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).
The stocking rate should be increased when grain supplements are fed, since the cattle will eat less pasture. In University of Oklahoma studies, it took 15 to 30 pounds of grain for each pound of gain increase if grass was surplus, but only 8 to 10 pounds if grass and cattle were balanced.