Many factors influence the feed cost for dairy cows. Grazed grass is considered the cheapest form of feed, followed by forage silages as the most economical feed source and concentrate supplements being the most expensive.
No crop can be ensiled and consumed with 100% efficiency. There is bound to be some losses during harvesting, storage or during feed-out, amounting to > 40% of the available grass DM. One way to combat this is the use of additive or having good management systems such as big baling.
The most sustainable feeds are on-farm produced feeds as transportation is minimised. In addition to that, "home grown" silage gives traceability as well as security for farmers.
One of the biggest advantage of preserving forage in times of plenty is that it allows flexibility to cope with shortages during unpredictable weather patterns.
Fertiliser and cereal-based concentrates are getting more expensive and it will soon be under pressure to be used for growing food for human consumption due to the growing population.