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).
10 to 15 lbs per acre to fill in where one generally has a lot of volunteer ryegrass every year. 20 to 30 lbs per acre for normal late winter and spring grazing or for making hay. 40 to 50 lbs per acre for quicker fall growth for early fall and winter grazing.