It's just that keeping your heart rate in the target range during regular aerobic exercise has been shown to be safe and effective for increasing your aerobic fitness. Exercising above this range can be very uncomfortable and may increase your risk of injury.
Typical daily weight gains for feedlot animals ranges between 0.5kg to 2.5kg per day, while grass fed animals are somewhat lower than this. The following data was collected from a small beef cattle company in Dallarnil in Queensland (approximately 300km north of Brisbane).
This scenario assumes you will carry the 400-pound calf to a sell weight of 750 lbs. and sell at $2.05/lb. However, if the sell price for 750-pound calves was $2.43/lb., then the net margin per calf increases to a net gain of $168.04 per head, and the net return per acre increases to $306.85 per head.