The easiest would be to maintain a separate warehouse for each batch of calves. Add the calves as an item into the warehouse with the appropriate valuation. Now add all the spends on the calves as a stock item into this same warehouse. So the Stock Balance Valuation will give you the total spend and that divided by the number of calves you started with will give you the valuation per calf. When the calves have grown to be cows, pass a “manufacturing” entry to “consume” up the calf and all the items that were used to spend on the calves - fodder, doctor’s services, medicines, etc. etc. and produce a milking cow. Now you retire that MilkingCow item (Stock entry purpose Material Issue and charge it to a temporary account) and create a MilkingCowAsset as a Fixed Asset and charge it to the same account. This part may be a bit tricky as you may need to create the asset using a Purchase Receipt.
Alternatively, you can do the same thing, but create a project for each batch of calves and charge everything to this Project.
The other option is to keep using the Asset Revaluation feature.
Of all of these, I think the Warehouse approach is the simplest and the easiest, but I’m aware that it’s an engineer’s solution and maybe be inadequate from an accounting perspective.
Hope this helps.