In response to a comment on the RWAF facebook page, Richard Saunders BSc(hons) BVSc CertZooMed CBiol MiBiol MRCVS (for those who don't already know, the RWAF vet expert advisor) has written the following, which you may find interesting. He intends to write a complete article on this subject which will appear soon in Rabbiting On, the quarterly journal of the Rabbit Welfare Association and Fund which is free to members.
Calcium content and diet.
It is widely accepted and well known that rabbits require a diet high in fibrous forage. This is best supplied as grass, hay or dried grass products, although some rabbits with dental disease will struggle to eat any or enough of these items, and other fibrous plants such as dandelions and greens may be helpful. It is vitally important to realise that what we think of as "grass" is not a single entity, neither is the hay that is made from it. In the same way that it used to be suggested that the Inuit had 200 words for snow, a rabbit is able to differentiate between a myriad varieties of "grass". Different species of grasses, at different points in their growth, and of different qualities depending on storage, have significantly different qualities and compositions. Ryegrasses are the most commonly used in the UK, with timothy and fescue also popular. Multiple cuts of grass are taken, and the timing of cut, and the storage after cutting affect composition greatly, with every years' growth being subject to smaller variations due to weather conditions. Rabbits certainly appreciate variety in their diet, achieved by giving them different grass species. And sourcing a variety helps them to select preferred types, and reducing the chance of them fixating on particular varieties which may have seasonal availability. However, palatability is important, and rabbits with mouth pain or other factors which prevent them eating some foods may need an option which is easier or more tasty to eat. As it is almost impossible to know the exact composition of a particular batch of hay, and because it is also possible that rabbits "know" what they need from their food, and may select accordingly, providing a variety is helpful to iron out any deficiencies or excesses in any one type, where possible. Timothy is generally thought to have the best Calcium:Phosphorus ratio, and is should probably form the majority of their diet. Meadow fescue and ryegrass is the most common, and therefore widely available. Various less common species are available from specialist haymakers, and are worth investigating to provide choice, variety and to tempt the "difficult" rabbit. Soft straws are often eaten by rabbits. Dried grasses are convenient, good quality and palatable, and have a protein and calcium level towards the upper end of the range required by adult rabbits. They are great for rabbits who are growing, lactating, pregnant, underweight, and can be safely eaten by most other rabbits, but should not be given as a sole forage source to overweight rabbits, those not eating all their caecotrophs, or with a tendency towards "sludgy" urine, but given in equal proportions with hay or fresh grasses. Alfalfa is high in protein and calcium, and should be given only sparingly to rabbits, although it can be useful in greater quantities for those who are pregnant or lactating. Fresh grass is preferred to hay or dried grass in rabbits with urinary tract problems, to increase water intake and dilute the urine. Offering a variety allows some self selection and fine tuning of the rabbits nutrition, but offering ad lib good quality palatable forage is the most important factor for a healthy rabbit diet, and the precise type is less important.
Pellets are generally moderately high in Calcium, and, if fed in high proportions of the total diet, can lead to an overall high Calcium level which may predispose the rabbit to sludgy urine. On the other hand, selectively feeding from a "muesli" mix type concentrate can lead to a significantly low Calcium level if fed in excess. These diets should be fed as a small amount of the total diet (eg 5% of the total diet by weight, usually amounting to no more than an egg cup full for a 2.5kg rabbit), and rabbits should not be fed enough of the mixes to allow selection of tastier components, but should have to clear their bowls. (Or ideally feed a pellet/nugget)