Elk calves have a fairly high mortality rate, but that should not influence your decision on whether to harverst a cow or calf. As previously noted, orphaned calves will be adopted by other elk.
20% mortality during the first 13 weeks is pretty standard and that doesn't deviate much from one study to another. That 20% is during summer - well before the calves first winter. Even farm raised elk have 5% to 15% calf mortality. Winter survival is far less predictable and has far greater variance.
Also consider that even in those areas where the 1 year survival rate is considered low (20 surviving calves per 100 births) that is still enough in many areas for the total herd size to actually increase.
