Fatty acid composition of beef from Nguni steers supplemented with Acacia karroo leaf-meal

Journal of Food Composition and Analysis 24 (2011) 523–528

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Original Article

Fatty acid composition of beef from Nguni steers supplemented with Acacia karroo leaf-meal C. Mapiye a, M. Chimonyo b,*, K. Dzama c, A. Hugo d, P.E. Strydom e, V. Muchenje a a

Department of Livestock and Pasture Science, University of Fort Hare, P. Bag X1314, Alice 5700, South Africa Discipline of Animal and Poultry Science, University of KwaZulu-Natal, King Rd, P. Bag X01, Scottsville 3209, South Africa c Department of Animal Sciences, Stellenbosch University, P. Bag X1, Matieland 7602, South Africa d Department of Microbial Biochemical and Food Biotechnology, University of Free State, P.O. Box 339 Bloemfontein 9300, South Africa e Meat Industry Centre, Department of Nutrition and Food Science, Agricultural Research Council, Private Bag X2, Irene 0062, South Africa b

A R T I C L E I N F O

A B S T R A C T

Article history: Received 12 January 2011 Accepted 20 January 2011 Available online 12 February 2011

The objective of this study was to determine fatty acid composition of beef from Nguni cattle supplemented with Acacia karroo leaf-meal. Thirty 19-month-old steers were randomly assigned to A. karroo leaf-meal (AK), sunflower cake (SF) and the control with no supplement (CN) diets. The m. longissimus thoracis et lumborum was sampled for analyses. Highest a-linolenic acid and docosapentaenoic acid, and lowest n6/n3 ratios were recorded in beef from steers that received AK diet (P < 0.05). Myristic acid and palmitic acid proportions were lower (P < 0.05) while n3 PUFA proportions were higher (P < 0.05) in beef from steers that were given AK and CN diets than those on the SF diet (P < 0.05). It was concluded that the inclusion of A. karroo in cattle diets can improve fatty acid composition of beef from a human health perspective. ß 2011 Elsevier Inc. All rights reserved.

Keywords: Eicosapentaenoic acid Human health a-Linolenic acid Nguni beef Palmitic acid Food analysis Food composition

1. Introduction Recently, consumers have increased preference for naturally/ organically produced animal products that do not adversely affect their health (Alfaia et al., 2007, 2009; Muchenje et al., 2009a). Nutritionists recommend a reduction in total fat intake, particularly of saturated fatty acids (SFA) and trans fatty acids, which are associated with an increased risk of cardio-vascular diseases and some cancers (Burlingame et al., 2009; Brouwer et al., 2010; USDA and HHS, 2010). Besides reducing fat intake, nutritionists urge consumers to increase intake of polyunsaturated fatty acids (PUFA), particularly the n3 PUFA at the expense of n6 PUFA (Simopoulos, 2004; Harris et al., 2009; Griffin, 2008). In addition, Givens and Gibbs (2008) and Givens (2009) recommended an increased consumption of long chain n3 PUFA such as eicosapentaenoic (20:5n3) and docosahexaenoic (22:6n3) acids, which are linked to the development and functionality of nervous, vision and immune systems and have cardio-protective and anticarcinogenic functions (Smit et al., 2009; McAfee et al., 2010; Molendi-Coste et al., 2011). The PUFA/SFA and n6/n3

* Corresponding author. Tel.: +27 767309002; fax: +27 33 260 5067. E-mail addresses: [email protected] (C. Mapiye), [email protected] (M. Chimonyo), [email protected] (V. Muchenje). 0889-1575/$ – see front matter ß 2011 Elsevier Inc. All rights reserved. doi:10.1016/j.jfca.2011.01.018

PUFA ratios have, therefore, become some of the most important parameters in evaluating the nutritional value and healthiness of foods (Aldai et al., 2005; Alfaia et al., 2007; Riediger et al., 2009). The fatty acid composition of beef is mainly influenced by genotype, feeding regime, age and gender (Aldai et al., 2007; Zapletal et al., 2009). Appropriating certain biological types of cattle to proper dietary regimens hold the most potential to improve fatty acid profiles and, consequently beef healthiness (Baublits et al., 2006; Muchenje et al., 2008a; Orellana et al., 2009). In southern Africa, for example, research has shown that under rangeland conditions, the Nguni breed has favourable carcass and meat quality characteristics (Muchenje et al., 2008b,c; Strydom, 2008), fatty acid profile (Muchenje et al., 2009b) and sensory attributes (Muchenje et al., 2008a). Resource-poor beef producers in southern Africa use indigenous browse legume trees, such as the Acacia genus that contain high concentration of condensed tannins as feed supplements (Mokoboki et al., 2005). Dietary condensed tannins have been reported to protect dietary lipids from biohydrogenation in the rumen (Khiaosa-Ard et al., 2009) and reduce microorganisms that are responsible for ruminal biohydrogenation (Molan et al., 2001; Vasta et al., 2009a). Condensed tannins, thus, positively influence meat fatty acids composition (Min et al., 2005; Vasta et al., 2009b). In southern Africa, leaves from Acacia karroo, a condensed tannin-rich (80–110 g/kg DM), densely populated and widespread indigenous leguminous tree, are being used as a protein supplement

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by smallholder cattle producers (Aganga et al., 1998; Mokoboki et al., 2005) with the potential to manipulate beef fatty acid composition. A. karroo leaves are rich in minerals (Aganga et al., 1998) and have antihelmintic effects (Xhomfulana et al., 2009). Although A. karroo is not readily browsed by cattle due to the presence of thorns and condensed tannins (Mokoboki et al., 2005), its leaf-meal has been reported to improve growth performance, carcass characteristics (Mapiye et al., 2009), protein content and fresh appearance of meat (Mapiye et al., 2010) from Nguni cattle. No reported work, however, has evaluated the effect of feeding A. karroo leaves on fatty acid composition of beef from local cattle genotypes raised under rangeland conditions. The objective of the current study was, therefore, to determine fatty acid composition of meat from Nguni cattle supplemented with A. karroo leaf-meal.

2.4. Chemical analyses of feed ingredients A. karroo, sunflower cake, veld hay and fresh herbage were assessed for dry matter (DM), crude protein (CP), calcium and iron using the Association of Official Agricultural Chemists (AOAC) (2003) procedures. Neutral detergent fibre (NDF) and acid detergent fibre (ADF) of the feed ingredients were determined according to Van Soest et al. (1991). In vitro dry matter disappearance and in vitro NDF disappearance after 48 h were determined using the Daisy ANKOM system based on Tilley and Terry (1963) and modified by Van Soest and Robertson (1985). Condensed tannins (CT) assays were performed colorimetrically with butanol-HCl method (Bate-Smith, 1981) using purified CT from Desmodium intortum as reference standard. Total phenolics were assayed colorimetrically according to Price and Butler (1977).

2. Material and methods 2.5. Average daily gains and body condition scores 2.1. Study site The study was conducted at the University of Fort Hare farm, Alice, South Africa. It lies along longitude 328780 E and latitude 268850 S at an altitude of 450–500 m above sea-level. It is located in the False Thornveld of the Eastern Cape Province, which is characterised by mean annual rainfall of 480 mm and mean annual temperature of 18.7 8C, respectively. Most rain falls in summer. Cattle graze on natural pastures mainly composed of the following grass species; Aristida congesta, Cympopogon plurinodis, Cynodon dactylon, Digitaria eriantha, Sporobolus africanus, Sporobolus fimbriatus, Themeda triandra and Eragrostis species. A. karroo, Scutia myrtina and Maytenus polyacantha are the dominant tree species. 2.2. Treatments and feeding management Thirty 19-month-old Nguni steers, which had a mean weight of 241.5  14.62 kg, were randomly assigned to three dietary treatments (10 steers per feeding treatment): A. karroo leaf meal (AK), sunflower cake (SF) and the control diet with no supplement (CN), from April 2008 to June 2008. The A. karroo leaf-meal was prepared between February and March 2008 as described by Mapiye et al. (2009, 2010) In addition to natural pasture, steers on the AK and SF diets were offered 1500 g and 650 g of feed, respectively, to supply 150 g of protein per day. Natural pasture hay (300 g) was mixed with 1500 g of A. karroo foliage to improve palatability. The hay used for the AK treatment group was harvested in the same paddocks where the steers were grazing. Steers on the CN diet relied entirely on natural pasture without any feed supplement. 2.3. Animal management All the steers were ear-tagged for easy identification. Steers on the same treatment were kept in one paddock. Steers on supplementary diets were allowed 21 days to adapt to their respective diets prior to the 60-day supplementary feeding trial. Each steer on the supplementary diet was fed individually. These steers were trained for 14 days during the adaptation period to feed from individual troughs. The feed supplements were offered daily at 0830 h. All the steers were released daily for grazing at 1000 h and penned at 1730 h throughout the trial period. The three treatment groups were continuously rotated on three paddocks (2 ha each) every seven days. This was done to avoid variations in quality and quantity of forage consumed that is attributable to differences in plant growth or micro-environmental condition of the paddocks. Clean tap water was freely accessible to the experimental animals. All steers were neither de-wormed nor dipped throughout the trial.

Individual weights of the steers were measured using a heavy duty scale (Cattleway, Johannesburg, South Africa) every fortnight. Average daily gain (ADG) (g/day) between the initial weight and slaughter weight was computed for each steer. Body condition scores (BCS) were visually appraised using a 5-point scale (1-very thin and 5-very fat) every fortnight (Nicholson and Butterworth, 1986). 2.6. Slaughter procedures Steers were slaughtered using the captive bolt method in June 2008 at 21 months of age, following the commercial procedures at the East London Abattoir. The m. longissimus thoracis et lumborum (LTL) of the right side was sampled, a day after slaughter, from the 10th rib in the direction of the rump and a 100 mm thick piece of the posterior side of the right LTL was taken and vacuum-packaged at 3 8C, pending fatty acid analysis. 2.7. Determination of fat, fatty acid profiles of feed ingredients and meat samples Total lipid from AK, SF and CN were extracted according to AOAC (2003) procedures for determination of fatty acids. Total lipid from muscle samples were quantitatively extracted, according to the method of Folch et al. (1957), using chloroform and methanol in a ratio of 2:1. An antioxidant, butylated hydroxytoluene was added at a concentration of 0.001% to the chloroform: methanol mixture. A rotary evaporator was used to dry the fat extracts under vacuum and the extracts were dried overnight in a vacuum oven at 50 8C, using phosphorus pentoxide as a moisture adsorbent. Total extractable intramuscular fat was determined gravimetrically from the extracted fat and expressed as percent fat (w/w) per 100 g tissue. The extracted fat from feed and muscle was stored in a polytop (glass vial, with push-in top) under a blanket of nitrogen and frozen at–20 8C pending analyses. Approximately 10 mg of extracted lipid was transferred into a Teflon-lined screw-top test tube. Fatty acid methyl esters (FAMEs) were prepared for gas chromatography by methylation of the extracted fat, using methanol–BF3 (Christie et al., 2001). FAMEs were quantified using a Varian GX 3400 flame ionization GC, with a fused silica capillary column, Chrompack CPSIL 88 (100 m length, 0.25 mm ID, 0.2 mm film thickness). Analysis was performed using an initial isothermic period (40 8C for 2 min). Thereafter, temperature was increased at a rate of 4 8C/min to 230 8C. Finally an isothermic period of 230 8C for 10 min followed. FAMEs n-hexane (1 ml) were injected into the column using a Varian 8200 CX Autosampler with a split ratio of 100:1. The injection port and detector were both maintained at 250 8C. Hydrogen, at 45 psi,

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Table 1 Nutritive value of the dietary components. Nutritive value (g/100 g DM)

N Dry matter Crude protein Fat Neutral detergent fibre Acid detergent fibre 48 h DM disappearance 48 h NDF disappearance Condensed tannins Total polyphenols Calcium Iron (mcg/g DM)

Supplement

Natural pasture

s.e.m.

A. karroo

Sunflower

Hay

March

April

May

June

5 89.7a 14.9b 2.0b 50.2a 29.0b 58.0b 44.0b 7.4b 30.5c 0.4b 336.8b

5 91.1ab 35.9c 2.5c 51.6a 18.0a 43.0a 41.0 a 0.1a 12.3b 0.1a 240.3a

5 92.5b 3.6a 2.2 a 65.6b 40.4c 55.0b 51.0c 0.1a 3.9 0.1a 421.6b

5 92.4b 4.0 a 2.2 a 64.9b 38.4c 54.0b 50.0c 0.2a 7.5 0.1a 432.6b

5 92.5b 3.2 a 2.2 a 63.7b 40.8c 53.0b 50.0c 0.1a 5.6a 0.1a 370.1b

5 92.1b 3.4a 2.2 a 62.6b 39.6c 51.0b 50.0 c 0.2a 6.1a 0.1a 310.7b

5 92.7b 3.3 a 2.2 a 64.8b 41.7c 54.0b 51.0c 0.2 a 3.8a 0.1 a 326.8b

0.70 1.13 0.16 1.52 2.04 1.55 1.08 0.60 3.08 0.05 45.52

Values in the same row with different superscripts are different at P < 0.05. s.e.m.: standard error of means.

functioned as the carrier gas, while nitrogen was employed as the makeup gas. Varian Star Chromatography Software recorded the chromatograms. Fatty acid methyl ester samples were identified by comparing the retention times of FAME peaks from samples with those of standards obtained from Supelco (Supelco 37 Component Fame Mix 47885-U, Sigma-Aldrich Aston Manor, Pretoria, South Africa). Conjugated linoleic acid (CLA) standards were obtained from Matreya Inc. (Pleasant Gap, Unites States). These standards included: cis-9, trans-11; cis-9, cis-11, trans-9, trans-11 and trans-10, cis-12-18:2 isomers. All other reagents and solvents were of analytical grade and obtained from Merck Chemicals (Pty Ltd, Halfway House, Johannesburg, South Africa). Fatty acids were expressed as the proportion of each individual fatty acid to the total of all fatty acids present in the sample. The following fatty acid combinations were calculated: omega3 (n3) fatty acids, omega6 (n6) fatty acids, total saturated fatty acids (SFA), total monounsaturated fatty acids (MUFA), polyunsaturated fatty acids (PUFA), PUFA/SFA ratio (P/S) and n6/n3 ratio.

highest in A. karroo. Highest dietary total SFA were found in the control diet followed by A. karroo and sunflower cake, in that order. Sunflower cake had the highest total MUFA, n6 PUFA, PUFA/SFA ratio and n6/n3 ratio compared to other diets. The n3 PUFA proportion was highest for A. karroo (P < 0.05) compared to SF and CN diets. 3.2. Supplementary feed intake, average daily gain and body condition scores Steers which were supplemented with sunflower cake diet consumed all their daily feed allocation. Steers that received SF diet had the highest average daily gains followed by those on the AK diet (P < 0.05; Table 3). Body condition scores at slaughter were higher (P < 0.05) in steers that received the AK and SF diets than those on the CN diet (Table 3). Highest slaughter weight was recorded in steers supplemented with sunflower cake followed by those supplemented with A. karroo leaf-meal. Nguni steers that were supplemented with SF diet had higher (P < 0.05) intramuscular fat than those that received AK and CN diets.

2.8. Statistical analyses 3.3. Meat fatty acid composition The nutritive value of the dietary components was analysed using General Linear Model procedure of SAS (2003). A similar model was used to determine the effect of diet on overall ADG, BCS at slaughter, slaughter weight, intramuscular fat and meat fatty acid composition. The significant differences between least square group means were compared using the PDIFF option of SAS (2003). 3. Results 3.1. Nutritive value of the experimental diets The nutritional composition and amount of condensed tannins in the experimental diets are shown in Table 1. Sunflower cake had the highest crude protein content (3.6 g/100 g DM) followed by A. karroo (14.9 g/100 g DM). A. karroo had the highest ADF (2.9 g/ 100 g DM), condensed tannins (7.4 g/100 g DM), total polyphenols (30.4 g/100 g DM), and calcium (0.38 g/100 g DM) concentrations. In vitro dry matter digestibility, NDF disappearance after 48 h and iron concentration were higher (P < 0.05) for the A. karroo diet than for sunflower cake. Table 2 shows the fatty acid composition of the dietary components. The most abundant fatty acids in A. karroo, sunflower cake and natural pasture were 18:3n3, 18:2n6 and 16:0, respectively. The proportions of 16:0 and 18:0 were highest for A. karroo compared to SF and CN diets. Concentrations of 18:1c9 and 18:2n6 were highest in sunflower cake while 18:3n3 was

Least square means of fatty acid proportions in beef from Nguni steers given three supplementary diets are shown in Table 4. Overall, the predominant fatty acids in intramuscular fat of Nguni Table 2 Least square means and standard error of means (s.e.m.) of fatty acids composition (% total fatty acid) of the natural pasture, Acacia karroo and sunflower cake. Fatty acid

Control

A. karroo

Sunflower cake

s.e.m.

n 12:0 14:0 16:0 18:0 18:1c9 18:2n6 18:3n3 Total saturated fatty acids (SFA) Total Monounsaturated Fatty Acids (MUFA) Total polyunsaturated fatty acids (PUFA) PUFA/SFA n6/n3

10 11.00b 3.88b 25.35b 7.11b 9.32b 22.96b 20.40b 47.38c

10 1.15a 3.82b 28.74c 9.16c 5.80a 16.94a 34.36c 42.88b

10 a nd 0.12a 9.91a 4.28a 31.50c 53.00c 0.93a 14.31a

0.107 0.038 0.191 0.107 0.095 0.159 0.061 0.232

9.44b

5.86a

31.65c

0.111

a

b

b

0.216

3.77c 56.86c

0.017 0.215

43.35

0.91a 1.12b

51.29

1.20b 0.49a

53.94

Values in the same row with different superscripts are different at P < 0.05. s.e.m.: standard error of means. a nd: not detected.

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526

Table 3 Least square means and standard error of means (s.e.m.) of average daily gain (ADG), body condition score at slaughter (BCS), slaughter weight, carcass traits and intramuscular fat of Nguni steers given three different diets. Parameter

N ADG (g/day) BSC at slaughter Slaughter weight (kg) Intramuscular fat (%)

Diet Control

A. karroo

Sunflower cake

s.e.m.

10 270.3a 1.5a 259.4a 0.87a

10 305.4b 1.8b 280.1b 0.88a

10 380.0c 1.9b 294.5c 1.2b

33.09 0.03 3.65 0.11

Values with similar superscripts in a row are not different (P < 0.05). s.e.m.: standard error of means.

steers were palmitic acid (16:0), stearic acid (18:0) and oleic acid (18:1c9), in that order. Meat from Nguni steers supplemented with sunflower cake had higher (P < 0.05) 14:0 and 16:0, and lower (P < 0.05) 15:1c10 proportions than those that received AK and CN diets. Nguni steers that received AK diet had lower (P < 0.05) (18:1t11) and higher (P < 0.05) 18:3n3 than those that received SF and CN diets. Proportions of 20:4n6 and 20:5n3 were higher (P < 0.05) in meat from steers that received the AK diet than those that were given the SF diet. Highest 20:3n3, 22:2n6 and

Table 4 Least square means and standard errors of fatty acid composition in percentage by weight of total identified fatty acids from the longissimus thoracis et lumborum muscle of Nguni steers given three different diets. Fatty acid (% total fatty acid)

Diet Control

A. karroo

Sunflower

s.e.m.

N 14:0 14:1c9 15:0 15:1c10 16:0 16:1c9 17:0 17:1c9 18:0 18:1c11 18:1c9 18:1t11 18:2c9t11a 18:2n6 18:3n3 20:0 20:1c11 20:3n3 20:4n6 20:5n3 22:0 22:2n6 22:5n3 22:6n3 Total saturated fatty acids (SFA) Total Monounsaturated Fatty Acids (MUFA) Total polyunsaturated fatty acids (PUFA) Total omega6 fatty acids (n6) Total omega3 fatty acids (n3) PUFA/MUFA PUFA/SFA n6/n3

10 1.99a 0.25 0.46 0.12b 24.31a 2.78 1.06 0.07 18.22 1.37 30.05 2.29b 0.29 6.14 1.94a 0.21 0.06 0.58b 3.38ab 1.56ab 0.31 0.25b 2.20b 0.11 46.56

10 1.88a 0.31 0.36 0.15b 24.22a 2.95 0.94 0.11 16.28 1.46 30.69 1.87a 0.32 6.32 2.59b 0.16 0.05 0.63c 3.74b 1.81b 0.31 0.33c 2.43c 0.11 44.15

10 2.39b 0.37 0.45 0.05a 25.71b 3.38 1.00 0.06 17.29 1.39 31.84 2.34b 0.39 5.44 1.53a 0.16 0.03 0.48a 2.54a 1.09a 0.21 0.14a 1.60a 0.10 47.22

0.147 0.054 0.036 0.024 0.096 0.264 0.059 0.022 0.955 0.515 0.895 0.149 0.034 0.598 0.212 0.027 0.031 0.045 0.364 0.173 0.143 0.041 0.213 0.040 1.266

36.97

37.58

39.47

1.129

16.47b

18.27b

13.31a

1.574

10.07

10.71

8.51

0.947

6.40

b

ab

0.44 0.36ab 1.58b

b

7.56

b

0.51 0.42b 1.44a

4.80

a

0.644

a

0.537 0.043 0.448

0.35 0.28a 1.78c

Values in the same row with different superscripts are different at P < 0.05. s.e.m.: standard error of means. a 18:2c9t11 corresponds to sum of c9t11 + t8c10 + t7c9.

22:5n3 proportions were recorded in meat from steers on the AK diet followed by those that were on the control diet (P < 0.05). Polyunsaturated fatty acids and total n3 fatty acids proportions were significantly higher in meat from steers on the AK and CN diets than those that received the SF diet. Meat from steers given the SF diet had lower PUFA/MUFA and PUFA/SFA ratios than those on the AK diet (Table 4). The lowest n6/n3 ratio was recorded in meat from steers that received the AK diet (P < 0.05; Table 4). 4. Discussion The observed in vitro DM and NDF disappearances of A. karroo leaves were consistent with previous reports (Mokoboki et al., 2005) and could be associated with their content of potential digestible material (NDF). The reported A. karroo values for calcium and iron are relatively higher than those of related Acacias such as A. tortilis and A. nilotica (Aganga et al., 1998) and are above the recommended levels for beef cattle (National Research Council, 2000). Condensed tannins and total polyphenolics concentrations of A. karroo were comparable to those reported by Aganga et al. (1998) and Mokoboki et al. (2005) and were higher than the concentrations of between 40 and 80 g/kg DM that promote postruminal digestion (Makkar, 2003). The finding that 16:0 and 18:3n3were part of the dominant fatty acids in A. karroo is consistent with previous reports (Siddhwaju et al., 1995). The observation that PUFA, particularly 18:3n3 constituted the bulk of the fatty acids in A. karroo agrees with earlier reports on Acacia species (Vijayakumari et al., 1994). Since the quantity and composition of fatty acids in meat are related to the presence of their precursors in the diet (French et al., 2000), beef from steers that were supplemented with A. karroo leaf-meal is likely to be rich in PUFA. The individual fatty acids proportions except 18:2n6 were within the normal range of values for beef cattle (Muchenje et al., 2009b; Alfaia et al., 2009). The finding that 18:0, 16:0 and 18:1c9 were among the most abundant fatty acids in intramuscular fat of beef cattle is consistent with previous reports (Partida et al., 2007; Muchenje et al., 2009b; Orellana et al., 2009). The observation that beef from steers that were given the AK diet had the highest proportions of 18:3n3 and 22:5n3 could be partly related to dietary fatty acid composition. Part of dietary n3 fatty acids such as 18:3n3, which were found in higher proportions in AK diet, could have escaped ruminal biohydrogenation, absorbed unchanged and deposited in the tissues (Min et al., 2005; Scollan et al., 2006). The higher proportions of 18:3n3 and 22:5n3 in the AK diet could also be ascribed to the presence of condensed tannins, which protect dietary lipids from biohydrogenation in the rumen (Vasta et al., 2009b), and/or inhibit growth and metabolism of ruminal bacteria responsible for ruminal biohydrogenation (Molan et al., 2001; Min et al., 2005; Vasta et al., 2007a,b, 2009a). Thus, protection of fatty acids or inhibition of ruminal bacteria activity by condensed tannins in the AK diet could have produced higher escape of 18:3n3 and 22:5n3 from rumen to tissues. Once incorporated into tissues, 18:3n3 can be left intact, oxidized or subjected to elongation and desaturation (Nu¨rnberg et al., 2005; Wood et al., 2008). Part of 18:3n3 is converted by desaturase and elongase enzymes into nutritionally important long-chain (20–22) n3 PUFA such as 20:5n3, 22:5n3 and 22:6n3 (Burdge and Calder, 2005; Brenna et al., 2009; Harris et al., 2009). Thus, apart from ruminal production, the higher amount of 22:5n3 obtained in intramuscular fat of steers fed the AK diet, compared to steers given other diets, might have arisen from increased endogenous biosynthesis of this fatty acid from 18:3n3 in the muscle. However, recent reviews have reported

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that the metabolic conversion efficiency of 18:3n3 to n3 PUFA in humans is low and as result n3 PUFA are now regarded as dietary essential (Burdge and Calder, 2005; Harris et al., 2009; Molendi-Coste et al., 2011). Despite the limited capacity of metabolic conversion of 18:3n3 to n3 PUFA (Burdge and Calder, 2005; Givens, 2009, 2010), it has many roles in human health that are independent from its conversion to n3 PUFA (Nannicini et al., 2006; Zhao et al., 2007; Hassan et al., 2010). The long chain n3 PUFA, predominantly 20:5n3, 22:5n3 and 22:6n3 play a significant role in prevention and treatment of certain diseases and disorders such as cardiovascular diseases, hypertension, type 2 diabetes, irritable bowel syndrome, muscular degeneration, rheumatoid arthritis, asthma, psychiatric disorders and several cancers (Givens, 2010; McAfee et al., 2010; Micha et al., 2010). To reduce the risk of these diseases in humans, nutritionists recommend dietary intake for 18:3n3 of between 1.1 and 2.5 g/day (Givens and Gibbs, 2008; EFSA, 2009; Smit et al., 2009) and between 200 and 600 mg/day for 20:5n3 + 22:6n3 (EFSA, 2009; USDA and HHS, 2010; MolendiCoste et al., 2011). Epidemiological research to determine the biological significance of the n3 PUFA in beef from animals supplemented with A. karroo leaves, especially 18:3n3, and 22:5n3, which were found in higher proportions in the AK diet than in other diets is recommended. Use of dietary condensed tannin-rich feeds such as A. karroo leaves to increase tissue concentrations of n3 PUFA on a population level may result in a decrease in mortality and health care costs associated with such illnesses. The observed higher intramuscular fat content in beef from steers that received SF diet could partly explain its high 14:0 and 16:0 proportions, and low PUFA proportions, PUFA/MUFA and PUFA/SFA ratios compared to AK and CN diets. De Smet et al. (2004) reported that as fatness increases, the levels of SFA and MUFA increase faster than the PUFA levels, leading to an increase in the relative proportions of SFA and MUFA and decrease in PUFA and PUFA/SFA ratio. The observation that n6/n3 ratio was lowest in beef from steers that were given the AK diet compared to those that received other diets was probably due to the higher proportions of n3 PUFA, particularly 18:3n3 in the former diet. The PUFA/SFA and n6/n3 ratios are commonly used to assess the nutritional value and healthiness of beef fat for human consumption (Simopoulos, 2004; Aldai et al., 2005; Alfaia et al., 2007). In general, a ratio of PUFA/SFA above 0.4 (Higgs, 2002) and a ratio of n6/n3 below 4.0 (Raes et al., 2004) are recommended in human diets to prevent the development of cardiovascular diseases and some chronic diseases including cancer (Griffin, 2008; Givens, 2010; Molendi-Coste et al., 2011). The current findings show that the PUFA/SFA and n6/n3 ratio in beef from steers given AK diet were within the optimum values for human diets. These findings could imply inclusion of condensed tannin rich feeds such A. karroo in rations could be one strategy of balancing n6/n3 PUFA ratio, and consequently improve beef healthiness from a consumer perspective. The low ratios of n6/n3 (1.44–1.78) observed in this study are a characteristic of fat from ruminants that are fed forage-based diets which contains high levels of 18:3n3 (French et al., 2000; Nu¨rnberg et al., 2005; Muchenje et al., 2009a). Contrary to current findings, Muchenje et al. (2009b) found slightly higher PUFA/SFA ratio for Nguni cattle that entirely subsisted on rangeland. The variation could be attributed to age differences. Nguni steers in this study were older than those used by Muchenje et al. (2009b) and it is known that older animal have more SFA in their fat composition (Aurousseau et al., 2004). Further studies to determine the possible interaction between diet and age on fatty acid composition are warranted.

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