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 can be reduced by around 1kg per day if milking once a day. With grass in the diet, at least by day, a concentrate crude protein content of 14% is sufficient. A standard guideline for mid to late March is therefore 14kg grass plus 3kg high-energy concentrate per day.