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).
So if she is thin, she may need to add weight as the fetus develops - about 100# of gain before she calves. So, a 1200# cow needs roughly 24# per day of hay to maintain her weight, plus another 750# of feed over the next 3 months to compensate for the fetal growth - an additional 8# of hay per day.