Ruminants, like most mammals, do not require supplemental vitamin C because they can synthesize it internally themselves – at least this is what they taught us in graduate school.
The latest edition of Dairy Nutrient Requirements (NASEM, 2021) follows the same reasoning, excluding vitamin C supplementation from dairy nutrition, including calf milk replacers. (NASEM is the same as the NRC from the U.S., just with a different name.)
In practice, some – but not all – milk replacer powder products contain supplemental vitamin C. Surprisingly, and erroneously, some vitamin C is also included in dry starter feeds. If the purpose is to feed the bacteria in the developing rumen, then little can be said. However, this is the least of the errors I have seen; for example, milk in the starter is a grave one, but I digress. So, back to milk replacer powders.
The main reasoning behind the supplemental vitamin C in some milk replacer is a limited amount of research that points to the fact that this extra vitamin C helps calves overcome disease-related stress (less days with diarrhea, etc). If that is the case, I am fully supportive. But we need to understand why we do this and, above all, not to overlap with other health-related measures. In fact, and this is my preference, vitamin C is usually added into electrolyte packs that are easily dissolved into the milk of sick or sickly calves, and this seems like a good measure to ensure economy. But it might not work as well in mega operations with thousands of calves.
So, we need to realize calves do not require vitamin C as a nutrient, vitamin C can be part of a “health” pack of measures against common stress factors, and the way to administer this “functional nutrient” can vary depending on individual farm circumstances.
