Background

Methodology

Diets for finishing cattle were defined to include no (control) corn milling co-products, or to provide 2% (low) or 6% (high) added corn oil from corn germ. These levels of germ were fed in high-starch or high-fiber finishing diets. The higher fiber diets included substitution of chopped, high-moisture earlage and dried corn-gluten feed for rolled, high-moisture corn and dry, whole-shelled corn. These diets were evaluated in 144 steers with an initial body weight of 349 kg selected based upon uniformity of weight. To allot steers to treatments, steers were ranked by body weight and then sequentially assigned to high- or low-starch diets. Steers were then sorted by starch diet (high or low), ranked by body weight and randomly assigned to one of three germ levels. The process was repeated to assign steers to pen replicates.

Individual body weights were determined initially and at 28-day intervals during the study except during the last 25-day period. Revalor-S implants were administered to all steers during the day 28 weighing process. The feeding study duration was 137 days. Ultrasound measurements of ribeye area, ribfat depth and percent intramuscular fat (IMF) were acquired 77 and six days prior to harvest.

An MRatio can be calculated as a means of comparing marbling to either preliminary yield grade or to total carcass fat (M2Ratio). The following equation was used:

The MRatio reported used marbling and ribfat depth as variables 1 and 2, respectively. For MRatio calculations, percent carcass fat derived from 9-10-11 rib sections was used as variable 2. A value of 0.0 signifies a ratio was not altered. Positive values indicate a favorable level of marbling to fatness. Negative MRatio values indicate marbling depression has occurred.

Glucose (GLU), non-esterified fatty acids (NEFA), insulin (INS), plasma urea nitrogen (PUN), triglycerides (TG), cholesterol (CHOL), high-density lipoproteins (HDL) and low-density lipoproteins (LDL) were determined in plasma. The remaining lipoprotein fraction, which included very-low density lipoproteins (VLDL), intermediate density lipoproteins and chylomicrons, was calculated by subtracting the HDL and LDL fractions from the total CHOL.

To evaluate plasma hormone and metabolite concentrations relative to the ability of cattle to deposit intramuscular versus subcutaneous adipose tissue, cattle were separated based on their M2Ratio. The M2Ratio was used to identify 24 steers to be used as sera donors for satellite cell cultures. These included the three or four highest and lowest M2Ratios within each level of germ. Satellite cells used were isolated previously from the Longissimus dorsi muscle of a young, growing, high-energy-fed heifer. 

FINDINGS

Fiber has less net energy (NE) than starch, and the tabular NE values were 62 and 57 Mcal/cwt. for control and higher-fiber diets, respectively. Predictably, the higher fiber diets resulted in 5% higher dry matter intake (DMI), 10% lower average daily gain (ADG) and 14% poorer efficiency. The only carcass trait affected (P < 0.01) was hot carcass weight (HCW) with weights of 832 lb and 801 lb, respectively. Fiber levels did not alter blood glucose, insulin or lipids. Improved performance has been observed when feeding distiller’s grains. The fiber aspect of the current experiment suggests that the fiber fraction of distiller’s grains is not the source of this response and that the fiber level does not alter carcass fatness or fat distributions.

Adding oil to the diet via germ increases the energy density. In this study, this had little effect on live animal production traits, except that the high germ inclusion caused lower ADG. The HCW differed (P < 0.05) due to germ level with values of 816 lb, 823 lb and 810 lb for control, low and high germ diets, respectively. Germ also reduced marbling (P < 0.05) while having no effect on ribfat thickness. The most pronounced effect on marbling occurred on the low germ diet. 

Feeding germ resulted in negative M2Ratio values (P < 0.10). Figure 1 illustrates carcass fat content and corresponding M2Ratios for the three levels of germ in this study. Feeding the low level of germ also caused a slight increase in blood glucose levels (63 vs. 68 mg/dl; P < 0.05). When germ was fed, blood lipids increased dramatically (P < 0.01). TGs, HDLs, LDLs, VLDLs and NEFAs all increased (P < 0.05) with increasing dietary germ. The relationship between this shift in lipid metabolism and the causes for the shift and accumulation of IMF could be investigated further. 

IMPLICATIONS

This project shed light on new concepts to pursue as the industry strives to improve quality grades in beef carcasses. Dietary oil causes dramatic shifts in blood lipid profiles that have not been previously considered when studying IMF accretion. Besides diet, those steers that produced more highly marbled carcasses had substantially different serum lipid profile responses than steers producing less marbled carcasses. Serum from high-marbling steers provided more growth stimulation to satellite cells involved in muscle growth. This is in contrast to the perceived antagonisms between muscle growth and IMF accretion. These observations will be useful in ongoing efforts to solve the puzzle of utilizing diet, management and genetics to improve beef quality grades.