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).
A good rule of thumb is 4% of body weight per head per day of good hay for mature dry cows ( 1st and 2nd trimesters of pregnacy). 4% mature weight per head per day of 50/50 good quality grass/alfalfa in the last trimester of pregnancy and early lactation. Watch their condition and adjust feeding accordingly.