, NC) was used for all analyses. To analyze differences among corn oil levels for all variables, the sum of squares for the corn oil level effect was further partitioned by preplanned linear and quadratic orthogonal contrasts. If the overall F-test of corn oil level x adipose tissue was significant (P ≤ 0.05), a t-test was performed to discern differences among corn oil level within adipose tissue and among adipose tissue within corn oil level. For total CLA and CLA cis-9, trans-11 content analyses, the model was adjusted to account for heterogeneous variances among dietary treatments by using the "GROUP = treatments" option of the REPEATED statement in PROC MIXED. Least squares means are reported. The REG procedure of SAS was used to compute the regression equation and determine the R2 between tissue VA or CLA cis-9, trans-11 and total dietary linoleic acid intake.


   RESULTS
Top
 Abstract
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 LITERATURE CITED
 
Daily FA intake of total diet, supplement plus forage, is shown in Table 1Go. Total FA intake increased (linear, P < 0.01) as expected with corn oil supplementation. Corn oil supplementation also linearly increased (P < 0.01) intake of palmitic (C16:0), stearic (C18:0), oleic (C18:1), and linoleic (C18:2) acids, whereas linolenic (C18:3) acid intake was linearly decreased (P = 0.03). Myristic acid (C14:0) was unaffected by oil supplementation. Linoleic acid intake increased by 174 g in medium and 316 g in high with respect to none. Corn oil supplementation to grazing steers did not alter (linear, P = 0.46 to 0.87; quadratic, P = 0.16 to 0.98) LM proximate composition (Table 2Go). The average moisture, CP, ash, and total lipid content across dietary treatments were: 74.1, 23.3, 1.4, and 2.5%, respectively. Cholesterol content of the LM was unchanged by level of corn oil supplemented to the steers (linear, P = 0.75, quadratic, P = 0.20; 58.3 mg/100 g). The lack of oil supplementation effect on LM cholesterol content is in agreement with Rule et al. (1997)Go. Total lipid content of s.c. adipose was also unchanged (linear, P = 0.56; quadratic, P = 0.64) and averaged 66.7%.


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  		Table 1. Daily fatty acid intake of total diet (supplement plus pasture) for steers grazing endophyte-free tall fescue supplemented with increasing levels of corn oil
 

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  		Table 2. Effect of corn oil supplementation to grazing steers on LM and s.c. adipose tissue proximate composition
 
Corn oil supplementation and adipose tissue effects on FA profile are presented in Table 3Go. The 2-way interaction between corn oil supplementation and adipose tissue was nonsignificant for the majority of FA; however, the interaction was significant (P < 0.05) for odd chain FA, total CLA, CLA cis-9 trans-11 isomer, and CLA cis-11 trans-13 isomer (Table 4Go). Increasing corn oil supplementation to grazing cattle resulted in a linear decrease (P < 0.01) in myristic and palmitic acids, 2 atherogenic FA. Myristic acid was reduced by 0.30 ± 0.14 percentage units with each g/kg of BW increment of corn oil supplementation, and palmitic acid by 1.27 ± 0.42. Thus, the high supplementation generated an 18 and 8% reduction in myristic acid and palmitic acid proportions, respectively. Concentrations of other saturated FA [lauric (C12:0) and stearic (C18:0) acids] as well as total SFA were unchanged with corn oil supplementation. Corn oil supplementation did not alter (P > 0.05) myristoleic (C14:1) acid, oleic acid, or total MUFA weight percentages.


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  		Table 3. Effect of corn oil supplementation to grazing steers on fatty acid composition of LM and s.c. adipose tissue
 

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  		Table 4. Effect of corn oil supplementation to grazing steers on odd-chain fatty acids (OCFA) and conjugated linoleic acid (CLA) percentages fatty acid composition of LM and s.c. adipose tissue1
 
Trans-10 octadecenoic acid and CLA isomer trans-10, cis-12 linearly increased (P ≤ 0.01) with oil supplementation. Linear (P = 0.02) and quadratic (P = 0.01) corn oil level effects were significant for VA proportion. Compared with none, the medium supplementation increased VA by 46% and high by 32%. Oil supplementation did not alter (P > 0.05) the proportion of total PUFA or linoleic, eicosapentaenoic (C20:5), docosapentaenoic (C22:5), or docosahexaenoic (C22:6) acids. Linolenic acid proportion linearly decreased (P = 0.004) with corn oil supplementation. The lack of effect of corn oil supplementation on total SFA and total PUFA percentages resulted in similar (P > 0.10) PUFA:SFA ratio across dietary treatments. Likewise, total n-6 and total n-3 FA percentage did not differ (P > 0.05) across dietary treatments. Nonetheless, n-6:n-3 ratio was increased (P = 0.03) by 0.35 ± 0.15 units per g/kg of BW of corn oil supplemented.

Total saturated FA, myristic acid, and arachidic (C20:0) acid proportions were greater (P ≤ 0.05) in s.c. adipose than LM, whereas the proportions of the major SFA in beef fat, palmitic, and stearic acids, as well as that of lauric acid, were similar (P > 0.05) between s.c. adipose and LM. Total MUFA proportion was 15% greater (P < 0.01) in s.c. adipose than in LM. In addition, trans-10 octadecenoic acid and VA, intermediates of linoleic acid ruminal biohydrogenation, were in greater (P < 0.01) proportions in the s.c. adipose than LM. Cis-octadecenoic acids formed during ruminal biohydrogenation were present in greater (P < 0.01) proportions for LM than s.c. adipose.

Longissimus muscle had greater (P < 0.01) total PUFA, n-6, and n-3 FA proportions than s.c. adipose. Linoleic acid, linolenic acid, and longer chain PUFA (eicosapentaenoic acid, docosapentaenoic acid, docosahexaenoic acid) were in greater (P < 0.01) proportions in the LM than in the s.c. adipose. The n-6:n-3 ratio was greater (P < 0.01) in LM than in the s.c. adipose. Total proportion of trans, trans isomers of CLA were also greater (P = 0.04) in the LM than in the s.c. adipose. In contrast, total content of cis, cis isomers and trans-10, cis-12 isomer of CLA were greater (P < 0.01) in the s.c. adipose than in the LM.

For pentadecyclic (C15:0) acid, margaric (C17:0) acid, and total odd chain FA (OCFA), the response to corn oil supplementation varied with tissue (Table 4Go). In the s.c. adipose, pentadecyclic acid percentage decreased (P < 0.05) with increasing oil supplementation level, whereas pentadecyclic acid percentage was lower (P = 0.04) in the LM for high than none with medium being intermediate (P > 0.05). The proportion of pentadecyclic acid in s.c. adipose from high was similar to those in the LM from none and medium supplementation (P > 0.05). In the s.c. adipose, total OCFA and margaric acid percentage decreased (P < 0.01) with increasing oil supplementation level; in the LM, total OCFA, and margaric acid percentages were greater (P < 0.01) in none than medium and high supplementation. Proportion of total OCFA in the s.c. adipose from the medium concentration was similar (P > 0.05) to that in the LM from none, and that in the s.c. adipose from the high supplementation was similar (P > 0.05) to the proportions in the LM from none and medium supplementation.

Interactions (P ≤ 0.05) were observed between corn oil level and adipose tissue for total CLA, cis-9, trans-11 CLA and cis-11, trans-13 CLA proportions. Cis-11, trans-13 CLA was only detected in the s.c. adipose of carcass from steers fed high. Oil supplementation, both medium and high, increased (P < 0.01) total CLA and cis-9, trans-11 CLA percentages compared with none in s.c. adipose; however, only medium oil supplementation level increased (P < 0.01) total CLA and cis-9, trans-11 CLA percentages compared with none in the LM.

The effects of total linoleic acid intake (g/d) on LM and s.c. adipose tissues VA and CLA cis-9, trans-11 proportions are presented in Figure 1Go. Quadratic equations better explained the variation observed in longissimus muscle VA proportion (R2 = 0.53, P = 0.005) and in s.c. adipose VA (R2 = 0.42, P = 0.02) and CLA cis-9, trans-11 (R2 = 0.35, P = 0.05) proportions than the respective linear equations. Neither the linear (R2 = 0.15, P = 0.12) nor quadratic (R2 = 0.18, P = 0.24) regression was significant for the CLA cis-9, trans-11 in the LM and total linoleic acid intake.


Figure 1
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  		Figure 1. Effect of linoleic acid intake (g/d) in grazing steers supplemented with increasing corn oil supplementation on (A, B) vaccenic acid and (C, D) CLA cis-9, trans-11 proportion in the (A, C) LM or (B, D) s.c. adipose tissue. Filled or open circles represent the actual trans-11 vaccenic acid or CLA proportion, whereas the line represents the predicted proportion. Linear and quadratic regressions between the CLA proportion in the LM and total linoleic acid intake (panel C) were not significant (linear, R2 = 0.15, P = 0.12; quadratic, R2 = 0.18, P = 0.24).

 
Stearoyl-CoA desaturase activity in the s.c. adipose from the tail-head region did not differ (P = 0.35) among corn oil supplementation levels (Figure 2Go). Similarly, SCD mRNA level did not differ (P = 0.60) among oil supplementation levels. Corn oil supplementation to grazing steers did not alter (P > 0.10) adipocytes per milligram of s.c. adipose tissue (Table 5Go). Neither mean and peak cell diameters nor cell volumes were affected by corn oil supplementation (P > 0.10). A biphasic adipocyte cell diameter distribution was observed for all dietary treatments (data not shown); on average, a first peak cell diameter was observed at 30 to 40 µm and second larger peak was observed at 112 µm. No differences (P > 0.10) in cell frequency were observed across dietary treatment at the different diameter classes (10-µm increment).


Figure 2
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  		Figure 2. Effect of corn oil supplementation to grazing steers on s.c. adipose stearoyl-CoA desaturase (SCD) activity (A; SEM = 0.44, n = 6) and mRNA level (B; SEM = 0.50, n = 6).

 

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  		Table 5. Effect of corn oil supplementation to grazing steers on cellularity of s.c. adipose tissue
 

   DISCUSSION
Top
 Abstract
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
LITERATURE CITED
 
Conjugated linoleic acid and VA are intermediates formed during ruminal biohydrogenation of dietary linoleic acid (Bauman et al., 1999Go). Several studies have been conducted to increase beef CLA cis-9, trans-11 proportion through supplementation of high concentrate diets with plant oils rich in linoleic acid; however, responses have been limited. Others (Beaulieu et al., 2002Go; Griswold et al., 2003Go; Hristov et al., 2005Go) did not observe changes in VA or CLA cis-9, trans-11. Similarly, Madron et al. (2002)Go did not observe any change in VA or CLA cis-9, trans-11 proportions of LM, eye of round, or chuck tender samples when cattle were supplemented with 12.7% extruded full-fat soybeans in a high-concentrate diet. However when inclusion of extruded full-fat soybeans increased to 25.6% of diet, VA and CLA cis-9, trans-11 content increased 0.38 and 0.11 percentage units with respect to the control diet. Vaccenic acid and CLA cis-9, trans-11 content in 3 different adipose tissues (i.m., s.c., and perirenal adipose) increased 0.14 and 0.06 percentage units, respectively, when 4% corn oil was added to high-concentrate diet (Gillis et al., 2004Go). In this study, corn oil supplementation to steers grazing endophyte-free tall fescue increased VA and CLA by 1.88 and 0.45 percentage units, respectively, when supplemented at 0.75 g/kg of BW. The cis-9, trans-11 isomer of CLA was increased by 0.17 and 0.45 percentage units in the LM and s.c. adipose tissue, respectively, when 0.75 g/kg of BW of corn oil was supplemented to grazing steers.

The greater response to dietary linoleic acid observed in our study compared with those that utilized high-concentrate diets is due to differences in ruminal environment generated by the basal diets. High-concentrate diets favors production of trans-10, cis-12 CLA and trans-10 octadecenoic acid as intermediates of linoleic acid biohydrogenation as opposed to the cis-9, trans-11 CLA and VA produced with high-forage diets (Bauman and Griinari, 2003Go). Sackmann et al. (2003)Go observed that ruminal outflow of trans-10 octadecenoic acid was almost 12 times that of VA, whereas trans-10 octadecenoic acid ruminal outflow decreased and VA increased linearly as forage in the diet increased from 12 to 36%. Increasing dietary linoleic acid in high-concentrate diets increased ruminal outflow (Duckett et al., 2002Go; Sackmann et al., 2003Go) and adipose tissue concentration (Gillis et al., 2004Go; Hristov et al., 2005Go) of trans-10 octadecenoic acid to a greater extent than VA. In the current study, trans-10 octadecenoic acid also increased with dietary linoleic acid, but changes were minor compared with VA.

The lack of an additional increase in VA proportion in either tissue by doubling the amount of corn oil supplemented from medium to high would be partially explained by the lower increase in dietary C18:2 proportion between medium and high than between none and medium. Dietary linoleic acid proportion was 27% of total FA in none, 42.2% in medium, and 48.6% in high due to reductions in forage intake with oil supplementation. Duodenal flow of VA was reduced from 78% of linoleic acid intake when dairy cows were fed ad libitum to only 47% when DMI was restricted to 80% of ad libitum (Qiu et al., 2004Go). In our study, total DMI for high was 83% of the total DMI in medium (Pavan et al., 2007Go).

The interaction between dietary treatment and tissue on cis-9, trans-11 CLA content was due to the high variation in cis-9, trans-11 CLA percentages observed in high. Cis-9, trans-11 CLA percentage was 27 and 48% greater in the LM and s.c. adipose, respectively, for medium than none. As suggested by Noci et al. (2005a)Go, the lower level of cis-9, trans-11 CLA in the i.m. fat could be related to lower i.m. fat deposition. Noci et al. (2005a)Go found that cis-9, trans-11 CLA is preferentially accumulated in the neutral lipid fraction, which is positively associated with the amount of i.m. fat in the muscle (Duckett et al., 1993Go). In addition, Santora et al. (2000)Go observed that endogenously generated cis-9, trans-11 CLA was only stored in triacylglycerol fraction in mice. According to Gillis et al. (2003)Go, endogenously generated cis-9, trans-11 CLA represents 86% of the total tissue content based on the ratio of VA to cis-9, trans-11 CLA in the duodenal digesta vs. adipose tissues. In this study, endogenously generated cis-9, trans-11 CLA represented 83% in the LM and 85% in the s.c. adipose tissue of the total cis-9, trans-11 CLA present based on the ratio of VA to cis-9, trans-11 CLA in abomasal digesta vs. tissue samples.

The high variability in cis-9, trans-11 CLA content in the high treatment may be related to the response to high fat supplementation levels by the animal. Others have shown large variability in CLA concentrations in milk fat when cows consumed fresh-pasture (Kelly et al., 1998Go) or extruded soybean supplements (Peterson et al., 2002Go). Peterson et al. (2002)Go suggested the source of the variation in milk cis-9, trans-11 CLA levels to be related to ruminal biohydrogenation and SCD activity in the mammary gland. Several authors (Madron et al., 2002Go; Daniel et al., 2004Go; Noci et al., 2005aGo) have reported a high positive linear relationship among cis-9, trans-11 CLA and VA concentrations in adipose tissues, suggesting that CLA content is dependent on VA concentrations. In our study, the linear regression within tissue including all dietary treatments was significant (P < 0.01) with R2 of 0.37 and 0.54 for the s.c. adipose and LM, respectively. However, the relationship within dietary treatments did not exist (P > 0.05), suggesting again the existence of individual variation in ruminal biohydrogenation of VA.

Despite the increase in dietary linoleic proportion with corn oil supplementation, tissue linoleic acid, or total PUFA proportion were not affected. Others (Engle et al., 2000Go; Beaulieu et al., 2002Go; Santos-Silva et al., 2003Go) also observed no change in tissue linoleic acid content with increasing dietary linoleic acid supplementation. In contrast, Andrae et al. (2001)Go and Gillis et al. (2004)Go observed an increase in tissue linoleic acid content when feeding high-oil corn or supplemental corn oil. Differences in the results may be related to the degree of ruminal biohydrogenation obtained with the different diets or to different FA composition of the base diets. Ruminal biohydrogenation extent was increased when dietary forage (Kucuk et al., 2001Go) or dietary lipid content increased, and when oil was fed instead of oilseed (Duckett et al., 2002Go). Linolenic acid percentage was 9.2 and 21% lower (P < 0.01) with the medium and high treatments, respectively. These reductions in tissue linolenic acid percentage with corn oil supplementation are similar to the reductions in dietary linolenic acid intake of 8.7 and 32% for medium and high treatments, respectively, due to a depression in forage intake with oil supplementation (Pavan et al., 2007Go). Despite no significant effects on total n-6 and n-3, corn oil supplementation increased the ratio of n-6:n-3 in both tissues. However, the highest n-6:n-3 ratio (3.6) obtained in the LM from animals fed the high treatment was still below the maximum recommended level of 4 for human consumption (Enser, 2001Go).

Although corn oil supplementation did not have any effect on total SFA or PUFA:SFA ratio, the negative effect of corn oil supplementation on myristic and palmitic acids is noteworthy. These 2 SFA are considered to have hypercholesterolemic effects on humans, whereas stearic acid, the other predominant SFA in beef, is considered neutral in this regard (Ulbricht and Southgate, 1991Go). Reductions in palmitic acid proportions were also observed when lipid intake was increased in lambs (Bolte et al., 2002Go) or beef cattle diets (Andrae et al., 2001Go; Beaulieu et al., 2002Go; Madron et al., 2002Go). Mir et al. (2002)Go had previously suggested the occurrence of a feedback inhibition of lipogenesis by adding 6% of sunflower oil to the diet. In our study, the linear reduction in tissue myristic and palmitic acid content would be the result of the reduction of their proportions in the diet, but also the reduction in de novo FA synthesis by an increase in exogenous FA supplied by the diet (Vernon, 1981Go). The reduction in OCFA percentage with corn oil supplementation in this study would suggest lower ruminal propionate production or lower de novo FA synthesis. Odd-chain FA are produced when propionate is substituted for acetate in de novo FA synthesis (Garton et al., 1972Go). According to Jenkins (1993)Go, a reduction in fiber digestion as observed in this study (Pavan et al., 2007Go) would result in a lowering of the acetate to propionate ratio in the rumen. Thus, the reduction in OCFA percentage would agree with a reduced de novo FA synthesis with exogenous FA supplementation.

A decrease in de novo FA synthesis with increasing corn oil supplementation would have offset an increase in stearic acid absorption, resulting in the similar proportion of stearic and oleic acids observed in the adipose tissues across dietary treatments. The effect of increasing dietary linoleic acid through vegetable oil supplementation on tissue stearic and oleic acid content is variable across studies. In agreement with our results, Andrae et al. (2001)Go did not observe changes in stearic or oleic acid percentage when feedlot diets containing typical corn were replaced by high-oil corn. Madron et al. (2002)Go observed an increase in stearic acid and decrease in oleic acid percentage in LM when extruded full-fat soybean were included in high-concentrate diet at increasing levels. In contrast, Gillis et al. (2004)Go detected similar proportions of stearic acid and lower oleic acid when heifers were fed a high-concentrate diet for 60 d with 4% corn oil.

Stearoyl-CoA desaturase is located in the adipose tissues of growing ruminants (Martin et al., 1999Go) and is responsible for the rate-limiting step in the biosynthesis of MUFA. Stearoyl-CoA desaturase, also known as the {Delta}9 desaturase, catalyzes the synthesis of monounsaturated FA by inserting a cis double bond between the 9th and 10th carbon (counting from the carboxyl terminal) in long-chain FA of 12 to 19 carbons (Ntambi, 1999Go). Palmitoyl- and stearoyl-CoA are the major substrates for SCD and are converted to palmitoleoyl- and oleoyl-CoA, respectively. Vaccenic acid is also a substrate and can be converted to cis-9, trans-11 CLA by SCD. In this study, over 80% of the total cis-9, trans-11 CLA resulted from the desaturation of VA to CLA based on the ratio of VA to CLA cis-9, trans-11 in digesta from abomasum and tissue samples. The activity and expression of SCD in ruminant adipose tissues directly impacts the FA composition of adipose and cell membranes, thereby regulating membrane fluidity. In rodents, diets rich in PUFA have been shown to reduce hepatic abundance of SCD (Ntambi, 1992Go, 1999Go; Jump and Clarke, 1999Go). Results from this study indicate that SCD activity or expression in s.c. adipose tissues was not negatively impacted with oil supplementation. This was also confirmed indirectly from the measurement of abomasal and tissue CLA:VA ratios, which did not differ among dietary treatments. In addition, these results suggest a parallel response between activity and expression of SCD in s.c. adipose tissues.

1 Corresponding author: sducket{at}clemson.edu

Received for publication November 6, 2006. Accepted for publication April 2, 2007.


   LITERATURE CITED
Top
 Abstract
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 LITERATURE CITED
 


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