Background

Consumer acceptance of beef is determined by perceived tenderness, juiciness, and flavor and is high when the mean rating for these traits is favorable (Platter et al., 2003). Marbling is significantly related to consumer acceptance such that the likelihood of consumer acceptance is increased 10% with each full marbling score increase (Platter et al., 2003). While some consumers prefer lowly marbled beef, a majority of consumers prefer highly marbled beef and are willing to pay more for it when tenderness is equal (Killinger et al., 2004; Platter et al., 2005). As a result, marbling is important to the profitability of beef producers. Hot carcass weight and marbling are the two most important drivers of profitability when fed cattle are marketed in a grid system and alternate in importance depending on the choice-select spread (Lawrence et al., 2003). Therefore, the beef industry should pay close attention to management practices which may affect marbling because those practices may affect beef demand, and industry profitability.

It has been known for some time that glucose provides up to 75% of the acetyl units for intramuscular fat lipogenesis (Smith and Crouse, 1984). More recently, in vivo studies have demonstrated that marbling expression is related to serum glucose concentrations in growing cattle when marbling is measured by ultrasound (Schoonmaker et al., 2003). These observations are congruent with production systems which have been developed to maximize starch intake throughout the life of early weaned calves to improve quality grade (Myers et al., 1999). Therefore, a working hypothesis has been developed that suggests dietary management strategies that reduce serum glucose levels also have the potential to detrimentally affect marbling.

The availability of distillers grains (DGs) will increase rapidly as the ethanol industry continues to expand. Current projections suggest that ethanol production will more than double from the current annual production of 6.5 billion gallons to 15 billion gallons produced annually by 2022. This level of ethanol production could result in an annual production of approximately 40 million tons (DM basis) of DGs. Feedlots will likely use much of the DGs because large quantities of the product can be transported to a single location and ruminants utilize highly fibrous feeds more effectively than non-ruminants. The US slaughters approximately 27 million fed cattle (steers and heifers) per year (USDA, 2008). If we assume fed cattle consume an average of 25 lbs of DM per day to account for northern and southern yards and cattle are on feed for an average of 150 days, then the US produces approximately 50 million tons of finished feed for feedlot cattle (DM basis). Therefore, feedyards would theoretically need to feed diets containing 80% DGs to utilize all of the DGs that will be produced. Although the feedyard industry will clearly not utilize all of the DGs, it is likely that DGs may be included in the diets of many of the fed cattle in the US in the future based on availability and increased demand for corn.

There is currently little concrete information available on the effects of DGs on marbling. Based on the previously stated hypothesis, inclusion of DGs in the diet of finishing cattle may reduce marbling because starch intake and digestibility are decreased in DGs diets (Pingel and Trenkle, 2006). However, dry-rolled corn based diets which include DGs have increased ruminal propionate concentrations (Erickson and Klopfenstein, 2006) and propionate is a precursor for glucose. Additionally, production studies have shown no differences in marbling scores when DGs were included at 0 to 50% of dietary DM (VanderPol et al., 2006a), although some have suggested that marbling is reduced when using yield grade as a covariate (Corah and McCully, 2006). There is currently a great deal of concern related to potential effects of distillers grains inclusion in finishing diets on marbling, but to date, few studies have been conducted to specifically study DGs and marbling.

Corn processing method also has the potential to affect marbling. Owens and Gardner (1999) suggested that steam-flaking reduced marbling based on an analysis of 552 published research articles. This potentially has large implications for Southern Plains beef producers because a majority of cattle finished in the Southern Plains region are fed steam-flaked corn-based diets. However, this analysis may be confounded with region of the country and grain type since more steam-flaking takes place in the southern region where cattle tend to have lower quality grades. Also, certain grains, such as sorghum, are more often steam-flaked than are other grain types. Nevertheless, the hypothesis that steam-flaking reduces marbling may have some merit because steam-flaking increases starch digestibility in the rumen (Zinn et al., 2002). However, steam-flaking also increases total starch digestibility such that the amount of starch digested in the small intestine may not greatly differ from dry rolled corn (Zinn et al., 2002).

If DGs inclusion and steam-flaking have the potential to reduce marbling, the combination of distiller’s grains in a steam-flaked corn diet could have devastating effects on marbling. This could have major implications for Southern Plains beef producers since DGs used in this region would likely be fed in steam-flaked corn based diets. Vander Pol et al. (2006b) compared several corn processing methods in diets containing 30% DGs. Compared to dry rolled corn diets, marbling scores were reduced in the steam-flaked corn diets and percent cattle grading upper 2/3 choice was reduced from 29.4% to 6.7%. However, cattle consuming the steam-flaked corn diet were marketed at 0.51 inch 12th rib fat thickness while cattle consuming dry rolled corn were marketed at 0.62 inch 12th rib fat thickness. Since marbling is related to fat thickness within genetically comparable cattle (Gwartney et al., 1996), marbling comparisons need to be made at equal fat endpoints. Therefore, information is needed to compare the effects of distillers grains and corn processing method within one trial where genetic variation is minimized and cattle are finished at an equal fat endpoint.

Methodology

Experimental design and treatments.
Eighty pre-conditioned steer calves (246 kg) were purchased from a single ranch to determine effects of corn processing method and wet distiller’s grains plus solubles (WDGS) inclusion on animal performance and carcass characteristics, as well as marbling attributes, sensory ratings, and shelf-life of longissimus muscle (LM). The experiment was designed as a randomized incomplete block with treatments arranged in a 2X2 factorial. Factors will include inclusion or absence of WDGS derived from fermentation of corn in the finishing diet of steers with corn processed by steam flaking (SFC; bushel weight = 27 lb/bu) or dry rolling (DRC). Three loads of WDGS were purchase from Chief Ethanol Fuels (Hastings, NE), were delivered together, and were bagged in an agricultural bag within 36 hours of being produced at the ethanol plant.

Cattle procurement and management.
Animal procedures were approved by the Cooperative Research, Education, and Extension Triangle Institutional Animal Care and Use Committee. Breed makeup of the steers was ¼ Hereford and ¾ Angus. Steers were received and allowed 35 days for adaptation. Fifty-four steers of uniform weight and mild disposition were selected from the pool and trained in the Calan gate system. During training, steers were blocked by weight and assigned to one of six pens, each containing nine individual Calan gates. Steers selected the gate from which they preferred to feed and experimental treatments were randomly assigned to gates within pen. The research facility had 54 Calan gates available for use. Therefore, each treatment was assigned to two to four animals per pen and pen was considered an incomplete block. Remaining steers were retained in the event they were needed to replace steers that did not adapt to the Calan gate system. Two steers failed to adapt to the system and were replaced prior to trial initiation. All steers were fed a common high forage growing diet consisting of 31% steam-flaked corn, 30% wet corn gluten feed, 30% alfalfa hay, 5% supplement, and 4% crude glycerin until initiation of the trial.

Upon trial initiation, steers (308 ± 8 kg) were weighed for three consecutive days after being limit-fed (1.8% BW) the common receiving diet for 5 days to minimize variation in gut fill. They were then stepped up onto their experimental finishing diets (Table 1) in 21 days in three steps containing 40%, 30%, and 20% alfalfa hay. Corn-based WDGS was included in the finishing diet at 35% (DM basis) and partially replaced corn (steam-flaked or dry-rolled), cottonseed meal, yellow grease, urea, and limestone in the diet. Finishing diets were formulated to contain a minimum of 13.5% CP with a positive degradable intake protein balance. Diets were also formulated to contain 0.70% Ca and 6.00% ether extract thereby attempting to equilibrate fat across diets. All diets contained 10% alfalfa hay as a roughage source and 0.70% supplement which provided trace minerals, vitamins, monensin, and tylosin (Elanco Animal Health; Greenfield, IN) at levels common with industry standards. Diets were offered once daily in the morning throughout the finishing period. Steers were initially implanted with 14 mg estradiol and 200 mg progesterone (Synovex S®; Fort Dodge Animal Health, Fort Dodge, IA) at trial initiation, and terminally implanted with 16 mg estradiol and 80 mg trenbolone acetate (Revlor IS®; Intervet Inc., Millsboro, DE) on day 84.

Ingredient samples were collected weekly for dry ingredients and thrice weekly for wet ingredients (WDGS and SFC) for DM determination. DM was determined in a 60°C oven for 24 hours. Weekly samples were composited by month and ground. Monthly samples were then composited to encompass the duration of the study. Chemical analysis of dietary ingredients was conducted at a commercial laboratory (Servitech Laboratories, Amarillo, TX), with the exception of NDF of the WDGS which was determined on a composite of the delivered product in an ANKOM 200 fiber analyzer (ANKOM Technology Corp., Fairport, NY) with 10 g/L sodium sulfite included in the NDF solution.  Steers were weighed every 28 days and received ultrasound scans for 12th rib fat thickness and marbling every 56 days to characterize growth and changes in body composition. Ultrasound measurements were taken at the 12th rib using an Aloka 210 (Wallingford, CT) B-mode instrument with a 3.5-MHz general purpose transducer array. Marbling and 12th rib fat thickness were estimated using image analysis software described by Brethour (2000). On d 55, 56, and 57, and again on day 111, 112, and 113, blood glucose levels were monitored twice daily (once in the a.m. and once in the p.m. for each steer) such that blood glucose estimates were collected at 0, 2, 4, 6, 8, and 10 hours post-feeding at two mid-points in the feeding period. Blood glucose was measured chute side using a human self-monitoring system (True Track Smart System®; Home Diagnostics, Inc., Fort Lauderdale, FL) as validated by Rumsey et al. (1999). Each blood sample was tested by three monitors and estimates with a CV greater than 6% were re-tested.

All steers were harvested at one time when the mean 12th rib fat thickness reached 13 mm. Steers were harvested at a commercial abattoir and carcass data collected by the West Texas A&M University Cattlemen’s Carcass Data Service. Individual animal identification was preserved and one loin per animal purchased for further analyses.

Sensory Evaluation.
One 2.54 cm sensory steak was removed from the 13th rib end of the loin from each animal. Steaks were vacuum packaged and aged for 14 days prior to freezing. Sensory analysis was performed in the Sensory Testing facility at Texas A&M University (College Station) with trained panelists seated in separate booths to prevent communication between panelists. An eight-member beef descriptive flavor attribute panel was selected and trained according to the SpectrumË procedures (with scale 0 = absent; 15 = extremely intense). During panel training, terminology development sessions were conducted based on a standard lexicon for beef flavors that characterize aromatic notes and chemical feeling factors. Aromatics (cooked beef/brothy, serum/bloody, cowy/grainy, cardboard, painty, fishy and liver/organy), mouth-feels (metallic and astringent) and basic tastes (sour, bitter, sweet, and salty) of each cooked steak were identified. Panelists were seated in separate booths with red filtered lights to mask color variation in samples (AMSA 1995). Up to 8 panelists evaluated up to 12 samples per day with 6 samples evaluated in a session. Panelists were provided with a 20 minute break between sessions and samples were served at least 4 minutes apart to reduce tastebud fatigue. Samples were served in a randomized order using 3-digit identification codes. Distilled water and ricotta cheese were provided for cleansing their pallets.

Prior to evaluation, steaks were thawed for two days at 4° C. On the day of evaluation steaks were cooked on an open hearth grill to an internal temperature of 70° C. Temperature was monitored using iron-constantan thermocouples inserted into the geometric center of each steak. Steaks were turned once upon reaching 35° C. After cooking, steaks were cut into 1.27 cm cubes and two cubes were served to each panelist through a bread-box hood.

Fatty Acid Composition.
Fatty acid composition of the longissimus muscle (trimmed of s.c. adipose tissue) was measured as described by Chung et al. (2006). Total lipids were extracted by the method of Folch et al. (1957), and fatty acid methyl esters were generated as described by Morrison and Smith (1964). Fatty acid methyl esters were measured with a Varian gas chromatograph (model CP-3800 fixed with a CP-8200 autosampler; Varian Inc., Walnut Creek, CA, U.S.A.). Separation of the fatty acid methyl esters occurred on a silica capillary column CP-Sil88 [100 m X 0.25 mm (i.d.)] (Chrompack Inc., Middleburg, The Netherlands). Helium was used as the carrier gas (flow rate = 2 mL/min). After 32 min at 180° C, oven temperature was increased at 20° C /min to 225° C and held for 13.75 min. Injector and flame ionization detector temperatures were 270 and 300° C, respectively. Identity of fatty acid will be based on retention times in comparison to standards (GLC-68D, NuChek Prep, Inc., Elysian, MN, U.S.A.).

Shelf-Life.
Loin samples were cut into five-2.54 cm steaks and steaks were randomly assigned to storage day of 0, 1, 3, 5 or 7 days. Steaks were individually packaged in Styrofoam tray (CRYOVAC, Sealed Air Co., Saddle Brook, NJ) and over-wrapped with polyvinyl chloride (PVC) film (Stretchable meat film 55003815; Prime Source, St. Louis, MO). Steak packages were then placed in a refrigerated cooler (2°C) under standard supermarket fluorescent lighting (Sylvania F40N, Osram Sylvania, Danvres, MA; Color Temperature = 3600K). Steaks across live animal dietary treatments and storage times were randomly assigned location and had similar access to light. On each storage day, steaks were analyzed for thiobarbituric acid reactive substances (TBARS) and color after 0, 1, 3, 5 and 7 days of shelf life. For the measurement of TBARS, the modified distillation TBARS method of Rhee (1978) was modified to include addition of 5 ml of a 0.5% solution of propyl gallate and EDTA for each 10 g sample in the blending process (Chae et al., 2004). Twenty grams of sample in 10 ml of 0.5% propyl gallate/EDTA was boiled in 0.1 N HCl for 15 min in 110 mL total volume. The volume was brought to 480 mL with distilled water and color was developed by the addition of 5 mL of 0.02 M 2-thiobarbituric acid. After heating in boiling water for 35 min, the samples were cooled in tap water for 10 min and the optical density of the sample against the blank was read at a wavelength of 530 nm. To obtain TBARS values, the sample absorbance was multiplied by a constant (K) (Tarladgis et al., 1960). TBARS are reported as mg malonaldehyde per kg of beef. The surface color of each patty before and after cooking was measured with a Minolta Colorimeter (CR-200, Minolta Co., Ramsey, NJ, U.S.A.) using L* (lightness), a* (redness), and b* (yellowness) color space values. Calibration was conducted on a white tile prior to use, and the calibration values were L* 96.03, a* 0.11, and b* 1.97. Three random spots were measured on the surface of each sample and the mean value was calculated.

Enzyme activities.
The activities of NADP-malate dehydrogenase and 6-phosphofructokinase were measured in 1-g samples of s.c. adipose tissue as described by Smith and Prior (1981) and Rhoades et al. (2005), respectively.

Cellularity Adipose tissue samples were sliced into 1-mm thick sections and fixed with osmium tetroxide as described by Etherton et al. (1977) as modified by Prior (1983). The fixed cells were filtered through 250-, 62-, and 20-μm nylon mesh screens with 0.01% Triton X-100 in 0.154 M NaCl to prevent cell clumping. Cell fractions collected from the 62- and 20-μm mesh screens were used to determine cell size, volume, and cells per gram of tissue with a Coulter Counter, Model ZM and Coulter Channelyzer 256 (Beckman Coulter, Miami, FL).

Statistical Analysis.
Data were analyzed using the Mixed procedures of SAS (SAS Inst. Inc., Cary, NC) with pen considered to be a random effect. Data were analyzed by ANOVA as an unbalanced randomized incomplete block design with individual steer as the experimental unit and pen as the block. The unbalanced design was necessary because there were six pens with nine gates per pen available. The model included WDGS inclusion, corn processing method, and their interaction. If the F-test was significant was significant for the interaction of WDGS inclusion and corn processing method (P < 0.05), simple means were separated using a T-test. For responses measured across time, a repeated measures statement was added. Covariance patterns were selected by their reduction of Akaike’s criterion relative to the unstructured pattern (Littell et al., 2002). Liner, quadratic, and cubic terms were added to the model to determine the response to time. Slope and intercepts were separated as suggested by Littell et al. (2002).

Findings  

The study was required an unbalanced design because the facility had only 54 Calan gates were available. However, one steer stopped consuming feed after the trial was initiated and was removed from the study and one loin was not retained at the abattoir. Therefore, for live animal response variables and carcass characteristics, sample size = 13, 14, 14, 12 for SFC + 0% WDGS, DRC + 0% WDGS, SFC + 35% WDGS, and DRC + 35% WDGS, respectively, and for response variables measured in the loin, sample size = 12, 14, 14, 12 for SFC + 0% WDGS, DRC + 0% WDGS, SFC + 35% WDGS, and DRC + 35% WDGS, respectively. Therefore, SE estimates are provided parenthetically following treatment means in the tables.

Chemical analysis of diet ingredients.
The chemical analysis of ingredients suggests both the SFC and DRC had higher than expected CP concentrations and lower than expected ether extract concentrations (Table 1). Additionally, the SFC had slightly greater ether extract and lesser CP compared to DRC. From an energetic intake standpoint, this may have put the DRC diets at a slight disadvantage. The lower than expected ether extract content of the DRC and SFC also resulted in diets containing WDGS to have slightly greater ether extract contents even though we attempted to equilibrate fat. Additionally, the WDGS had lower Ca than expected which resulted in reduced Ca levels in diets containing WDGS. However, the Ca:P ratio was still greater than 1 for the WDGS diets leading us to believe there were no negative impacts of the reduced Ca levels in the WDGS diets.

Animal performance.
There were no differences in final BW or ADG calculated from an actual live animal (final BW measured live and shrunk 4%) or carcass adjusted (final BW calculated as HCW/0.63) weight basis (P > 0.25; Table 2). Steam-flaking corn resulted in reduced DMI and improved feed efficiency compared to DRC in diets with and without WDGS (P < 0.01). Additionally, while the addition of 35% WDGS did not significantly impact DMI or ADG (P >0.26) slight shifts in these variables resulted in significant improvements in F:G (P = 0.03) due to the inclusion of WDGS. It is well documented that steam-flaking increases the energy availability of cereal grains resulting in improved feed efficiency from reduced DMI, improved ADG, or both (Zinn et al., 2002). The current data set resulted in a 13.3% improvement in feed efficiency when feeding SFC rather than DRC when comparing diets without WDGS. These observations are consistent with previous observations comparing SFC and DRC. The current data set also suggests an improvement in feed efficiency from feeding 35% WDGS. Vander Pol et al. (2006a) fed 0% to 50% WDGS in DRC-based diets and found optimal inclusion to be approximately 40% based on greatest feed efficiency. Similarly, Corrigan et al. (2007) fed 0% to 40% WDGS in diets utilizing three corn processing methods and saw a liner increase in G:F when adding WDGS to DRC-based diets but no change in G:F when adding WDGS to SFC based diets. The observations of Corrigan et al. (2007) suggest WDGS has an energy value similar to SFC. Conversely, Depenbusch et al. (2008a) observed reduced feed efficiency when feeding 25% corn WDGS in SFC-based diets and suggested that the relative response to WDGS in SFC-based diets may be lesser compared to DRC-based diets because of the energetic differences associated with these two corn processing methods. Our data are unique in that the inclusion of WDGS improved feed efficiency in both DRC- and SFC-based diets.

Carcass characteristics.
Dietary treatment had no effect on fat thickness, HCW, or dressing percent (P ≥ 0.27; Table 2). The inclusion of WDGS resulted in a concomitant reduction in % KPH (P = 0.02) and LM area (P < 0.01) which resulted in no difference in calculated USDA yield grade (P ≥ 0.39). Zinn et al. (1997) reported a linear decline in LM area as dietary sulfur concentrations increased from 0.15% to 0.25%. Conversely, Loneragan et al. (2001) reported a linear increase in LM area with increasing water sulfur intake. Although differences reported in these studies are unclear, the reduction of LM area in the current study may be related to dietary sulfur. The sulfur concentration of diets containing WDGS was 0.34% compared to 0.12% for diets containing no WDGS (Table 1). However, reduced KPH % has not been reported in studies investigating sulfur intake (Loneragan et al., 2001; Zinn et al., 1997) or dietary WDGS inclusion (Depenbusch et al., 2008a; Depenbusch et al., 2008b), however, the % KPH is not consistently reported.

There was a tendency for an interaction between WDGS inclusion and corn processing method on marbling score (P = 0.10). This interaction would suggest that cattle fed SFC without WDGS had greater marbling and other dietary treatments were not different. However, analysis of ultrasound estimated marbling scores collected at the initiation of the study suggested steers assigned to the SFC without WDGS treatments also had greater marbling at the time the study began (P < 0.03; data not shown). Therefore, initial marbling score was used as covariate in the analysis of marbling score and differences in marbling score became insignificant (P ≥ 0.22). Serial ultrasound estimates of marbling score suggest greater s.c. and i.m. fat accretion in cattle fed SFC- compared to DRC-based diets (Figures 1 and 2). This is consistent with greater energy intake in steers fed more intensely processing corn. Conversely, dietary inclusion of 35% WDGS resulted in similar s.c. fat accretion (Figure 3), but decreased i.m. fat accretion, especially late in the feeding period (Figure 4) compared to steers fed diets without WDGS. A major objective of this study was to determine if WDGS influenced marbling. Given the variation in marbling scores among individuals, even when genetic variation is minimized, it is not surprising that we did not observe a difference in marbling of carcasses. However, serially ultrasound reduces individual variation because the same steer is measured multiple times. This technique provided evidence that the inclusion of WDGS did reduce marbling.

Subcutaneous adipose tissue development.
The s.c. adipose tissue activities of NADP-malate dehydrogenase (NADP-MDH) and 6-phosphofructokinase (PFK) provide estimates of lipogenic and glycolytic capacities, respectively. Feeding DRC tended (P = 0.10) to increase plasma glucose and WDGS significantly (P < 0.01) depressed plasma glucose (Table 3). This is consistent with greater post-ruminal starch absorption from DRC-based diets and lesser from diets containing WDGS. However, there was no difference in PFK activity across treatment groups, indicating no difference in glycolytic flux in response to the changes in plasma glucose concentrations.

Steam-flaked corn increased (P ≤ 0.02) s.c. adipose tissue NADP-MDH activity relative to activity in s.c. adipose tissue from cattle fed DRC. Prior and Scott (1980) demonstrated that glucose infusion into beef cattle significantly increased the activities of acetyl-CoA carboxylase and fatty acid synthase, which limit the rate of lipid synthesis in bovine adipose tissue. However, glucose infusion had no effect on the activities of ATP-citrate lyase or NADP-MDH (Prior and Scott, 1980). This is consistent with the results of the current study, that changes in NADP-MDH activity were independent of differences in plasma glucose concentrations.  

Steamed-flaked corn also increased the rate of s.c. adipose tissue accumulation (Figure 1) and numerically increased 12th rib fat thickness and s.c. adipocyte volume, relative to DRC. An increase in ME intake in steers strongly elevates NADP-MDH activity in s.c. adipose tissue, as well as in activities of all key enzymes that regulate fatty acid synthesis in bovine adipose tissue (Smith et al., 1992). The increase in NADP-MDH activity in cattle fed SFC is consistent with the greater increased energy availability from SFC, relative to DRC. Neither corn processing method nor WDGS inclusion affected the mg protein per gram of adipose tissue, consistent with the lack of a significant effect of dietary treatment on adipocyte size (Table 3). We previously demonstrated that even a 25% reduction in feed intake did not significantly reduce s.c. adipocyte size in growing heifers (Smith et al., 1992). Only when the heifers were restricted to 30% ad libitum intake was adipocyte volume measurably depressed. Therefore, the differences in ME intake across dietary treatments were apparently insufficient to elicit large differences in adipocyte volume in this study. However, ultrasound measurement of 12th rib fat thickness indicated divergence of fat thickness after approximately 120 days on feed (Figure 1); cattle fed DRC had less 12th rib fat thickness than cattle fed SFC. This is consistent with the depression in NADP-MDH activity in s.c. adipose tissue of cattle fed DRC, and indicates that there was a biological effect of feeding DRC. We conclude that DRC depressed NADP-MDH primarily due to its lower digestibility, relative to SFC.

Intramuscular adipose tissue development.
With increasing animal age there is a concomitant increase in lipid filling of intramuscular (i.m., marbling) adipocytes, and this is associated with an elevation in the monounsaturated:saturated fatty acid (MUFA:SFA) ratio (Chung et al., 2006; Smith et al., 2006). The MUFA:SFA ratio of lipids from longissimus muscle was strongly decreased by WDGS, indicating that i.m. adipocyte differentiation was depressed in cattle receiving these treatments. There was a tendency (P = 0.10) for WDGS to depress marbling scores in cattle fed SFC, and WDGS significantly depressed the rate of marbling accretion, as assessed by ultrasound (Figure 4). Additionally, the ratio of marbling score:yield grade was 174 in cattle not receiving WDGS, and was 161 in cattle fed WDGS. Thus, the relative proportion of carcass adiposity represented by i.m. adipose tissue was depressed by WDGS.

The 16:1/18:0 ratio is a reliable estimator of Δ9 desaturase enzyme activity in bovine adipose tissue (Smith et al., 2006). The Δ9 desaturase enzyme is responsible for the conversion of saturated fatty acid as well as trans-vaccenic acid (18:1 trans 11) to their respective Δ9 desaturated products. The most abundant fatty acid in bovine muscle and adipose tissue, oleic acid (18:1 cis 9) is the product of the desaturation of stearic acid (18:0). Similarly, palmitoleic acid (16:1) is the desaturation product of palmitic acid (16:0). WDGS strongly depressed the concentrations of both of these MUFA. The gene that encodes the Δ9 desaturase, SCD, is expressed at higher levels as adipose tissues develop (Chung et al., 2007). We interpret the fatty acid data to indicate that WDGS depressed SCD gene expression and concomitantly reduced Δ9 desaturase activity in i.m. adipose tissue, resulting in more saturated lipids in the longissimus muscle. Additionally, a reduction in SCD gene expression by WDGS may have caused a depression in i.m. adipocyte differentiation.

Sensory analysis.
Sensory attributes for steaks from cattle fed either SFC or DRC and either 0 or 35% dietary WDGS are presented in Table 5. The interaction between corn processing method and WDGS inclusion was significant for palatability attributes and cooked beef fat aromatics. Steers fed DRC without the inclusion of WDGS were slightly drier, tougher and more intense in overall flavor than steaks from the other treatments. Within steers fed 35% WDGS, steaks from steers fed SFC had slightly less cooked beef fat flavor than steaks from steers fed DRC. Steaks from steers fed 0% WDGS were intermediate in cooked beef fat flavor. Main effects for corn processing affected cooked beef/brothy, serumy/bloody, cardboardy, livery/organy aromatics; chemical burn and metallic mouthfeels; and bitter, salt and sweet basic tastes. Steaks from animals fed SFC had more intense cooked beefy/brothy and livery/organy flavor aromatics and bitter, salt and sweet basic tastes; and less intense serumy/bloody and cardboardy flavor aromatics, and metallic mouthfeels. These results indicate that corn processing method affected sensory properties of steaks; however, differences were consistent, but minimal. While an expert, trained sensory panel could detect small differences in flavor attributes of steaks from steers fed SFC versus steaks from steers fed DRC, consumers may not detect these slight differences. Steaks from steers fed 0% WDGS were higher in cooked beefy/brothy and browned/burnt flavor aromatics, chemical burn and astringent mouthfeels, and bitter basic tastes when compared to steaks from steers fed 35% WDGS. Additionally, steaks were lower in cardboardy and livery/organy flavor aromatics and sweet basic tastes from steers fed 0% versus 35% WDGS. These results indicate that sensory differences were detected in steaks from steers fed differing levels of dietary wet distiller’s grain plus solubles; however, differences were slight and it is unknown if the combination of these differences at the levels detected by expert, trained sensory panelists would be detected by consumers.

Shelf-life.
Corn processing method did not affect the color of steaks; however, steaks from steers fed DRC corn had slightly higher TBARS values indicating that these steaks were slightly more susceptible to lipid oxidation. Although not statistically significant (P = 0.25), the numeric effects of corn processing method and WDGS inclusion indicate a reduction in TBARS when steers consumed SFC without WDGS (Table 6). This is consistent with lower concentration of linoleic acid (18:2) observed in the lipid profile of steers consuming SFC without WDGS (Table 4). Steaks from steers fed 35% WDGS were darker and had slightly higher levels of lipid oxidation. Within steaks from steers fed SFC, the steaks from steers fed 0% WDGS were slightly more red than the steaks from steers fed 35% WDGS. Interactions of main effects were not significant for color space values, except for the previously discussed interaction for CIE a* color space values. Storage day had the greatest effect on color space values (P < 0.01; Table 6). With increased storage, steaks became darker, less red and more yellow as would be expected. Steaks were similar in color after 0 and 1 days of storage. After 3 days of storage, steaks were slightly lighter, but less red and yellow. After 5 days of storage, steaks were darker and had increased yellow values. After 7 days of storage, steaks had the lowest red values. Color changes reported in this study are as expected and based on these results, steaks had appreciably changed in color after 3 days of storage. Interestingly, dietary treatments did not affect color changes during storage indicating that steak color during storage was not influenced by animal diet. This was not true for the effect of dietary treatment on rate of lipid oxidation. As expected, steaks had higher TBARS values with increased storage; however, there was a storage day by dietary wet distiller’s grain treatment interaction (Figure 5). Steaks from steers fed 0% WDGS had similar TBARS values compared to steaks from steers fed 35% WDGS at 0, 1, 3, and 5 days of storage. However, after 7 days of storage, steaks from steers fed 0% WDGS had lower TBARS values. These results indicate that steaks from steers fed 35% WDGS would have higher rates of lipid oxidation after extended storage. It is interesting that differences in lipid oxidation did not occur until steaks had been stored for extended periods. Color shelf-life changed at 3 days of storage, but differences in lipid oxidation were not noted until 7 days. Most likely steaks would be pulled from the retail case after color deterioration and therefore, differences in lipid oxidation may not be found as lipid oxidation differences occurred 4 days later. These results indicate that when lipid oxidation is occurring at a rapid rate as found from 5 to 7 days in the current study, steaks from animals fed 35% WDGS oxidized at a more rapid rate. An unexpected finding of this study was that both DRC and WDGS increased the muscle concentration of linoleic acid (18:2n-6). Linoleic acid is the most abundant polyunsaturated fatty acid in beef, and increasing its concentration may have made the beef lipids more susceptible to oxidation.

Implications  

Feeding wet distiller’s grains to feedlot cattle may impact marbling and shelf-life of beef, but likely has minimal impacts on beef taste and quality. Research conducted by Texas AgriLife Research investigated the effects of feeding 35% distiller’s grains to steers consuming diets made up of steam-flaked or dry-rolled corn. While effects of feeding steam-flaked or dry-rolled corn could be explained by energetic differences inherent to those processing methods, the addition of distiller’s grains may have altered the activity of enzymes important in the deposition of intramuscular fat (marbling). As a result, beef from cattle fed distiller’s grains was higher in saturated and lower in monounsaturated fatty acids, relative to cattle fed diets without distiller’s grains. In addition, steaks from steers fed distiller’s grains were darker in color and were more susceptible to lipid oxidation after 5 days of storage. However, when a trained taste panel evaluated steaks for palatability attributes, only subtle differences could be found. These results indicate that while the inclusion of distiller’s grains may impact marbling, fatty acid composition, and length of time beef is displayed in the retail case, consumers should not expect feeding distiller’s grains to change the eating quality of their beef.