Does vitamin C need to be added to pig feed?

Does vitamin C need to be added to pig feed?
In the formulas I've seen from some teachers, none of them include vitamin C.
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Learning... looking forward to an expert's answer...
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Vitamin C's role is its anti-stress effect. Some manufacturers add it. Most manufacturers don't add it.
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Adding vitamin C at certain stages and in specific seasons can reduce pigs' stress response!