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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.
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