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ANIMAL PRODUCTS |
* Agriculture & Agri-Food Canada, Sherbrooke, Québec, J1M 1Z3, Canada;
and
Département des Sciences Animales, Université Laval, Québec, Québec, G1V 0A6, Canada;
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Department of Animal & Poultry Science, University of Guelph, Guelph, Ontario, N1G 2W1, Canada;
and
Agriculture & Agri-Food Canada, Kapuskasing, Ontario, P5N 2Y3, Canada; and and
# Agriculture & Agri-Food Canada, Saint-Hyacinthe, Québec, J2S 8E3, Canada
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Abstract |
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 Top Abstract INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONLITERATURE CITED |
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0.05) the proportion of C18:0, C20:0, trans isomers of C18:1, and cis-9, trans-11 C18:2. Exclusive feeding of forages increased the proportion of cis-9, cis-12, cis-15 C18:3 as well as several other isomers of the n-3 family and decreased in the ratio of n-6 to n-3 fatty acids in the LD muscle as compared with supplementing grain (P < 0.05). In addition, the forage-based diet increased (P < 0.01) the concentration in the intramuscular fat of several intermediates (cis-9, trans-11, cis-15 C18:3; trans-11, cis-15 C18:2; trans-11 C18:1) of ruminal biohydrogenation. Forage feeding also increased the proportion of cis-9, trans-11 C18:2 (P < 0.01) and decreased the concentration of trans-10 C18:1 in the LD muscle (P = 0.03). It is concluded that quality demands of health-conscious consumers can be met through a forage-finishing and growth promotants-free beef production system.
Key Words: beef cattle • beef production system • beef quality • fatty acid • shear force • taste panel
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INTRODUCTION |
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONLITERATURE CITED |
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) and is the response of beef producers to the requests from a growing segment of consumers for healthier animal products (Bech-Larsen and Grunert, 2003). Although the use of ionophores has been shown to have a limited effect on meat quality (Boucqué et al., 1982; Burnham et al., 1997), the use of hormonal implants may increase the incidence of dark cutters and beef toughness (Scanga et al., 1998; Roeber et al., 2000; Barham et al., 2003). However, implants can have variable impacts on intramuscular fatty acid profile depending on composition and duration of implant used (Kennett and Siebert, 1987; Duckett et al., 1999).
Meat quality traits and shelf life were improved in several past studies (O’Sullivan et al., 2002; Gatellier et al., 2005) when steers were finished on forage diets as compared with finishing with concentrates. In fact, Mandell et al. (1997, 1998) found forage finishing to produce tender beef in continental breeds (Charolais or Limousin cross), although flavor intensity differed from grain-fed beef. However, previous research has also shown that young bulls and steers of an early maturing breed type (Hereford x Shorthorn) can produce beef with similar palatability attributes on grass silage versus feeding concentrates (Fortin et al., 1985). Recently, feeding high-concentrate diets was shown to decrease the proportion of CLA and n-3 fatty acids in intramuscular fat (IMF) as compared with forage finishing with conserved forages or pasture (Raes et al., 2004). Therefore, our objective was to compare the effects of conventional and natural beef production systems on meat quality, palatability, and fatty acid profile of beef from forage or grain-finished steers with or without the use of growth promotants.
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MATERIALS AND METHODS |
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONLITERATURE CITED |
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.
Details on the growing-finishing phase including implant procedures, diet and feed ingredient composition, feeding regimen, and on-farm and preslaughter animal handling are described in Berthiaume et al. (2006). Briefly, a total of 40 Angus cross steer calves were purchased in Western Canada and were transported to the Kapuskasing Beef Research Farm in Northern Ontario. Upon receipt, steers were vaccinated and treated for internal and external parasites (Cydectin, Wyeth Animal Health, Guelph, Ontario, Canada). Five management regimens were evaluated in the study (Table 1), with the regimens in the growing phase including grass silage only (GS), growth promotants used with grass silage feeding (GS/GP), grass silage fed with low amounts of concentrates (4% soybean meal; GS + LCON), grass silage fed with high amounts of concentrates (8% soybean meal; GS + HCON), and growth promotants used with feeding of grass silage and high concentrates (8% SBM; GS/GP + HCON). Grass silages are considered a poor substrate for rumen microbial protein synthesis and are low in ruminally undegradable protein (Titgemeyer and Loëst, 2001). Veira et al. (1994, 1995) demonstrated that the addition of soybean meal (between 4 and 8%, DM basis) to a grass silage-based diet resulted in a similar increase in ADG (+14%) for implanted growing cattle.
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) distributed at random within each of these groups. Each pen was equipped with Calan (American Calan, Northwood, NH) electronic headgates to allow individual measurement of feed intake. Before initiation of the study, steers were fed a grass silage diet and allowed to adapt to the feed and the electronic headgates for 21 d. Also, during the first 14 d of this period, steers administered growth promotants (GS/GP and GS/GP + HCON) were given the adaptation dose (11 mg of DM/kg) of monensin (Rumensin, Elanco, Division Eli Lilly Canada Inc., Guelph, Ontario, Canada). After that period, the dose was increased to 33 mg of DM/kg for the duration of the study.
The 5 management regimens each included separate growing (d 0 to 98) and finishing (d 99 to slaughter) phase treatments. On d 0 of the study, steers were introduced to their respective diets as described previously (Table 1). Animals assigned to treatments GS/ GP and GS/GP + HCON were implanted with Revalor G (40 mg of trenbolone acetate + 8 mg of estradiol; Hoechst-Roussel Agri- Vet, Somerville, NJ) and were reimplanted 70 d later with Revalor S (120 mg of trenbolone acetate + 24 mg of estradiol; Hoechst-Roussel Agri-Vet). During the growing phase (d 0 to 98), steers on all management regimens had ad libitum access to grass silage. Solvent-extracted soybean meal was fed at 4% of the diet to GS + LCON steers or at 8% of the diet to GS + HCON and GS/GP + HCON steers. The total mixed ration was fed once daily (0900 h) at an estimated 110% of the previous day’s ad libitum intake with water continuously available. The quantity of feed placed before every steer and the quantity left uneaten were weighed daily. After weighing steers on d 98, the diets of steers on treatment GS + LCON, GS + HCON, and GS/GP + HCON were gradually changed to energy-dense diets. Animals on treatment GS + LCON were offered a total mixed ration composed of grass silage and rolled barley (60:40, DM basis), whereas steers on treatments GS + HCON and GS/GP + HCON were offered a total mixed ration composed of grass silage and rolled barley (30:70, wt/wt, DM basis). Cattle were adjusted gradually to grain-based diets by offering 75% silage and 25% barley for 7 d followed by 60% silage and 40% barley for 4 d. Thereafter, the proportion of barley was increased by 10% every 4 d until it reached 80% of the diet DM. However, when barley was increased from 70 to 80%, steers on treatment GS + HCON, which received no ionophores, reduced their voluntary intake and exhibited signs of rumen acidosis. As soon as the proportion of barley was reduced to 70%, all animals resumed eating. It was therefore decided to limit barley to 70% of the diet (DM basis).
Cattle were slaughtered, using a backfat endpoint, after attaining at least 8 mm of backfat (measured by ultrasound) at the 3/4 position over the longissimus muscle between the 12th and 13th ribs. This backfat thickness has been shown to yield Canada A grade carcasses (CBGA, 2007) in previous trials (Veira et al., 1983; Petit and Flipot, 1992; Petit et al., 1994). Ultrasound determinations of backfat were conducted every 14 d, when half of the steers had deposited 6 mm at the 3/4 position over the longissimus dorsi (LD) muscle. After the final weighing, steers were given a mixture of electrolytes (1 kg/steer; Nutri-Charge, Research Management Services, Edmonton, Alberta, Canada) and feed. Steers were transported more than 900 km to a commercial abattoir near Sherbrooke, Québec, Canada.
Meat Quality Measurements
After carcass grading (6th day after slaughter), the striploin (13th rib-5th lumbar vertebra) was removed from both sides of each carcass and taken to the cutting room of the Agriculture and Agri-Food Canada Research Centre in Sherbrooke, Québec. The striploin from the right side of the carcass was frozen (–18°C) to be used for sensory evaluation. Each striploin from the left side of the carcass was stripped of fat, bone, and superficial muscle tissue leaving the LD muscle to be prepared for meat quality evaluation. Muscle pH was measured in the LD muscle at the interface between the 12th and 13th ribs on the 6th day postmortem using an Oakton Instruments Model pH 100 Series pH meter (Oakton Instruments, Nilis, IL) fitted with a spear-type electrode (Cole Parmer Instrument Company, Vernon Hills, IL) and an automatic temperature compensation probe. Two steaks (25-mm thick) were cut from the LD muscle. The first steak was exposed to atmospheric oxygen for a 1-h bloom period according to Honikel (1998). An instrumental color measurement was then recorded at the exposed surface using a Minolta Chromameter CR 300 (D65 light source with 0° viewing angle geometry, Konica Minolta, Tokyo, Japan) according to the reflectance coordinates (L*, a*, b*) of the Commission International de l’Éclairage (1976). A second steak was preweighed into a polystyrene tray, overwrapped with an oxygen-permeable film, and stored for 48 h at 2°C for gravimetric drip loss measurement. An adjacent chop (10 cm in length) was frozen for later analysis of cooking losses and Warner-Bratzler shear force (WBSF).
On the day of these analyses, LD chops were thawed at 4°C, trimmed, and squared to provide an approximately 300-g rectangular sample. Sample weight was recorded, and the samples were vacuum-packed in individual polyethylene bags. The cooking and shear force determination procedures were conducted following the standard protocol described by Boccard et al. (1981). In brief, cooking was performed in hot water at 75°C until a final cooking temperature of 72°C was attained in the core of the sample as determined with a Type T thermocouple (Cole Parmer Canada, Anjou, Quebec, Canada) inserted in the geometric center of each sample. A 40-min cold shower was immediately applied to the samples to stop the cooking process. The bags were then opened, and the meat samples were blotted and weighed for the calculation of cooking losses. After trimming 1 cm of meat from each side of the chop, ten 1-cm2 cross-sectional square cores that were parallel to the longitudinal orientation of the muscle fibers axis were taken at approximately the same location from each chop. Shear force measurements were carried out with a Warner-Bratzler device attached to a TA.XT.plus Texture Analyzer (Texture Technologies Corp., Scarsdale, NY). Cores were sheared across the fiber axis, with a 30-kg cell at a speed of 1.5 mm/sec and the 10 readings averaged.
The remainder of the fresh LD muscle was ground, vacuum-packed, and frozen (–20°C) pending proximate analysis. Dry matter content was determined by the air-drying method (method 950.46; AOAC, 1990) at temperatures of 100 to 102°C. Intramuscular fat content was measured by Soxtec extraction (Soxtec System HT6, Perstorp Analytical/Tecator Inc., Herndon, VA) with ethanol and dichloromethane as solvents. Protein content was analyzed (Leco Analyzer FP 2000, Leco Corp., Lakeview, MI) by a combustion method. Hydroxyproline concentration was measured according to Matissek et al. (1992) using a correction factor of 8 to convert hydroxyproline to collagen. Collagen solubility was determined using the procedure of Hill (1966). Total hematin was analyzed as described by Hornsey (1956), whereas the myofibrillar fragmentation index was measured according to the method described by Hopkins et al. (2000).
For the analysis of fatty acids, lipid extraction was carried out according to a modification of the procedure of Folch et al. (1957), using methylene chloride as a substitute for chloroform (Carlson, 1985). Methyl esters were prepared from extracted fat by base-catalyzed transmethylation, according to the method of Chouinard et al. (1997). Composition analyses of the fatty acids were carried out with a gas chromatograph (HP 5890A Series II, Hewlett Packard, Palo Alto, CA) equipped with a 100-m CP-Sil 88 capillary column (i.d., 0.25 mm; film thickness, 0.20 µm; Chrompack, Middelburg, the Netherlands) and a flame ionization detector. At the time of the sample injection, the column temperature was 80°C for 1 min and then was ramped at 2°C· min–1 to 215°C and maintained for 30 min. Inlet and detector temperatures were 220 and 230°C, respectively. The split ratio was 100:1. The flow rate for hydrogen carrier gas was 1 mL·min–1. Each fatty acid peak was identified and quantified using pure methyl ester standards (Nu Chek Prep., Elysian, MN). The column and method used gave a satisfactory separation of the major isomers of C18:1 and C18:2 fatty acids (Figure 1).
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). The cubes were placed into amber glass containers with plastic covers (2 cubes per container). This method, that was described previously by Graveline et al. (2006), allows uniform cooking temperature because the pieces of meat have the same size, reduces sample handling after cooking, fastens the presentation to judges, and preserves the flavors inside the container. Meat cubes were cooked in a preheated, forced-air steam oven [i.e., the Voss cooking cell (Voss 813-S, VossGesllschaft mbH, Stardedt, Germany)]. Meat cubes were cooked at a temperature of 68°C for 15 to 18 min, until an internal temperature of 65°C was reached in the core of the sample, as determined with a Dual J_T_E_K thermocouple thermometer (Barnanat Co., Barrington, IL) inserted in the geometric center of each sample. The samples were then taken to the sensory evaluation laboratory and were immediately placed in incubators at a temperature of 50°C, where they remained until 30 min before the trials took place.
Sensory attributes were analyzed by descriptive analysis, as defined by Meilgaard et al. (2007). Palatability attribute definitions and scale values included the following: toughness, which is the resistance of the sample to a very slight opening and shutting of the jaws (1 being very soft and 15 very tough; Meilgaard et al., 2007); juiciness, which is the overall impression of moistness perceived in the mouth after 5 to 7 chews (0 being very little and 15 being very much; Meilgaard et al., 2007); and flavor, which is the amount of full meaty flavor present after 8 chews (0 being very weak and 7 being very intense; Wood et al., 1995).
Scores were given using the notation test method (Meilgaard et al., 2007). For this notation test, 11 expert sensory evaluators were recruited and trained. After the performance assessments, 10 judges were finally selected to participate in the sensory panel. Panelists took part in 3 replications, during which all beef from all 5 management regimens was presented monadically in a randomized complete block design. The panelists rated the intensity of the texture attributes by making a mark on a 15-cm horizontal line that corresponded to the amount of the perceived stimulus. The panelist rated the intensity of the global flavor by making a mark on a horizontal line (7 cm), the left end of which corresponded to none and the right end to very strong.
Statistical Analysis
Five pens with 8 Calan gates per pen were divided in 2 sides, with 4 Calan gates per side, resulting in 10 pen-side combinations (blocks). The 5 management regimen treatments were assigned to calves according to a balanced, incomplete block design. Each steer (n = 40) was considered an experimental unit in a statistical model that included block as a random effect and management regimen as a fixed effect. The MIXED procedure (SAS Inst. Inc., Cary, NC) was used to analyze a 2 x 2 factorial separation of growth promotants (none versus implants and ionophores) and concentrates (none with finishing exclusively on forages versus feeding forage and grain) including the interaction to study differences between treatments GS, GS/GP, GS + HCON, and GS/GP + HCON. Furthermore, a single contrast was added to compare treatments GS/GP vs. GS + LCON to determine if growth promotants (GS/ GP) could be replaced by feeding moderate amounts of concentrates (GS + LCON) in the growing-finishing phase. The data obtained from the sensory score test were analyzed with PROC MIXED of SAS (SAS Inst. Inc.) using management regimen as a fixed effect and sessions and judges as random effects.
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RESULTS AND DISCUSSION |
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONLITERATURE CITED |
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; Scanga et al., 1998; Roeber et al., 2000), in which ionophores were usually incorporated into the diet. In the present study, 2 dark cutters (based on pH > 6) were observed, with both occurring in steers administered growth promotants. In agreement with Foutz et al. (1997) and Roeber et al. (2000), no differences in muscle pH and color lightness (L*) were found in this study when growth promotants were used (Table 2). However, a growth promotant x concentrates interaction (GP x CON) was present (P = 0.04) for the a* value as muscle redness increased in grass silage-fed cattle administered growth promotants in contrast to a decrease in muscle redness when growth promotants were used with the high (70%)-concentrate diet. This interaction is supported by a similar interaction (P = 0.04) in the hematin content of LD muscle with increases in hematin with use of growth promotants with grass silage feeding as compared with decreased hematin when growth promotants were used with the high concentrate diet.
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in that the cooking losses were greater (P = 0.02) when growth promotants were administered to steers fed grass silage as compared with cattle fed low amounts (40%) of concentrates. Growth promotants increased (P < 0.01) WBSF regardless if cattle were finished on grass silage or a high-concentrate diet. Our results contrast with those of Xiong et al. (1996), in which growth promotants improved beef tenderness in semimembranosus muscle (top round) aged at least 5 d. Aging should not be a factor of variation between studies, because tougher beef was still found at 14 d aging time in implanted steers (Roeber et al., 2000; Scheffler et al., 2003). The different response to growth promotants on WBSF values between Xiong et al. (1996) and the present study may be due to muscle differences for the effects of growth promotants on beef tenderness (Ouali et al., 1988). Because IMF and collagen content did not differ (P > 0.05) with use of growth promotants, the increased tenderness of beef from steers fed without growth promotants may be mostly attributable to increased postmortem proteolytic changes during aging as demonstrated by the greater myofibrillar fragmentation index (P = 0.02) versus beef produced using growth promotants. Postmortem myofibril fragmentation accounts for more than 50% of the variation in beef tenderness (Culler et al., 1978). Wheeler and Koohmaraie (1992) previously reported a detrimental effect of growth promotants on postmortem myofibrillar protein degradation via a lower activity of the proteolytic enzymes resulting in increased meat toughness. Similar to other studies (Xiong et al., 1996; Foutz et al., 1997; Smith et al., 2007), meat composition was not influenced by growth promotants when examining DM, IMF, protein, and total and soluble collagen.
Beef from animals grazing grass is usually darker and tougher than beef from animals fed concentrates (Priolo et al., 2001). The reduced color lightness in the meat of grass-fed steers is usually linked to greater ultimate pH (pHu), lower marbling, or increased myoglobin content in the muscle; these traits are frequently associated with the older age at slaughter for steers finished on grass (Coulon and Priolo, 2002). The increase in the pHu value is either due to lower glycogen reserves in the muscle or reduced fat coverage leading to faster cooling rates and slower postmortem pH fall (Farouk and Lovatt, 2000; Immonen et al., 2000). In the present study, in which beef were slaughtered at the same backfat level, diet (grass silage or concentrate) had no influence (P > 0.05) on pHu, color lightness (L*), or drip loss, agreeing with several studies evaluating meat quality traits in grass- and concentrate-fed steers (French et al., 2001; Farouk and Wieliczko, 2003; Varela et al., 2004). According to Hedrick et al. (1983), the influence of diet composition on the color characteristics of beef is reduced by the transformation processes that occurs in the rumen. However, as observed before, either feeding a high (70%)-concentrate or a grass silage diet influenced the beef redness score (a* value) only in combination with growth promotants (GP x CON; P = 0.04). This result disagrees with Farouk and Wieliczko (2003), who observed a redder and more vivid color in grain- versus grass-finished beef.
As observed in previous studies (Bowling et al., 1977; Dinius and Cross, 1978; Varela et al., 2004), no differences in cooking losses were found in beef-fed grass silage or concentrate diets alone in this study. In contrast, Hedrick et al. (1983) and Mandell et al. (1998) reported either lower or higher cooking losses, respectively, in beef fed concentrates.
Improved tenderness in beef from cattle finished on grain diets is often reported in the literature (Bowling et al., 1977; Hedrick et al., 1983; Larick et al., 1987). This effect may be either explained by an increased fat deposition in cattle fed high-concentrate diets, which prevents cold-shortening of the carcass, or by the younger age of the animal at slaughter given the age effect on collagen maturity (Sheath et al., 2001). However, the present study found no differences (P > 0.05) in WBSF values due to diet, in agreement with previous research (Reagan et al., 1977; Xiong et al., 1996).
The feeding of the high-concentrate diet increased (P < 0.05) DM and IMF content while reducing (P < 0.02) total collagen concentration versus cattle finished on grass silage. Concentrate feeding also increased IMF content in previous studies (Schaake et al., 1993; Mandell et al., 1998). The reduced total collagen concentration can be explained by the greater growth rate of concentrate-fed steers and subsequently younger age at slaughter (McCormick, 1994; Berthiaume et al., 2006). However, soluble collagen content was similar (P > 0.05) across diets, indicating the lack of effect of the greater dietary energy intake on this variable as previously reported by Dikeman et al. (1986) and Mandell et al. (1998).
The lack of relationship between IMF and collagen content and beef tenderness has also been reported by Mandell et al. (1998) and French et al. (2001). The influence of IMF content on beef palatability reported in the literature is controversial (Nishimura et al., 1999; Jeremiah et al., 2003). In a comparative study between breeds containing the same IMF content in the LD muscle, Chambaz et al. (2003) reported variation in beef tenderness and thus concluded that the effect of IMF content on beef palatability is limited. According to Listrat et al. (1999), the effect of collagen content on meat tenderness is muscle-dependent, with the relationship being greater in muscles with greater collagen content (i.e., the semitendinosus muscle), compared with muscles containing low amounts of collagen, such as the LD muscle evaluated in the present study.
Fatty Acid Composition
The use of growth promotants (ionophores and implants) affected the fatty acid profile of the LD muscle (Table 3), most likely due to their combined effects on ruminal fermentation and animal physiology. In the current study, the use of growth promotants increased (P < 0.05) the proportions of C17:0, C18:0, C20:0, trans isomers of C18:1, and cis-9, trans-11 C18:2 tended to decrease (P = 0.06) the proportion of cis-9 C14:1 and reduced (P = 0.03) cis-9 C16:1. The addition of ionophores increased microbial lipid synthesis in vitro (O’Kelly and Spiers, 1990), which may explain the greater proportion of C17:0 synthesized by ruminal microorganisms using odd-chain volatile fatty acids as substrate. The increased concentration of trans isomers of C18:1 is likely the result of the ionophore supplementation, which is known to modify ruminal bacterial populations and can interfere with the process of ruminal biohydrogenation. Fellner et al. (1997) and Jenkins et al. (2003) reported increases in the concentration of trans C18:1 in ruminal contents during in vitro incubations with the ionophore monensin as compared with control incubations. Jenkins et al. (2003) further identified that the effect of monensin was specific to trans-10 C18:1, which was not apparent in the current experiment.
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reported an increase in saturated fatty acids (including C18:0) and a decrease in monounsaturated fatty acids with hormonal implants. The use of growth promotants in the present study increased (P < 0.05) C18:0 and C20:0 while decreasing cis-9 C14:1, cis-9 C16:1, and the desaturase indexes for C16:0 and C18:0. These effects are most likely attributable to the use of hormonal implants, although the mechanism involved remains to be elucidated. There were very few interactions between diet composition (grass silage with or without concentrate) and growth promotants (ionophores/implants) except for the increase in total trans C18:1 content in the LD muscle observed with growth promotants, which tended to be more pronounced with feeding a high-concentrate diet as compared with feeding a grass silage-based diet (P = 0.06). However, this tendency was not apparent when trans C18:1 isomers were considered individually.
The feeding of the low-concentrate (40%) diet modified the fatty acid profile of LD muscle as compared with cattle fed grass silage in conjunction with growth promotants. When P-values for the management regimen were significant (P < 0.05), numerical values for the proportions of individual fatty acids in beef from cattle fed the low-concentrate diets tended to be intermediate between values observed for beef from cattle fed grass silage and high-concentrate diets.
The feeding of high-concentrate diets increased (P < 0.01) the proportion of cis-9 C14:1, cis-9 C16:1, and cis-9 C18:1 and decreased (P = 0.04) the concentrations of C18:0 as compared with forage feeding. Daniel et al. (2004) reported that feeding growing lambs a high-concentrate diet increased plasma insulin concentration as compared with feeding a diet based on grass pellets. It was also previously observed that insulin significantly increased the expression of the enzyme 
9-desa-turase in adipose tissue of the same animal model (Ward et al., 1998). This effect of high-concentrate diets on blood insulin concentration may explain greater proportions of monounsaturated fatty acids and lower de-saturase indices in IMF as compared with grass silage-fed cattle in the present study, although the mechanism was not assessed by actual insulin measurements. The forage-based diets increased (P < 0.01) the proportion of cis-9, cis-12, cis-15 C18:3 in the LD muscle as compared with feeding concentrates, agreeing with previous studies with beef cattle comparing alfalfa silage (Mandell et al., 1998) or pasture (French et al., 2000) feeding with diets containing large amounts of grain. Although Sackmann et al. (2003) noted that more than 90% of dietary cis-9, cis-12, cis-15 C18:3 (
linolenic acid, ALA) may undergo biohydrogenation in the rumen, the rich sources of ALA found in the grass silage diet of the current study or in pasture (Lorenz et al., 2002) lead to an increase in the incorporation of several other isomers of the n-3 family in muscle. After absorption, ALA goes through a series of reactions involving chain elongation and desaturation (Raes et al., 2004), leading to the deposition of greater proportions of C18:4 (cis-6, cis-9, cis-12, cis-15), C20:4 (cis-8, cis-11, cis-14, cis-17), C20:5 (cis-5, cis-8, cis-11, cis-14, cis-17), C22:5 (cis-7, cis-10, cis-13, cis-16, cis-19), and C22:6 (cis-4, cis-7, cis-10, cis-13, cis-16, cis-19). Another explanation for the greater proportions of n-3 fatty acids observed in the LD muscle of cattle finished on grass silage might be that IMF content was lower with this treatment compared with the high-concentrate diet. The PUFA such as long-chain n-3 fatty acids are more commonly found in the membrane phospholipids (Bas and Sauvant, 2001; Raes et al., 2004), which represent a greater proportion of total lipids in leaner meat (Bas and Sauvant, 2001). Similarly, a greater proportion of triacylglycerols rich in saturated and monounsaturated C18 fatty acids may partially explain the greater proportion of those fatty acids in LD muscle of cattle finished on the high-concentrate diet.
The greater (P < 0.01) proportions of n-3 fatty acids combined with a lack of effect of management regimens on several n-6 fatty acids led to a decrease (P < 0.01) in the ratio of n-6 to n-3 fatty acids in the LD muscle of cattle fed the grass silage-based diet. Lowering the ratio of n-6 to n-3 fatty acids in food products has been recommended to prevent or modulate certain diseases in humans (Connor, 2000).
The grass silage diets also increased the proportion of several intermediates of ruminal biohydrogenation of PUFA in the LD muscle as compared with beef from cattle fed concentrate diets. According to Harfoot (1981), the biohydrogenation pathway of cis-9, cis-12, cis-15 C18:3, a major fatty acid found in grass forage (Boufaïed et al., 2003), involves the production of cis-9, trans-11, cis-15 C18:3; trans-11, cis-15 C18:2; trans-11 C18:1; and, ultimately, C18:0, which can all be absorbed during the process.
Forage feeding also increased (P < 0.01) the proportion of cis-9, trans-11 C18:2 in the LD muscle of steers. This CLA isomer is produced during ruminal biohydro-genation of cis-9, cis-12 C18:2 but not cis-9, cis-12, cis-15 C18:3 (Harfoot, 1981). Griinari and Bauman (1999) have shown that cis-9, trans-11 C18:2 can also be synthesized endogenously from trans-11 C18:1 through the activity of the enzyme 9-desaturase. Feeding a high-forage diet may therefore have increased the rate of appearance of trans-11 C18:1 in the rumen, providing more substrate for the endogenous production and deposition of CLA in bovine tissues. This hypothesis is consistent with Sackmann et al. (2003), who observed an increase of trans-11 C18:1 concentration with no effect on cis-9, trans-11 C18:2 in duodenal contents of Hereford steers fed increasing levels of grass hay.
Contrary to the other trans isomers of C18:1, feeding high-concentrate diets increased (P = 0.03) the concentration of trans-10 C18:1 in the LD muscle as compared with grass silage-based diets. The feeding of high-concentrate diets alters ruminal fermentation and is associated with modification of rumen biohydrogenation processes. In particular, Griinari and Bauman (1999) reported a shift in major hydrogenation pathways when high-concentrate diets are fed, characterized by a decrease in the formation of trans-11 C18:1 and an increase in the production of trans-10 C18:1 in the rumen of dairy cows. The same phenomenon seems to be apparent in the growing steers used in the current experiment.
Overall, management regimen did not affect (P > 0.05) proportions of saturated medium-chain fatty acids (C12:0, C14:0, and C16:0) and proportions of several PUFA of the n-6 family (cis-9, cis-12 C18:2; cis-6, cis-9, cis-12 C18:3; and cis-8, cis-11, cis-14 C20:3).
Sensory Data
Except for toughness, management regimen did not affect (P > 0.05) juiciness or flavor attributes of beef (Table 4). As previously observed for WBSF (Table 2), the use of growth promotants tended to increase (P = 0.06) toughness in the striploin of implanted beef. The detection of the panelists of tougher beef in implanted steers and the relative confirmation of the WBSF results are surprising based on the different sensitivity between the 2 methods for picking up differences in sensory traits (Lorenzen et al., 2003). Differences in the methodology are represented by the cooking method (Obuz et al., 2004) and time of toughness evaluation relative to completion of cooking (Caine et al., 2003). Warm beef was, in fact, used for sensory evaluation of toughness, whereas beef samples at room temperature were used in the determination of WBSF. In this study, the lower endpoint cooking temperature (65°C) of striploin samples prepared for the sensory evaluation compared with that for the WBSF analysis (72°C) may have reduced the effects of growth promotants on sensory toughness. Indeed, according to Obuz et al. (2004), WBSF values linearly increase from an endpoint temperature of 60°C.
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; Lochner et al., 1980; Fishell et al., 1985). However, in this study, this explanation cannot be confirmed, because steers fed concentrates grew faster and produced heavier carcasses compared with forage-fed ones (Berthiaume et al., 2006). The literature either reports differences or no differences in juiciness and flavor between forage and grain-fed beef (Oltjen et al., 1971; Young and Kauffman, 1978; Mandell et al., 1998; Muir et al., 1998; French et al., 2001). As in French et al. (2001), the lack of effect of the management regimen on these sensory traits in the present experiment may be explained by the narrow range of variation in concentrations of IMF and linolenic acid (C18:3). In this study, the use of growth promotants produced tougher meat and dark cutters. It also tended to increase the proportion of saturated fatty acids when expressed as a percentage of total fatty acids in muscles, but with no effect on beef flavor. Contrary to previous studies, forage feeding did not produce darker beef. However, it increased the proportion of n-3 fatty acids and CLA when expressed as a percentage of total fatty acids in muscles as compared with feeding high-grain diets. Overall, these findings will be of interest to beef producers who invest in the niche production of forage-finished, natural beef to meet the quality demands of health-conscious consumers.
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Footnotes |
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1 Corresponding author: faucitanol{at}agr.gc.ca
Received for publication November 26, 2007. Accepted for publication March 21, 2008.
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LITERATURE CITED |
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