First off, what exactly are Iron Rations? The idea of a pre-prepared food ration for soldiers has been around since the early 1800's but only got fully established in the 20th century. Often these were designed as emergency rations for when troops were not able to be fed and supplied by an army canteen. Rations often include a meal of some kind plus the necessary makings of a hot drink such as coffee or chocolate. The meal might be a dried food item designed for field cooking or it might be a ready-to-eat item such as an energy bar. One thing that is clear is that nothing like this existed in the quasi-medieval world in which D&D is set. The second questions is, what would be a better alternative for our PC's? Tolkien developed the ideal alternative in the form of the Elven Lembas Bread. This is often referred to as waybread and may have been based on the well known (but much less nutritious) hardtack biscuits used by the navy. This would complimented by the invigorating Ent-draught, a drink brewed from mountain spring water by the Tree Ents of Middle Earth.
Of course you don't have to plunder Tolkien for ways to feed your Characters on their adventures. There were plenty of preserved food alternatives used by medieval travelers that would easily fit into a standard game setting.
- Dried meat - Made by soaking strips of meat in salt brine or rubbing with dry salt. The meat is dried and needs to be re-hydrated before eating.
- Hard cheeses: These have a high calorific value and remain edible for a long time. They are also versatile being suitable for cooking.
- Sausages: The sausage has been around for centuries and is a common way of preserving meat during the winter months. Usually made by mixing ground meat with spices and then put in a casing made from animal intestine.
- Bread: The elves are not the only race to enjoy bread. Bread is one of the oldest foods and is high in nutritional value. Bread can be extended in life by twice-baking (or four times in the case of hardtack).
- Pickled Vegetables: The art of pickling is almost as old as the art of bread making.
- Dried Fruits: This was a common way of preserving fruits in the ancient world and would ensure bountiful harvests lasted all year. They are also a good way of ensuring good nutrition for the weary traveler.
I expect many roleplayers will consider Rations the same way they do Encumbrance, something they can safely ignore because the GM is focused on storytelling not bookkeeping. But I think this is a great pity and a missed opportunity for side adventures and a little 'local colour'. Its a chance for the Ranger to show off his woodsman skills or the Archer to develop his hunting technique.