The extraction would surely involve some heavy duty restraints for a wild mammoth, but a domesticated variety would likely be more manageable. As for the ejaculate itself it may prove rather appetizing and nutritious.
Healthy semen (that is, cum produced by a well-nourished, disease-free male) may in fact be not only safe to eat, but actually good for you. Seminal fluid, the base of cum, is a veritable nutrient-rich soup. Semen is a source of highly concentrated, high-quality protein. In dietary terms, it's comparable to egg whites or gelatin.
Besides protein, semen contains high concentrations of some minerals, such as zinc, and trace amounts of other important nutrients, like calcium and magnesium. While you shouldn't depend on a shot of cum to get your recommended daily allowances of vitamins and minerals, swallowing semen on a regular basis can't hurt as a dietary supplement. http://www.dontspitswallow.com/cum_nutrition.shtml