You mean like humans....?
Welcome and that's a good question. I guess we can assume they have looked at other species if indeed as some think they have been around a while. Among the 'alien cattle mute proponents' the theory is they are using the cattle parts for some scientific experiment. (Others have actually, perhaps jokingly, said they eat the parts.) But horses , sheep, cats, and dogs have also been said to have been mutilated mysteriously though those cases are poorly documented from what I have read online...currently the only source we have. I have also read years ago that cow/cattle blood and dna is somewhat similar to human materials on a biological and chemical level so those who support an alien conspiracy have also used this as a reason.
But as many have mentioned over the years if they need cattle parts why not just raise some of their own and have a ready supply for themselves? Fo these and other reasons I think most ufologists now reject the idea that aliens are taking cattle parts but the idea persists.