Ruminants ferment dietary carbohydrates to volatile fatty acids in the rumen, which provide over 90% of the energetic need of the animal. In the rumen, very small amounts of glucose is absorbed, but certain tissues are obligatory users of glucose and must be have glucose for survival. Storage of glucose as glycogen in the muscle is also a major determinant of meat quality. Therefore, ruminants rely on gluconeogenesis, the process of synthesizing glucose or glycogen from non-carbohydrate precursors. Glucose is an important substrate for intramuscular (marbling) fat deposition, while acetate is important for the deposition of subcutaneous fat. Thus, available substrates for intramuscular fat deposition may be limited in traditional finishing diets and supplementation of these substrates to the small intestine may increase marbling and glycogen deposition in the muscle, which would enhance carcass value and meat quality.
Crude glycerin is a byproduct of the biodiesel industry and can be utilized in livestock diets. Crude glycerin is approximately 80% glycerol but this can vary depending on the source of the byproduct. Glycerol is the three-carbon backbone of triglycerides and can be used as a substrate for gluconeogenesis. High fructose corn syrup is produced from corn milling and has become the leading sweetener utilized in the beverage industry. High fructose corn syrup is 42% glucose and 55% fructose, but can be further refined to enhance fructose levels. High intakes of fructose can lead to increased lipogenesis and fatty acid synthesis. However, if glycerol or fructose are fed to ruminants and enter the rumen, they are fermented into volatile fatty acids and only very small amounts of these substances would be transported to the small intestine for absorption. If these substrates were allowed to bypass fermentation in the rumen they may provide a readily available source of glycerol and fructose to the tissues for new fatty acid synthesis. Previous studies have estimated that 60-80% of water consumed by ruminants bypasses the rumen. They found greater bypass of drinking water for animals on high concentrate diets (80%) compared to hay and forage diets (62%). Therefore, drinking water provides an excellent means of delivery of substrates to the small intestine of finishing animals for direct absorption and utilization by the tissues.
The objective of this research is to supply substrates for gluconeogenesis (glycerol) and/or new fatty acid synthesis (glycerol and fructose) via drinking water to stimulate marbling deposition and enhance meat quality of finishing cattle.
Thirty-six steers were sorted by weight and randomly assigned to one of three treatments: 1) control, regular drinking water (CON), 2) 0.36 lb of crude glycerin per gallon of drinking water (GLYC) or 3) 0.36 lb of high fructose corn syrup per gallon of drinking water (HFCS). Steers were allowed ad libitum access to drinking water treatments for 25 days and then harvested at a commercial packing plant. At 24 hours postmortem, carcasses were graded by trained personnel and the rib section (IMPS 107) encompassing the 6th to the 12th rib from the left side of each carcass was identified, removed and shipped to the Clemson University Meat Laboratory. Steaks were obtained from each rib for subsequent analyses of adipocyte size, objective color, pH, water-holding capacity, tenderness, fatty acid composition and proximate composition.
Overall, drinking water supplementation of glycerol (GLYC) or fructose (HFCS) to finishing steers for 25 days had little to no effect on animal performance or meat quality. Mean intramuscular adipocyte diameter was greater (P < 0.05) for steers offered HFCS compared to GLYC with CON being intermediate. These differences in mean adipocyte size were related to changes in the adipocyte size distribution. There were greater proportions of small (20-30 μm) adipocytes in GLYC compared to HFCS and CON, which would indicate greater hyperplasia (increase in cell number). In contrast, HFCS and CON had greater proportions of medium (40-50 μm) adipocytes than GLYC, which would indicate greater hypertrophy (increase in cell size). Overall, HFCS or GLYC supplementation via drinking water did not alter carcass or meat quality variables but did alter the size and distribution of intramuscular adipocytes.
This study shows that adipocyte size and number may be altered with glycerol and fructose supplementation via drinking water. Further research would be needed in order to determine if a longer supplementation and/or higher concentrations of these substrates in the drinking water would translate to a greater amount of marbling deposition.
Figure 1. Intramuscular adipocyte diameter distribution of steers offered crude glycerin (GLYC) or high fructose corn syrup (HFCS) in drinking water or regular drinking water (CON).