Oxford Living Dictionaries (aka Oxford Online Dictionary or ODO) offers as a definition for cow
1.1 (loosely) a domestic bovine animal, regardless of sex or age.
But the same resource answers the vocabulary question What is the word for a cow that doesn't specify its sex?
The truth is that there is no noun in general use that refers equally to a cow or a bull.
Zoologists use two terms. The first is 'ox', which is often restricted to animals of the genus Bos (i.e. the wild cattle - gaur, banteng, yak, aurochs, and kouprey - as well as domestic cattle). In popular use, though, the word 'ox' often refers to a castrated male animal, so that isn't a perfect solution. The second zoological term is 'bovine', which is used as a noun to refer to any animal of the wider group that comprises cattle, buffaloes, and bison. But this would be a strange choice in most general contexts (emphases mine).
And the same source in a blog entry called The peculiar history of cows in the OED says:
...Very rarely [sic] do we stop and think about the fact that cows are not, technically speaking, a species. They’re only the female half...
...In the plural, we can say that they’re cattle (except when cattle is used to mean livestock generally). But the singular is messier. The word ox is one candidate, as it originally meant ‘a cow, a bull’, but now is more often specified to a ‘castrated adult male of this animal.’ Heifer is also sometimes used as a sex-neutral term, though this too is not strictly correct. Some may accuse such a position of pedantry, noting that the use of cow to refer to the species has grown so pervasive as to have changed its meaning, but that doesn’t mean the phrase ‘male cow’ is going to make scientific sense any time soon (emphases mine).
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When I am amongst my rural kin I have learned to call a cow a cow and a bull a bull and not to mix the two.