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).
Goats are not efficient converters of processed grain to weight gain. Seven pounds of grain produces one pound of weight gain. Determining average daily gain on a diet of forage/browse is how most producers measure the ability to grow fast.