Yes she has to be weaned. Chances are (and high chances at that) that the first-calf heifer won't wean the calf herself. She won't have the experience to wean the calf, for one. The best time to pull the calf off is one of two indicators: 1) ideal age to wean the calf, which would be at around 6 months of age; or 2) if the calf is pulling the first-calf heifer down in terms of weight and condition. If the heifer is losing condition or is thinner than she should be, the calf must be either weaned (best time is 3 to 4 months of age, at the earliest), or creep-fed with grain, or both.