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).
Depending on what levels of each category that you feed, you will find that goats will gain about .25 to .5 a pound each day. You can adjust your rations to have your goats reach a particular market weight in a specified amount of time. Goats will readily eat a variety of forages.