Background

Propiogenic diets (such as high-grain diets) promote greater marbling development than acetogenic diets. Researchers predicted that “glucogenic” diets, which provide more glucose for absorption from the small intestine (e.g., dry-rolled corn), would promote marbling development, hence juiciness, to a greater extent than propiogenic diets. This hypothesis was tested by directly infusing isocaloric amounts of glucose and propionate into the digestive tract of young, growing steers. Glucose was infused into the rumen (reflecting a high-starch diet) and into the abomasum, the latter treatment bypassing ruminal metabolism. Acetate was also infused into the rumen to directly provide substrate for fatty acid biosynthesis in subcutaneous (SC) and intramuscular (IM) (marbling) adipose tissues. It was predicted that glucose infusion into the abomasum would increase carcass adiposity and beef juiciness, relative to ruminal infusions of acetate, propionate, or glucose. Total IM lipid and fatty acid composition of IM and SC adipose tissues were measured to confirm results for carcass marbling scores, as increased marbling is associated with a higher concentration of monounsaturated fatty acids in SC and IM adipose tissues. In vitro incorporation of glucose and acetate into fatty acids was also measured to confirm the effects of acetate, propionate and glucose infusions on SC and IM adipose tissue metabolism. 

The objectives of this study were to determine if increasing the delivery of glucose or a gluconeogenic precursor (propionate) to the liver would promote marbling development, and to establish the relationship between marbling scores, fatty acid composition and juiciness of beef.

Methodology

Angus crossbred steers (n = 24) were backgrounded on native pasture until approximately 22 months of age. The steers were fitted with ruminal cannulas at the Texas A&M University Animal Research and Teaching Center. Cattle were adapted to a standard, corn/sorghum finishing diet (Table 1) over a 2 week period while recovering from the placement of the cannulas. After the adaptation period, the following solutions were infused: Acetate, 1.077 kg/day (16.7 mol/day; 3.76 Mcal/day) infused into the rumen (n = 6) (control group), Glucose, 1.0 kg/day (5.55 mol/day; 3.76 Mcal/day) infused into the abomasum (n = 6), Glucose, 1.0 kg/day (5.55 mol/day; 3.76 Mcal/day) infused into the rumen; (n = 6) and Propionate, 0.758 kg/day (10.15 mol/day; 3.76 Mcal/day) infused into the rumen (n = 6).

The solutions were infused for the last 30 days of the finishing period. At the end of the infusion period, the steers were transported approximately 9 km to the Texas A&M University Rosenthal Meat Science & Technology Center, where the cattle were harvested by humane, industry standard procedures. Immediately post-exsanguination, a portion of the 5th-8th rib section of the Longissimus thoracis muscle was removed and transported to the laboratory. Pieces of IM and SC adipose tissue were removed by dissection while still fresh and incubated with 1 μCi [U-14C] glucose or 1 μCi [1-14C] sodium acetate. Fatty acid synthesis was measured in IM and SC adipose tissue. Radioactivity of lipid extracts was counted with a scintillation counter.

Carcasses were graded after chilling for 48 hours postmortem at 4°C, after which the Strip Loin was removed for sensory and fatty acid analyses. Strip Loin Steaks were individually vacuum packaged and aged for 14 days, after which they were stored at -40°C until sensory evaluation. Descriptive flavor analysis was conducted by a trained, 4-member descriptive panel. Steaks were cooked in electric skillets set at 204°C to an average internal temperature of 71°C. Once steaks reached 71°C they were cut into eight individual pieces, providing two samples for each panelist. Samples were served a minimum of 4 minutes apart.

Findings  

Contrary to the hypothesis, acetate elicited the highest marbling scores, but also the highest yield grades, when infused over the 30-day period. Because ruminant species have evolved to utilize acetate as the primary precursor for fatty acid biosynthesis within their adipose tissues (which was confirmed in this study), the data suggest that providing additional acetate, rather than glucose increases both subcutaneous and intramuscular adipose tissue development. Finally, most beef flavor and textural values were highest in beef from the acetate-infused steers, suggesting that providing additional sources of ruminal acetate would promote the production of higher quality beef.

Implications  

The findings of this study do not support the premise that feeding additional starch over that typically found in feedlot finishing diets will increase beef carcass quality. Instead, providing supplemental sources of acetate may increase marbling scores, and beef flavor and textural scores, which is a novel and unique finding of this study. However, additional studies are required to establish practical and cost-effective means of supplementing acetate in feedlot diets.