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).
However, some 800-pound cattle fed 4 pounds of corn or 3 pounds of corn and 1 pound of soybean meal per head daily on these same pastures from mid-June to early September gained 0.54 pounds more per head daily than those without supplements. An extra pound of gain took 7.4 pounds of supplement.