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The Fatty Acid Composition of Muscle Fat and Subcutaneous Adipose Tissue of Pasture-Fed Beef Heifers: Influence of the Duration of Grazing

Posted on: Saturday, 30 July 2005, 03:01 CDT

ABSTRACT:

Our objective was to determine the effect of the duration of grazing before slaughter on the fatty acid composition of muscle fat and s.c. adipose tissue (SAT) of beef heifers. Sixty crossbred Charolais heifers (n = 15 per treatment) were assigned randomly to one of four dietary treatments: 45 animals (Treatments 1, 2, and 3, respectively) were housed at the beginning of the experiment, and 15 (Treatment 4) were fed at pasture. Two groups of 15 heifers were moved to pasture 40 d (Treatment 2) and 99 d (Treatment 3) before slaughter, respectively, resulting in preslaughter grazing periods of 0, 40, 99, or 158 d for Treatments 1, 2, 3, and 4, respectively. Before grazing the predominantly perennial ryegrass pasture, animals were housed and offered grass silage ad libitum and 3 kg of concentrate diet (650 g of grass silage/kg of total DMI). After slaughter, the fatty acid profile of the neutral (NL) and polar lipid (PL) fractions of muscle fat from the LM and the total lipids from SAT were analyzed by gas chromatography. Duration of grazing showed a quadratic tendency on mean carcass weight (P = 0.08), but did not affect growth (P = 0.27) or the lipid content (P = 0.13) of the LM. Increasing the duration of grazing led to a linear increase (P < 0.001) in the concentration (on fresh-tissue basis) of CLA in muscle fat (from 11.80 to 17.75 mg/100 g of muscle in NL, and from 0.52 to 0.82 mg/100 of g muscle in PL) and in SAT (from 3.98 to 10.23 mg/g of SAT; P < 0.001), and increased the concentration of C18:1trans-11 in both muscle fat fractions (P < 0.001) and in SAT (P < 0.001). In the total muscle lipids, the polyunsaturated to saturated fatty acid ratio (P:S) increased from 0.12 to 0.15 with increased duration of grazing following a linear (P < 0.05) and cubic pattern (P < 0.05). Increasing the duration of grazing led to a linear decrease in the n-6:n-3 ratio of muscle fat from 2.00 to 1.32 (P < 0.001), and from 2.64 to 1.65 in the SAT lipids (P < 0.001), mainly as a consequence of the increased concentration of C18:3n-3. It is concluded that muscle fat and SAT fatty acid profile was improved from a human health perspective by pasture feeding, and that this improvement depended on the duration of grazing.

Key Words: Beef Cattle, Conjugated Linoleic Acid, Fatty Acids, Grazing

Introduction

Lipids of ruminant origin are among the richest sources of CLA (Chin et al., 1992), a fatty acid that has received considerable attention in recent years because of its anticancer and other human health-promoting properties (Bauman et al., 2001). The CLAcis- 9,trans-11 isomer in particular has been reported to inhibit human cancer cells (Schultz et al., 1992) and to limit chemically induced tumor development in test animals (Belury, 1995). Other CLA isomers seem to possess different biological functions, including modification of the immune system and inhibition of body fat accretion (Park et al., 1997). Production of CLA occurs in the rumen as a result of incomplete biohydrogenation of dietary fatty acids, particularly C18:2n-6 (Kepler and Tove, 1967). However, a considerable proportion of the CLA found in ruminant tissues is derived from tissue desaturation of ruminally derived C18:1trans-11 (Santora et al., 2000) through the action of A9-desaturase, an enzyme active in the mammary gland and in adipose tissue (Griinari et al., 2000; Bauman et al., 2001).

Ruminai biohydrogenation of the predominant fatty acid in pasture (C18:3n-3) also leads to production of C18:1trans-11 and ultimately to CLA in tissue. French et al. (2000) showed that inclusion of pasture compared with grass silage or concentrates in the diets of finishing steers increased the proportion of CLA, increased the polyunsaturated:saturated fatty acid (P:S) ratio, and decreased the n-6:n-3 fatty acid ratio in fat from LM. In the study of French et al. (2000), steers grazed pasture for 85 d.

We hypothesized that the extent of the alteration in the fatty acid profile due to grazing is a function of the duration of grazing before slaughter. The objective of the present experiment was to determine the effect of time spent at pasture before slaughter on the fatty acid composition of muscle fat and s.c. adipose tissue (SAT) of beef cattle.

Materials and Methods

Animal Management and Sampling Procedures

Sixty Charolais crossbred heifers were housed in a slatted-floor shed and offered grass silage ad libitum for 2 mo before the start of the experiment. The animals were then blocked according to BW (mean = 332 kg 38.9 kg) and, within block, assigned randomly to one of four dietary treatments (n = 15 per treatment). Three groups of 15 heifers began the experiment indoors (in a slatted-floor shed, 3 m^sup 2^/animal, with natural light) (Treatments 1, 2 and 3) and were offered grass silage ad libitum and 3 kg of concentrate/heifer daily (as-fed basis). The silage was a first cut from a predominantly perennial ryegrass sward, harvested without wilting, and ensiled without an additive. The concentrate consisted of rolled barley (430 g/kg), molassed sugar beet pulp (430 g/kg), soybean meal (80 g/kg), molasses (45 g/kg), and a mineral and vitamin mix (15 g/ kg), on an as-fed basis. The mineral and vitamin mix contained Ca (28.5%), P (1.6%), Na (5.6%), vitamin A (500,000 IU), vitamin D^sub 3^ (125,000 IU), vitamin E (1,500 IU), cobalt carbonate (42 mg/kg), cupric sulfate (500 mg/kg), calcium iodate (10 mg/kg), iron sulfate (1,000 mg/kg), manganese sulfate (5,800 mg/kg), sodium selenite (16 mg/kg), and zinc sulfate (7,500 mg/kg) on an as-fed basis. One group remained indoors for the duration of the 158-d experiment. A second group of 15 animals was moved to pasture 118 d before slaughter, and the third group was moved to pasture 59 d before slaughter (Treatments 2 and 3). The fourth group of 15 heifers (Treatment 4) was at pasture for the entire 158 d of the experiment. The sward consisted of predominantly perennial ryegrass (Lolium perenne), but Poa trivialis and Trifolium repens were also present. Within each treatment, animals were managed as three groups of five animals each, formed according to BW block. At the beginning of the experiment, grazing animals were assigned a pasture allowance of 132 kg of DM per group. As the season progressed, daily pasture allowances were increased by increasing the land allocation such that each group was offered a sward of similar yield and quality. The target ADG was set by the group of heifers fed silage and concentrates, and the pasture allowances for the grazing animals were adjusted to ensure a mean similar growth rate for all treatments. The pasture intakes were estimated as described by French et al. (2000). Animals from all dietary treatments were slaughtered at the same time. On the day of slaughter, animals were weighed without fasting, transported 120 km to a commercial facility (Meadow Meats, Rathdowney, Ireland) within 3 h, and slaughtered within 60 min of arrival. After slaughter, the weight of the carcass and the perirenal adipose tissue were recorded. Carcasses were chilled for 24 h, and the LM and adipose tissue were removed. Duplicate samples of steaks (25 mm thick) from the region of the 7th rib of the LM were stored at -30C, whereas the samples of SAT from the 9th-rib region were stored at -20C.

Muscle Fatty Acid Analysis

Samples of the LM were defrosted and homogenized with a Robot Coupe R301 Ultra food processor (Robot Coupe S.N.C., Vincennes, France). Muscle fat was extracted from homogenized muscle as described by Folch et al. (1957), by adding 0.05% (wt/vol) butylated hydroxytoluene as antioxidant in the 2:1 (vol/vol) chloroform/ methanol mixture. Muscle (2 g) was homogenized in an 80-mL screw- cap test tube (25 mm 200 mm) with 20 mL of solvent using an Ultra Turrax T25 homogenizer (Janke and Kunkel, IKA Labortechnik, Staufen, Germany). The homogenizer was rinsed with 16 mL of the solvent solution, and this volume was added to the previous 20 mL. The tubes were stored overnight at 40C in darkness. The tube contents were filtered through Whatman No. 4 filter paper (Whatman, Ltd., Maidstone, U.K.). The tubes were rinsed with 5 mL of solvent solution, and the filter cake was rinsed with a further 5 mL of solvent. A volume of 0.02% CaCl2 solution in distilled water (wt/ wt) equivalent to 25% of the filtrate was added to the test tubes, which were shaken and left to separate overnight at 4C. The top aqueous layer was removed by vacuum and the bottom layer was poured through a funnel containing Whatman No. 4 filter paper and approximately 5 g of anhydrous Na^sub 2^SO^sub 4^. The filtrate containing the extracted i.m. lipid was collected into 50-mL screw- cap glass bottles and stored overnight at -30C. The lipid extract was dried to a constant weight under a stream of N^sub 2^, and redissolved in 1 mL of chloroform. The lipid samples were then applied to solid-phase extraction cartridges with 500 mg of aminopropyl packing (Bond-Elut 500 mg, 3-mL reservoir; Varian Instruments, PaIo Alto, CA) previously conditioned by a 3 mL 3 mL flush with chloroform. The neutral lipid (NL) fraction was eluted with 4 mL of chloroform and the eluate was collected. The cartridges were washed with1 mL of 1:1 chloroform/methanol (vol/vol), followed by 5 mL of methanol to extract the polar lipid (PL) fraction. The NL and PL fractions were dried to constant weight in preweighed glass tubes (12 mm 75 mm), and the weight of each fraction was recorded. The separated lipid classes were dissolved in 300 L of toluene for preparation of fatty acid methyl esters (FAME). The methylation procedure involved a combination of alkaline and acidic transesterification, as outlined by Kramer and Zhou (2001). The extracted lipid fractions were initially methylated with NaOCH^sub 3^, which was followed with a 4% solution of HCl in methanol to avoid possible isomerization of conjugated dienes associated with the use of BF3/ CH^sub 3^OH (Park et al., 2001). Both methylation procedures were carried out at 50C for 20 min. Tricosanoic acid (C23:0) methyl ester was used as an internal standard for fatty acid quantification. Deionized water (2 mL) saturated with hexane (95:5 water-hexane; vol/vol) was added to the tube containing the FAME, followed by 2 mL of hexane. The tubes were centrifuged (800 g) for 5 min, and the top layer containing FAME in hexane was removed and transferred to glass tubes (12 mm 75 mm). This step was repeated with a further 2 mL of deionized water saturated with hexane. The top layers were transferred to tubes containing approximately 0.75 g of Na^sub 2^SO^sub 4^, and centrifuged (800 g for 5 min). An aliquot of the supernatant (500 L) containing FAME was transferred into a 2-mL glass vial and further diluted with 500 L of hexane before injection.

Gas Chromatographic Analysis

The FAME were separated by gas chromatography using a Varian 3800 GC (Varian Instruments) equipped with a CP-SiI 88 capillary column (100 m 0.25 mm i.d., 0.2-m film thickness; Chrompack, The Netherlands) and a Varian 8400 autosampler. The injector and the flame ionization detector were kept at constant temperatures of 250 and 260C, respectively. The column oven temperature was held at 40C for 2 min, increased at 20C/min to 80C and held for 2 min, increased to 160C at 20C/min, to 220C at 4C/min, and to 240C at 2C/min and held for 8 min. The total run time was 43 min, and the carrier gas used was H2. For peak identification, a standard mix of 37 FAME (Supelco Inc., Bellefonte, PA) was used, and individual standards from Matreya (Matreya Inc., Pleasant Gap, PA) were used for identification of those FAME not contained in the mix.

Subcutaneous Adipose Tissue Fatty Acid Analysis

After thawing, the SAT was reduced to paste with a Robot Coupe R301 Ultra food processor. A 400-mg sample, inclusive of connective tissue, was homogenized in the chloroform-methanol mix with the Ultra Turrax as previously described for the muscle tissue, and filtered (Whatman Paper No. 4) to remove residual connective tissue. The procedure used was the same as that described for fatty acid analysis in muscle fat. No separation of NL and PL fractions was performed on the SAT lipids. The methylation was carried out as described for muscle samples.

Table 1. Chemical composition and fatty acid profile of feedstuffs fed to beef heifers

Feed Sampling and Chemical Analysis

Duplicate grass samples were collected daily, stored frozen at - 20C, and subsequently pooled within week of the study for general chemical and fatty acid analysis. Grass silage samples were taken daily and stored at -2O0C for subsequent pooling and subsampling. Samples of concentrate meals were taken twice weekly and stored frozen at -2O0C. They were pooled before performing chemical and fatty acid analysis. General chemical analysis of feeds was as described by Moloney et al. (1996). The fatty acid composition of feedstuffs was determined using the procedure described by Sukhija and Palmquist (1988). The feed FAME in toluene were analyzed by GC as described above.

Statistical Analyses

Data were subjected to ANOVA according to a randomized complete block design. The model used had block and duration of grazing as main effects and animal as the experimental unit. The 3 df for duration of grazing were separated into linear, quadratic, and cubic orthogonal polynomials. Treatment differences were considered significant at P < 0.05. The statistical analyses were performed using Genstat 5.0 (VSN Int., Ltd., Oxford, U.K.)

Results

Feed Chemical and Fatty Acid Composition

The chemical and fatty acid compositions of the feeds are reported in Table 1. Pasture and grass silage had similar DM and oil contents, but grass silage had the highest CP. Concentrates had the highest proportions of C16:0, C18:l, and C18:2n-6, and the lowest proportion of C18:n-3. The proportion of C18:3n-3 in pasture and grass silage exceeded 45% of FAME.

Table 2. Effect of duration of grazing on animal characteristics

Feed Intake and Animal Performance

Animal performance data are summarized in Table 2. Preslaughter weight, carcass weight, and ADG were not affected by the dietary treatments; however, there was a quadratic decrease in the dressing percent and the weight of perirenal fat as the duration of grazing increased. Daily concentrate consumption averaged 3.55, 3.29, and 2.81 kg/d DM during the housing phases, whereas the average DMI from silage was 4.10, 4.21, and 4.50 kg/d for animals that were housed for 158, 118, and 59 d, respectively. Pasture DMI averaged 8.52, 7.49, and 7.47 kg/d for animals that grazed for 40, 99, and 158 d, respectively. Average daily intakes of fat were 207, 218, 217, and 224 g for O, 40, 99, and 158 d at pasture, respectively. Animals at pasture for 158 d consumed, on average, 133 g of PUFA compared with 123 g/d for their housed counterparts. Animals grazing for 40 d increased their PUFA intake from 123, to 152 g/ d, whereas those grazing for 99 d increased their PUFA intake from 124 to 133 g/d after grazing pasture.

Fatty Acid Composition of Muscle Fat

No differences were detected between treatments for total muscle fat (2,980 mg/100 g of muscle on average) or fatty acid content (2,515 mg/100 g of fresh muscle on average).

Neutral Lipids. No differences were induced by the dietary treatments on the total fatty acid content of the NL fraction. The proportions of fatty acids in the NL fraction of muscle fat from the LM are shown in Table 3. The proportions of total and MUFA in the NL fraction were not affected by the dietary treatments, but the proportion of SFA decreased linearly with increasing duration of grazing. The proportion of PUFA, the P:S ratio, and the proportion of C18:3n-3, C20:3n-3, C20:5n3, C22:5n-3, and the total n-3 PUFA increased linearly with an increase in the duration of grazing (Table 3). The linear increase in C22:2n-6 and C18:3n-6 did not lead to an increase in total n-6 PUFA. An increase in the duration of grazing led to a linear decrease in n6:n-3 PUFA ratio and to a linear increase in the proportion of C18:Itrans-11, CLActs-9,rans- ll, and CLAtrans-W,cis-l2. The proportion of C14:0 and C14:l increased as the duration of grazing increased from O to 99 d, and then decreased as the duration was extended to 158 d, resulting in a quadratic response pattern (the cubic term being significant for C14:0). A quadratic response to increasing duration of grazing was also observed for the proportion of C20:1. A cubic effect of diet was detected for the proportion of C16:0.

Polar Lipids. The proportions of fatty acids in the PL fraction of muscle fat of LM are shown in Table 4. The PL fatty acids accounted for approximately 0.10 of the total fatty acids, which was consistent across the treatments. The concentration of total fatty acids and MUFA in the PL followed a quadratic and cubic pattern, respectively, with increasing duration of grazing. As the duration of grazing increased, n-3 PUFA linearly increased, but the proportion of n-6 PUFA decreased following a cubic pattern, whereas the n-6:n-3 ratio linearly decreased. Extending the duration of grazing resulted in a linear increase in C18:1trans-11, CLAcis- 9,trans-11 and led to an increase following a cubic pattern for C18:3n-3. The proportion of C20:1, C20:5n-3, and C22:2n-6 increased linearly with an increase in the duration of grazing, whereas the proportion of C20:4n-6 and C22:1 linearly decreased. The proportion of C14:1, C17:l, C22:0, and C22:5n-3 showed a quadratic response to the increasing duration of grazing.

Total Lipids. The proportions of total fatty acids in LM are shown in Table 5. No differences were detected for the total fatty acid content; however, the n-3 PUFA increased following a cubic pattern, with increasing duration of grazing, resulting in a linear decrease in the n-6:n-3 ratio similar to the NL and the PL fractions. Similar trends were observed in the total fatty acids to the NL for short and medium chain and fatty acids with 18 carbons. The concentration and the proportion of CLAcis-9,trans-1 and C18:trans-11 increased linearly with increasing duration of grazing, similar to the NL.

Fatty Acid Composition in SAT

Total fatty acids content in SAT was influenced by the duration of grazing following a cubic pattern. Data relating to fatty acid proportion in SAT are shown in Table 6. The proportion of PUFA, the P:S ratio, the proportion of n-3 PUFA, the proportion of C12:0, C18:1trans-9, C18:1trans-11, CLAcis-9,trans-11, CLA-trans-10,cis- 12, C18:3n-3, C20:2n-6, C20:5n-3, and C22:5n-3 were increased linearly with increasing duration of grazing. The proportion of SFA, the n-6:n-3 ratio, and the proportion of C16:0, C20:1, and C20:3n-6 were decreased linearly. Differences were induced by the dietary treatments in the proportion of C17:0, as the cubic term of the polynomial partition was significant. Eicosanoic acid, C22:0 and C22:1 showed a quadratic response to the extended duration of grazing.

Table 3. Effects of duration of grazing on fatty acid proportion of neutral lipids in fat from longissimus muscle of beef heifers

Discussion

Although different shor\t-term concentrate-feeding strategies (42 d) were examined by Griswold et al. (2003), and Duckett et al. (1993) examined the effect of extending the concentrate-finishing period for steers fed a grass-based backgrounding diet, there are no reports available on the effects of the duration of feeding fresh pasture on the fatty acid composition of beef. In the present experiment, the feeding strategy imposed was successful in ensuring a similar average carcass weight and muscle fatty acid concentration for all treatments. Leat (1978) observed a greater proportion of MUFA in the fat deposited by fatter animals, and the allowances offered in the current experiment were therefore designed to avoid such a confounding effect of fatness on the fatty acid profile.

Table 4. Effects of duration of grazing on fatty acid proportion of polar lipids in fat from longissimus muscle of beef heifers

Several studies have compared the effect of grain-finishing vs. forage-finishing on the fatty acid composition of muscle and SAT. The proportion of C18:3n-3 in pasture in the present study was similar to that of French et al. (2000), but the proportion of C16:0 was lower. The fatty acid profile of total lipids in LM of animals fed grass silage and concentrate or pasture for 158 d was similar to French et al. (2000), reflecting largely similar feed fatty acid composition, and similar to Rule et al. (1995) for C18:3n-3 in the SAT of forage-fed animals. Similar to the results of the current study, Itoh et al. (1999) reported greater proportions of C16:0 in the triacylglycerols and in the total i.m. lipids from grain- finished cattle than from cattle finished on perennial pasture. In this experiment, the response of C18:3n-3 to pasture-feeding was a consistent increase in muscle fat and SAT, where a 76% increase was observed in animals fed pasture compared with those fed concentrate and silage. Scollan et al. (2002) reported similar proportions of C18:3n-3 in muscle fat when steers were fed grass or a mixed pasture of grass and white clover, although animals were grazing for a period of 465 d. Other comparisons of pasture-finishing with grain- finishing of cattle found a lesser concentration of C20:5n-3 and C22:5n-3 in grain-finished animals than in animals finished at pasture (Rule et al., 1995; Itoh et al., 1999), which also were greater in animals fed pasture than in animals finished on concentrate and silage in the present experiment. Itoh et al. (1999) did not report the concentration of long-chain n-3 PUFA in the NL, but they found a substantial increase in C20:5n-3 and C22:5n-3 in the PL fraction due to forage feeding.

Table 5. Effects of duration of grazing on fatty acid proportion of total lipids in fat from longissimus muscle of beef heifers

Table 6. Effects of duration of grazing on fatty acid proportion in subcutaneous adipose tissue (SAT) of beef heifers

The P: S ratio in muscle fat and SAT increased linearly with increasing duration of grazing in the present study, and was similar to that reported by French et al. (2000). Overall, the proportions of SFA were only marginally influenced by increasing duration of grazing, whereas the main differences were observed in the proportion of PUFA in both muscle fat and SAT. The greater intake of PUFA through a grass-based diet resulted in an increasingly greater proportion of PUFA in the muscle in the NL and in the SAT as the duration of preslaughter grazing increased. Duckettet al. (1993), who investigated the effects of switching from pasture-feeding to concentrate feeding, reported a time-dependent increase in the proportion of C14:0 and C18:1, a marginal difference in the proportion of C16:0, and a time-dependent decrease in C18:0. In the present experiment, switching the animals from a silage and concentrate-based diet to a grass-based diet led to a time- dependent decrease in the proportion of C16:0 in total muscle fat, and a similar trend was observed in SAT.

Duckett et al. (1993) reported a higher P:S ratio (0.26) in the pasture-fed control group, in animals that had a fat content similar to the one observed in the present experiment (2.52% total lipid). This reflects the negative relationship between the fatty acid concentration in the muscle and the P:S ratio reported by Scollan et al. (2003), as very lean animals would have high ratios (Raes et al., 2001). The values obtained in the current experiment (0.15 for animals at pasture for 158 d) were within the range that could be expected according to Scollan et al. (2003) and were consistent with the findings of Enser et al. (1996).

Forage feeding had little effect on C18:3n-3 in the short-term (42 d) feeding trial described by Griswold et al. (2003). Although the increase in the duration of the feeding period before slaughter in the study by Duckett et al. (1993) resulted in an increase in carcass fatness, which tends to increase MUFA deposition irrespective of the fatty acid composition of the diet, some of the findings are in agreement with the results reported in the present study. The proportion of C18:3n-3 linearly decreased in the study by Duckett et al. (1993) and linearly increased in the present study. This effect could be attributed to the larger amount of C18:3n-3 consumed by animals grazing for 158 d, coupled with the longer grazing time compared with their counterparts that grazed for 40 d and 99 d, respectively.

The lipid classes were separated to allow for the assessment of dietary effects on NL and PL because the incorporation of C18:3n-3 and PUFA in general is higher in PL than in NL. Averaged across treatments, C18:3n-3 accounted for 4.8% of total fatty acids in the PL fraction, whereas C18:3n-3 represented only 0.7% of total fatty acids in the NL fraction. The combined proportion of C20:5n-3 andC22:5n-3 accounted for 0.1% of NL in cattle fed silage and concentrate and increased linearly to over 0.2% in cattle fed at pasture for 158 d, whereas in PL, these fatty acids increased from 5.1 to 5.8%. In the SAT, similar differences were noted, with a 111% increase in C20:5n-3 and a 68% increase in C22:5n-3, but, similar to the content in NL in muscle, these fatty acids accounted for a minimal proportion of the total fatty acids in SAT. This last result would suggest a relationship between the duration of feeding a diet rich in C18:3n-3 and the concentration of the long-chain n-3 PUFA in the muscle and adipose tissue. It is widely documented that, despite the high degree (92% on average) of ruminai biohydrogenation of dietary C18:3n-3 (Doreau and Ferlay, 1994), feedstuffs rich in C18:3n-3, such as pasture in this experiment or linseed, lead to an increase in the incorporation of long-chain n-3 fatty acids in the muscle and adipose tissue (Vatansever et al., 2000; Scollan et al., 2001), indicating chain elongation of C18:3n-3 that escapes biohydrogenation. The differences observed in the incorporation of PUFA between the NL and PL, largely in favor of the latter, would suggest that the effects of feeding strategies aimed toward increasing PUFA incorporation, and n-3 PUFA in particular, should focus preferentially on PL, where such an effect would be more highly visible.

The overall n-6:n-3 ratio (2.21) in the muscle of animals housed for 158 d was already acceptable according to the UKDH (1994), possibly due to the influence of grass silage feeding. This ratio is generally higher in concentrate-fed animals (Marmer et al., 1984; Enser et al., 1998). The value of the C18:2n-6:C18:3n-3 ratio in the present study was 2.83 for animals fed silage and concentrate but decreased linearly to 1.62 for animals fed pasture for 158 d. These results are similar to the findings of French et al. (2000; 2.33 after 85 d of pasture feeding) and Enser et al. (1996), where LM from pasture-fed steers had a C18:2n-6:C18:3n-3 ratio of 1.98 and a n-6:n-3 ratio of 1.32. This result confirmed the advantage of pasture-feeding on the n-6:n-3 ratio, but also showed the benefit of extending the time at pasture on the deposition of nutritionally beneficial muscle fatty acids.

The ruminal biohydrogenation pathway of C18:3n-3 (0.475 of total fatty acids in the pasture used in this experiment) leads to the formation of C18:1trans-11 (Harfoot and Hazelwood, 1988). Griinari and Bauman (1999) observed that, because only a small portion of CLA escapes from the rumen, where it is produced as an intermediate of C18:2n-6 biohydrogenation (Harfoot and Hazelwood, 1988), it also must originate from another product of ruminal metabolism, identified as C18:1trans-11. The increased proportion of CLA in both muscle and SAT in the present experiment could not be explained by incomplete biohydrogenation of C18:2n-6 and subsequent deposition in the adipose tissue because the dietary intake of C18:2n-6 was low, particularly for animals fed at pasture for 158 d. Figure 1 shows the relationship between CLA and C18:1trans-11 in the muscle fat and in the SAT in this experiment. The increased proportion of CLA in all tissues analyzed as the duration of grazing increased suggests a relationship between the time spent on a diet rich in C18:3n-3 as substrate for biohydrogenation, the concentration of C18:1trans-11 as a product of biohydrogenation, and the production of CLA in the tissues through the action of Δ^sup 9^-desaturase. Although the linear trends were similarly observed in the NL, PL, and SAT, the relative proportion of CLA in the NL fraction was greater than in the PL, as CLAcis-9,trans-11 accounted, on average, for 0.61% of total fatty acids in the NL and for 0.25% of total fatty acids in the PL. This suggests that CLA is preferentially incorporated in the NL fraction, which agrees with the results of Scollan et al. (2003). Madron et al. (2002) and Enser et al. (1999) observed a linear relationship between the concentration of CLA and C18:1trans-11 in muscle adipose tissue; however, information on the extent to which \16;^sup 9^-desaturase is responsible for the overall CLA production in the muscle adipose tissue is not available to date. In the present experiment, there was no difference in the index of Δ^sup 9^-desaturase activity in muscle fat across the treatments, which would suggest that the main reason for increased tissue CLA is an increase in ruminai production of C18:1 trans-11. This result agrees with studies showing a repressing effect of long- chain PUFA on the activity of the Δ^sup 9^-desaturase (Ntambi, 1999). Regression of the values for desaturase activity against the CLA proportion in the total lipid yielded an r^sup 2^ value of 0.13, lower than the value of 0.62 found in Figure 1, where the relationship between CLA and trans-vaccenic acid is shown.

Figure 1. Relationship between trans-11 octadecenoic acid and cis- 9,trans-11 CLA in muscle fat (A) and adipose tissue (B) of beef heifers.

French et al. (2000) showed a linear relationship between the proportion of grass in the diet and the CLA concentration in muscle fat, and some studies reported a response in CLA concentration in milk fat to increased pasture intake (Stanton et al., 1997; Kelly et al., 1998). The effects of pasture feeding on muscle fatty acid composition were confirmed by Shanta et al. (1997), who found an increase in the CLA concentration of the semi-membranosus muscle in cattle fed pasture compared with feedlot finishing. Although the proportion of CLA trans-10,cis-12 was very low both in muscle fat and SAT, the linear increase noted suggests a possible increase in ruminal outflow of C18:1trans-10 due to the increase in the duration of grazing. This result was surprising because the increased production of C18:1trans-10 has so far been associated with low- roughage diets high in concentrates and polyunsaturated fat (Piperova et al., 1997).

Inclusion of pasture in the diet may lead to alterations in the ruminal environment other than increasing C18:3n-3 supply. The type and the source of dietary carbohydrates could induce different ruminal conditions, leading to changes in microbial fermentation patterns and, ultimately, in the concentration of C18:1trans-11 for CLA production by desaturation. It could be hypothesized that a feeding regimen with a high forage:concentrate ratio or an exclusively grass-based feeding regimen could induce ruminal conditions, such as pH, available carbohydrates, and ruminal flow, that are more suitable to enhancing the production of C18:1trans-11 as precursor of CLA. Although CLA was linearly increased in PL, 95% of total CLA found in LM was found in the NL fraction, and this proportion was constant across the treatments, which agrees with the findings of Scollan et al. (2003). Therefore, modification of the fatty acid composition targeting a CLA increase more likely would be visible in fatter rather than leaner animals.

Implications

This study confirmed that replacing a silage/concentrate-based diet rich in C18:2n-6 with pasture rich in C18:3n-3 resulted in a favorable fatty acid profile in muscle and subcutaneous fat from a human health perspective. Increased incorporation of conjugated linoleic acid in both tissues via a C18:3n-3-rich diet supports the strategy for increasing conjugated linoleic acid by increasing ruminai production of C18:Itrans-11. Clarification of the expression and activity of Δ9-desaturase in converting C18:1trans-11 to conjugated linoleic acid in the muscle deserves further attention. The linear increase in the proportion of conjugated linoleic acid, C18:1trans-11 or C18:3n-3, in pasture-fed animals with increasing duration of grazing suggests a slow tissue turnover of fatty acids, and the optimal achievable proportion of beneficial fatty acids remains to be identified.

2005 American Society of Animal Science. All rights reserved. J. Anim. Sci. 2005. 83:1167-1178

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F. Noci*[dagger], F. J. Monahan*, P. French[dagger], and A. P. Moloney[dagger]2

* Department of Food Science, Faculty of Agriculture, University College Dublin, Dublin 4, Ireland; and [dagger] Teagasc, Grange Research Centre, Dunsany, Ireland

1 This research was supported through the European Union, 5th Framework Programme (Project QLRT-2000-31423, "Healthy Beef"). The technical assistance of V. McHugh, A. McArthur, and N. Blount, and the cooperation of Meadow Meats, Rathdowney, Ireland, are gratefully acknowledged.

2 Correspondence-phone: +353 46 90 61100; fax: +353 46 90 26154; e-mail: amoloney@grange.teagasc.ie.

Received March 26, 2004.

Accepted February 20, 2005.

Copyright American Society of Animal Science May 2005

Source: Journal of Animal Science

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