Meat lipid profile of steers finished in pearl millet pasture with different rates of concentrate

The objective of this work was to evaluate the meat lipid profile from Devon beef steers finished in pearl millet (Pennisetum americanum) pasture and fed at different rates of concentrate supplementary diet. Twelve steers weighing 270 kg, at 12‐month‐average initial age, were randomly distributed into three treatments: pearl millet pasture; and pearl millet pasture plus a concentrate equivalent at 0.5 or 1.0% of body weight, with two replicates. Total contents of saturated and unsaturated fatty acids, the polyunsaturated:saturated ratio and other relevant fatty acids as the vaccenic acid, conjugated linoleic acid, omega‐3, and omega‐6 were not affected by the consumption of a concentrate supplement at 0.5 or 1.0% live weight. However, the 0.5% supplementation level reduced the concentration of dihomo‐γ‐linolenic fatty acid (C20: 3 n‐6), while the 1.0% supplementation level elevated the content of docosahexaenoic (DHA) (C22: 6 n‐3) fatty acid, and the omega‐6:omega‐3 ratio in meat. Consumption of up to 1.0% energy supplementation increases the omega‐6:omega‐3 ratio in meat from Devon steers grazing on pearl millet pasture.

A supplementation diet with grain for cattle kept on pasture can be used as a strategy to increase individual weight gain, as well as to rise the stocking rate per hectare.With fat input from grain, energy supplementation can contribute with higher amount of unsaturated fatty acids (UFA), and also promote changes in the digestive processes in the rumen, like the passage rate increase, and ruminal pH reduction.Therefore, supplementation with grain predisposes to lower rates of biohydrogenation and larger input of UFA which reach the small intestine to be absorbed (Martínez Marín, 2007).Even so, the pasture type appears to be an important factor in the process of lipid deposition in beef, since tropical pastures show a lipid profile with lower levels of polyunsaturated fatty acids (PUFA), mainly C18:3 and smaller amounts of C14:0, in comparison to temperate pastures (Menezes et al., 2010).
The objective of this work was to evaluate the meat lipid profile from Devon beef steers finished in pearl millet (Pennisetum americanum) pasture and fed at different rates of concentrate supplementary diet.

Materials and Methods
The experiment was carried out in the Department of Animal Science, Universidade Federal de Santa Maria (UFSM), RS, at 29°43'S, 53°42'W, 95 m altitude.Climate is classified as humid subtropical (Moreno, 1961), and the soil mapping belongs to São Pedro unit, classified as Paleudalf (red-yellow podzolic) (Streck et al., 2002).
Twelve Devon steers, weighing 270 kg, with 12-month-average initial age, were kept in a pearl millet (Pennisetum americanum L. Leeke) pasture, and were subjected to the following feeding treatments: only pearl millet grazing; pearl millet plus concentrate at 0.5% of body weight; and pearl millet plus concentrate at 1.0% of body weight.
The continuous grazing system with variable stocking rate was used, as described by Mott & Lucas (1952), with an offer of leaf blades dry matter (DM) kept at 10% of body weight.Two pasture replicates per treatment were used, with two tester animals each one, where regulator animals were eventually also allocated.Samples of the concentrate supplement and of the pasture (by simulating grazing) were collected to determine the chemical composition and fatty acid profile of consumed materials (Table 1).
The pasture showed 29.90% dry matter (DM), 9.63% crude protein (CP), 2.87 Mcal digestible energy (DE) per kg DM, and 53.82% NDF.Animals exclusively finished on pasture had free access to salt.The animals of the supplementary treatment received a daily concentrate diet at 14 h, which was composed of wheat bran, limestone, sodium chloride and monensin, totaling an energy supplement with 13.9% CP and 71.4% TDN.
After 100 days of grazing, the animals with 375.8 kg average weight were slaughtered in a commercial slaughterhouse, after 14-hour complete fast.The slaughter process followed the normal flow of the establishment, in accordance with the Regulation of Industrial and Sanitary Inspection of Animal Products.After carcass cooling for 24 hours at 1°C, samples of the longissimus dorsi muscle were removed, packed, and frozen for subsequent analysis of fatty acid profile.
To determine fatty acids in the samples, lipids were initially extracted by using the method of Bligh & Dyer (1959) and, then, they were esterified according to Hartman & Lago (1973), and analyzed in an Agilent 6890 Gas Chromatograph (Hewlett-Packard, Palo Alto, CA, USA), equipped with a flame ionization detector (FID) and a capillary column SP-2560 100 m x 0.25 mm x 0.2 µm (Supelco, Sigma-Aldrich, São Paulo, SP, Brazil).The temperature gradient used for ester separation of fatty acids was: 140 o C for 5 min, increasing 1.6°C per min up to 210°C, remaining for additional 10 min; increasing 10°C per min up to 240 o C, staying for another 15 min, totaling a run of 76 min.Gas (N2) flow was 30 mL per min, and the volume of injection was 1 μL with 1:50 split ratio.Thus, the identification of fatty acids was carried out by comparing the retention time of fatty acids from the samples with known standards.
The experimental design was completely randomized.Data analysis of variance was done by GLM procedure, and variables were tested with Shapiro-Wilk normality.The mathematic model was: Y ijk = μ + β i + ε ijk -i, in which: Y ijk represents the dependent variables; μ is the overall mean of the observations; β i is the effect of dietary treatment, and ε ijk is the residual random error NID (0, σ 2 ).The means were classified by the F test, and the parameters with significant effect were compared by t test, at 5% probability.Statistical analyses were performed with the aid of SAS software.

Results and Discussion
The different levels of energy supplementation did not alter the levels of saturated fatty acids (SFA) in meat (Table 2).Using stored forage, Menezes et al. (2010) observed that the consumption of pearl millet silage, compared to annual ryegrass silage, promotes a higher intake of SFA; however, the total of SFA that reaches the small intestine for absorption is similar.This may explain the insignificant change in the levels of SFA in meat from cattle, in the present and in other studies, such as that by Prado et al. (2003).
Stearic fatty acid (C18:0) accounted for approximately 19.22% of total fatty acids (Table 2).This value was 18% lower than that reported by Prado et al. (2003) for meat of steers grazing pearl millet.Although saturated, this fatty acid is considered neutral because it can be transformed into oleic fatty acid (C18:1 cis9), which is considered a hypocholesterolemic acid because it acts to reduce LDL-cholesterol, and to increase high-density lipoprotein (HDL) levels.
Monounsaturated fatty acids (MUFA) were similar in treatments and amounted 36.38 g 100g -1 lipid (Table 3).The addition of unsaturated fatty acids in meat may result from the increase of unsaturated fat intake, since about 20% UFA ingested by ruminants reaches the small intestine without undergoing complete biohydrogenation (Martínez Marin, 2007).Furthermore, the increase of this type of fatty acids in beef may be originated from the action of factors that inhibit complete biohydrogenation, as the inclusion of  concentrates in the diet.In this case, nutritional factors, such as the increasing rate of passage of digesta, and ruminal pH reduction may hinder the action and development of rumen bacteria which act in the process of biohydrogenation like Butyrivibrio fibrisolvens, which requires a minimum 6.2 pH (Furlan et al., 2006).
The content of oleic acid (C18:1 n-9 cis) did not differ by the use of supplementation, likewise observed in the studies of Prado et al. (2003).It was expected that the raise of concentrate supplementation, whose content of polyunsaturated fatty acids is high (Table 1), could result in a small reduction in rumen pH, and, consequently, in an incomplete biohydrogenation of fatty acids (Loor et al., 2004); this fact would result in higher levels of monounsaturated acids; however, which did not occur in the present study.
The content of vaccenic acid (C18:1 trans-11) was similar between treatments and showed 1.79 g 100g -1 average intramuscular fat.This fatty acid is considered the precursor of the conjugated linoleic acids (CLA) that can be formed from the action of the enzyme Delta 9-desaturase, which is present in the adipose tissue of animals (Ledoux et al., 2000).The reducing effects of the total biohydrogenation process seem to be more intense, when the inclusion of concentrate in the diet is elevated, which results in the increase of duodenal flow of C18:0 trans-11 (Menezes et al., 2010).Therefore, energy supplementation used in the present study seems not to have been effective on ruminal biohydrogenation, since it caused no changes of the concentrations of vaccenic acid and CLA cis-9-trans-11.The production of CLA can also be related to higher intake of polyunsaturated fatty acids, especially linoleic acid (Duynisveld et al., 2006); however, the high content of this fatty acid in the concentrate fraction of the diet (Table 1) was not enough to promote changes in the CLA content in meat from animals grazing pearl millet.In grazing systems, other factors such as the proportions of leaf and stem, as well as the forage maturity, may determine the percentage of PUFA (Boufaïed et al., 2003).

Table 1 .
Fatty acid profile of pearl millet and concentrate (g 100g -1 fat tissue in natura).

Table 2 .
Profile of saturated fatty acids in longissimus dorsi muscle of beef steers grazing pearl millet and feeding a supplementation diet (0.5% and 1.0%) at different rates (g 100g -1 of fat tissue in natura).