The bitul of chametz in a mixture is the 60:1 because both of them were mutar before Pesach. The bitul of the declaration would not allow it to be eaten as that is similar to finding a piece of bread on Pesach that had been "nullified" by the kol chamirah. You would still have to destroy it on chol hamoed as it is still forbidden chametz. The difference would be a container of milk. If you had it from before Pesach, then the "chametz" in it (such as the vitamin D) would have been batel (while it was still mutar) and the entire container is mutar. If the vitamin D had been added after Pesach started, then it is chametz and asur.
Note that the mention of feeding the cows grain is due to the worry that pieces of grain that had stuck to the udders somehow got into the milk. This was mainly true during the days of hand milking, but it could occur when the vacuum tube is attached to the udder (although very very unlikely nowadays). However, due to the seriousness of the matter we are machmir even for such an unlikely event.
MILK ON MOED?
Q. Friends tell me they do not buy dairy products on Chol HaMoed Pesach. Why do they purchase these dairy products before the holiday?
A. We take a very strict approach to Passover, often going beyond the letter of the law to avoid chametz.
Dairy is a good example. Technically speaking, there is no chametz
concern when it comes to milk. But cows eat grain, and grain becomes
chametz upon prolonged contact with liquid. The custom to purchase
dairy products before Pesach is rooted in the fear that a chametz
particle may be present in the milk.
Why, then, does it make a difference if the milk is purchased before
Pesach? The answer lies in the concept of bittul - nullification. A
particle of grain would be indiscernible and also would constitute
less than one-sixtieth of the milk. Hence the grain is nullified in
the milk. However, nullification applies only before Pesach begins
(specifically, before midday on erev Pesach). On Pesach we do not
apply the rule of nullification. Therefore, if the milk was in the
possession of the Jew before noon on erev Pesach, any chametz in the
milk is batel; hence the custom to purchase milk before the holiday.
A dairy product that was present in a Jewish-owned store before Pesach
would not be of concern, since any chametz in it was nullified. The
custom to avoid purchase of dairy on Chol HaMoed pertains to dairy
found in stores owned by non-Jews.
Chalav Yisrael dairy products are made under the watchful eye of
observant Jews. When marked Kosher for Passover, these may be bought
on Chol HaMoed. One can be assured that the cows were not fed grain
during the period immediately before Pesach, so that there is no
suspicion that chametz is present in the milk.
When buying dairy products for Pesach, please make sure that they have