PATTERNS OF MUSCLE, BONE, AND FAT ACCRETION IN THREE BIOLOGICAL TYPES OF FEEDLOT BULLS FED THREE DIETARY ENERGY LEVELS

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Abstract
The effect of three types of diet on the carcass composition of 71 feedlot bulls of three biological types (Dairy Cross (DX), Hereford Cross (HX), and Beef Cross (BX)) was studied over a 2-yr period. Diets consisted of pelleted alfalfa with 0, 35 or 85% grain. Serial slaughter and carcass dissection into eight wholesale cuts and component tissues were carried out over an age range of 392–636 days. Interactions of biological type with diet-year were generally not significant for actual weight of muscle, bone and fat, or for weight of these tissues at a constant side subcutaneous fat weight of 16 kg. There were few significant effects of biological type or diet-year on the ratio of actual weight of subcutaneous to intermusclar plus body cavity fat. HX bulls had significantly less muscle and less bone than BX or DX, based on actual weights, but the rate of tissue accretion relative to side subcutaneous fat was the same for the three biological types. Animals on the lowest level of dietary energy had less weight of fat than did those on an 85% grain diet, but diet-year growth coefficients of tissue weights relative to side subcutaneous fat weight were generally homogeneous. It was concluded that effects of dietary energy were consistent over the biological types studied and for most of the wholesale cuts. Key words: Feedlot bulls, diet, biological types, tissue growth