Final answer:
The three factors used to determine USDA feeder cattle grades are thickness, frame size, and quality. Quality is a crucial element in predicting the potential for desirable meat production in terms of leanness and tenderness.
Step-by-step explanation:
The three factors used to determine USDA feeder cattle grades are thickness, frame size, and quality. When assessing the grade of feeder cattle, these criteria are essential in predicting the animal's ability to produce a desirable quantity of meat that meets consumer standards for leanness and tenderness. It is important to note that factors such as economy, efficiency, and thriftiness, while important to overall cattle management and production, are not the specific factors used in the USDA grading system.