I really need to ask if the following two English expressions are correct:

His twenty-four cows were killed.

His two dozen cows were killed.

I feel kind of weird about those sentences, but I don't know if we can actually say in this way. Please give me your opinion and I'll appreciate it.
Hi Viceidol

Yes, you could use those two sentences, and I prefer the first to the second. However, it might sound more natural to say this:
All twenty-four of his cows were killed.

Both are ok, although I'd be more likely just to say "all his cows were killed"
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How about "Two dozen of his cows were killed."?

Is that OK, too?

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Hi Viceidol

That's OK, but the meaning is different. That means that he started with more than 24 cows and then 24 of them were killed.
"Two dozen of his cows were killed."= "Twenty-four of his cows were killed".

Yes. A dozen is exactly equal to twelve (12). I get the impression that that is your central concern. Emotion: smile