Technically, you need to maintain a minimum effective NDF level to keep the rumen health.  Processed HM Corn has zero effective fiber, so in this case the hay will be the only source of e-NDF in the diet.  HM corn is also a very soluble starch source so can cause acidosis easily.  Using a mature grass hay would be a good choice as it is high in effective NDF.  In a ration with HM corn, some sort of co-product, and ground hay, you would want to feed about 8% of the as fed diet as ground hay.  For finishing cattle eating 22-24# of dry matter that means about 2.5# of ground hay.  For haylage, you can adjust for the moisture, but haylage also does not have as much effective NDF since it's "soft" so round up.   For corn silage, 8-10# is as low as I'd go if it's the only source of fiber in a HM corn diet.