This is an exerpt from a well know study on Yellowstone wolf predation.
The entire thing can be found here:
http://www.npwrc.usgs.gov/resource/2...rity/Intro.htm

The wolves in our study killed elk at rates similar to wolves hunting caribou (Rangifer tarandus; Mech et al. 1998:110) and tended to kill calves, old cows, and individuals with low marrow fat (unhealthy).

In summary, the relationships between reintroduced YNP wolves and previously wolf-free elk did not differ in any way that we could detect from wolf-prey relations in long-extant (this means systems that always had wolf-elk predation occuring) systems. This was true despite the high ratio of prey available to wolves and the large number of unculled prey. Furthermore, the degree of winter severity affected the new wolf-elk system in much the same way it affects long-extant systems. These findings suggest a dominating influence of winter severity on wolf predation patterns (Mech et al. 1998).