Beef represents a highly concentrated, natural source of valuable nutrients, including high quality protein, B-vitamins, and niacin, as well as highly available iron and zinc. However, beef also contains high concentrations of saturated fatty acids that are implicated in human cardiovascular health issues. Certain feedstuffs can help modify the fatty acid profile of beef to increase mono-unsaturated fatty acids, which may offer health benefits. The challenge in developing such ‘designer beef' diets is to incorporate these feedstuffs in economical formulas which also maintain animal performance and desirable carcass characteristics.
Recent research at the Lethbridge Research Centre in Alberta has focused on sunflower seeds (SFS) in beef finishing diets to increase conjugated linoleic acid (CLA) isomers. The work showed that diets for finishing steers can include SFS without decreasing productivity and lean meat yield. Moreover, SFS at up to 15% of the diet did not adversely affect key meat quality indicators.
Because of potential health benefits associated with consumption of CLA by humans (Azain, 2003), researchers have directed considerable effort at increasing the CLA content of ruminant-derived foods (Mir et al, 2004). Conjugated linoleic acids are intermediary products of biohydrogenation of polyunsaturated fatty acids (PUFA), such as linoleic acid (LA) by rumen bacteria such as Butyrivibrio fibrosolvens (Kepler et al, 1966). Also, they form as a result of bioconversion of vaccenic acid by the enzyme 9-desaturase (Griinari et al, 2000).
To increase CLA content of meat, animal diets must be supplemented with the substrate fatty acid, linoleic acid (LA). Sunflower seeds contain about 40% oil, of which over 66% is LA. Therefore, SFS can be an excellent dietary supplement to increase the CLA content in ruminant tissues. Work by numerous researchers shows that various oilseed sources of LA, including SFS, canola, and safflowe, rcan increase CLA content of milk and meat. However, such work also shows that other components of the diet can affect the amount of CLA formed and deposited in the muscle.
For example, inclusion of sunflower oil at 6% of diet dry matter (DM) into diets containing no silage led to at least three-fold increase in tissue CLA concentrations in steers (Mir et al, 2002). However, provision of similar levels of sunflower oil to steers fed silage containing diets resulted in a maximum of 30% increase in CLA (Mir et al, 2003b). This work showed it was essential to determine the amount of CLA, or its precursor fatty acid, that would be deposited in the tissues, if SFS were included in the diet to provide 6% sunflower oil on a DM basis along with a variety of other dietary components.
Although LA is an excellent substrate for increasing CLA in ruminant products, it is a PUFA, which can be toxic to many rumen bacteria and can reduce rumen fermentation rates and microbial protein yield (Madron et al, 2002). However, rather than diet supplementation with sunflower oil, what if SFS were fed? Then the rate of oil release in the rumen may be controlled by mastication. This more gradual intake of oil from dietary SFS may minimize the adverse effects of the free oil on rumen microbes.
In this work at Lethbridge, our first objective was to determine the extent of increase in CLA content of muscle and adipose of steers fed SFS supplemented diets containing barley, barley and silage, or hay. Whole SFS is high in fiber (Gibb et al, 2004), so our second objective was to evaluate whether dietary SFS would successfully eliminate the requirement of a roughage source such as barley silage. The third objective was to determine the effect of replacing barley silage, partially or totally, with chopped alfalfa hay on concentrations of CLA or its precursor, vaccenic acid, in fat from meat and subcutaneous adipose tissue. Finally, we sought to determine whether it was more effective to provide the SFS and hay as a convenient pellet rather than as individual ingredients.
Our hypothesis was that feeding SFS at 15% of diet DM would provide adequate amounts of LA to increase deposition of CLA and its precursor FA in beef. Also, the extent of increase of these FA would be affected by the other dietary components. We therefore designed experiments at the Lethbridge Research Centre to evaluate the effects of dietary SFS in combination with other dietary components, focusing on production performance, tissue FA composition, meat quality, meat appreciation by consumer and purchaser, along with objective assessment of stability of the meat by determining lipid oxidation products.
We used 72 non-implanted, mixed breed, yearling steers in a completely randomized experimental design. Steers were individually fed six finishing diets (n=12 per diet) in which, on a DM basis:
Diet 1, the control diet, contained 84% rolled barley and 15% barley silage;
Diet 2, the barley silage was replaced with SFS;
Diets 3, 4, and 5, the silage was linearly replaced from 15% to 7.5% to 0% with chopped alfalfa hay, with these three diets containing 15% SFS;
Diets 5 and 6 were identical in composition, but the hay and SFS were included as a pellet.
Supplementation with SFS (Diets 2-6) increased dietary net energy of maintenance (NEm) and net energy of gain (NEg) (2.39 and 1.46 Mcal kg-1 of DM) compared to the energy content of the control diet (2.16 and 1.29 Mcal kg-1 of DM, respectively). Replacement of barley silage with SFS did not change crude protein (CP) content of the diet but partial or complete substitution of barley silage with hay increased dietary CP content.
We similarly observed some increase in dietary neutral detergent fiber (NDF) and acid detergent fiber (ADF) contents due to replacement of the rolled barley in the experimental diets with SFS. However, the compositions of the diets were such that they met or exceeded the requirements defined for finishing diets for steers (NRC, 1996).
Sunflower seeds contain about 40% oil, which previous research found to consist of a variety of FA: 68.5% C18:2 cis9, cis12; 21.7% mixed C18:1; 5.5% C16:0; 3.5% C18:0; and, trace amounts of C18:3 and C20:0 (Palmquist, 1988). In the Lethbridge experimental diets, substitution of 15% rolled barley with SFS, which provided 6% added sunflower oil on DM basis, doubled the amount of dietary lipids and C18:2 cis9, cis12. The substitution of SFS decreased the proportions of C14:0, C16:0, C18:0, and C18:3, but increased C22:0.
The higher fat and weight percentage of LA (66% of total FA) of SFS supplemented diets over the control diet (51%), resulted in increased fat, energy and LA acid intake.
The average weight of steers at randomization to treatment diets was 398.6 kg + 4.5 kg, and, at the end of the 28-day adaptation period, their average weight was 434.2 + 5.1 kg, and not different among treatments. Although steers fed the SFS supplemented diets consumed less DM, their average energy intake ranged between 29.2 Mcal d-1 for Diet 6 and 35.1 Mcal d-1 for Diet 1, the control diet, which decreased (P < 0.02) the growth rate of steers fed the SFS. However, there was no difference in final weights (596.2 + 7.8 kg) or feed efficiency (9.0 + 0.2 kg DM F:G) among different treatment groups.
The difference in DM intake between the steers fed the control diet and those fed the barley and SFS diet led to a decrease (P = 0.03) in protein intake, which led to the relatively lower rate of gain of steers fed this diet. In a previous study at Lethbridge, better feed efficiency and average daily gain were observed in non-implanted beef steers receiving sunflower oil (6% of DM) than those fed the control diet during the backgrounding phase, but not in the finishing phase (Mir et al, 2002).
In the present study, SFS was supplemented only in the finishing phase, by which the results agreed with the previous observations. Although the final weights of the steers were comparable to those previously observed by Lethbridge researchers for similar types of cattle (Mir et al, 2003b), the rate of weight gain and feed to gain ratio were substantially lower in the present study, which may be due to the difference in the nature of the oil supplement (sunflower oil versus SFS) provided to the steers in the two trials.
Similar low rates of gain have been reported by other researchers for steers fed 14% SFS, despite use of growth promoting implants (Gibb et al, 2004). By contrast, other researchers found no decrease in rate of gain in cattle fed whole soybeans up to 24% of diet DM or approximately 4% added fat to the diet (Felton and Kerley, 2004).
Carcass and meat quality effects
In the present Lethbridge study, steers fed SFS-containing diets had reduced back fat depth (P = 0.02) at 13.8 mm versus 17.0 mm for steers on the control diet. Dietary supplementation with SFS increased weight percentage for CLA isomers (CLA cis9, trans11 and CLA trans10, cis12) in fat from the pars costalis diaphragmatis (PCD) muscle (0.43% versus 0.22%, P = 0.001) and brisket fat (0.74% versus 0.44%, P = 0.0004), with the increase being greatest in steers fed the silage-free diets.
We found that the increase in weight percentage of vaccenic acid was highest in tissues of steers fed hay along with SFS (11.7%) or barley and SFS (11.8%) relative to those fed the control diet (2.12%). Meat from steers fed the treatment diets had higher scores for juiciness (P = 0.02), while initial tenderness (P = 0.1) and overall tenderness (P = 0.08) tended to be higher compared with beef from control steers. The steaks from SFS fed steers appeared to have a darker color relative to steaks from control steers on day-7 (P < 0.0001). Feeding SFS in finishing diets of steers had little influence on meat discoloration, retail acceptance and thio-barbituric acid reactive substances.
The recent work at Lethbridge showed that supplementation of finishing diets for steers with SFS increased content of CLA isomers and vaccenic acid in tissues, without affecting feed conversion efficiency or meat quality. SFS ws especially effective in increasing the heart-healthy FA in beef when incorporated into silage-free diets.