SELECTING, PURCHASING AND STORING GRASS HAY
Grass hay should be the foundation of every guinea pig's diet. Guinea pigs who have unlimited amounts of hay are not only healthier but happier as well.
Hay -- Vital to Good Health
Grass hay is a staple of the guinea pig diet. Providing unlimited grass hay is one of the best things you can do for the health of your guinea pig. Unlimited hay:
- Provides the necessary fiber required by herbivores' digestive systems
- Keeps teeth properly ground.
Because the back teeth are continually growing, guinea pigs need to be constantly grazing and grinding to keep them a good length. Since hay is only a modest source of protein and nutrients it does not contribute significantly to obesity.
Alfalfa hay (a legume hay) is much higher in protein, calcium and carbohydrates and is primarily fed to young pups and pregnant or nursing guinea pigs. Healthy guinea pigs, who are often already be eating an alfalfa based pellet, do not benefit from alfalfa hays. Some guinea pigs prone to stones or suffering from specific medical illnesses should not be fed calcium rich hays.
Be sure your guinea pigs have hay 24/7.
Small Bags or Bales?
Many guinea pig owners buy small bags of hay at a pet store and dole it out sparingly to their animals. Purchasing larger quantities means it is available for bedding and food at reasonable cost. Space limitations may mean that although economical, a bale is not practical. But if you can, by all means purchase your hay in quantity.
Purchasing grass hay by the bale can be an inexpensive way to offer them all they desire. A bale can last as food and bedding for several months and reportedly retains its nutritional value if stored properly for a couple years.
Another alternative to purchasing small bags or bales (if you do not have good hay locally or a place to store it) is purchasing a large box online. 
Selecting Hay
Locating good quality hay can be a challenge. You may find leads to a farm or stable with high quality hay by calling your extension service or talking to a forage expert at a local farm supply. Horse owners are notoriously picky about the quality of hay they provide their animals and may be another source. The best hays, according to the extension service, are:
- Harvested before blossom or heading.
- Very leafy (poorer quality hays are more stemmy)
- Have the natural green color of the crop
- Fragrant (it will smell clean, no moldy/musty or burnt smell; not dusty)
- Very soft and pliable
- Free of trash, weeds, dirt and other foreign material
As the hay moves into early blossom or early heading, to late and the final seed stage (stemmy) much of the nutritional value goes into seed production. Soft pliable hay is much safer for your pet also, as stiff stems can and do cause injuries. The first harvest of hay is generally called the first cut. This hay has been slowly and irregularly growing throughout the winter. The second harvest (second cut) is usually more uniform. Farmers balance yield and quality in deciding when to harvest the crop. Weather conditions may affect optimum harvest time. The Oxbow Hay Company has information on its site explaining hay quality.
Nutritional Content
Most grass hays are basically similar in nutritional value. Some hays will be more palatable than others. Timothy and orchard grass hays are favorites. As long as whatever grass hay you have found is of high quality, it will probably be suitable for your guinea pig.
The Hay Chart provides general calcium, phosphorus, and magnesium values for a variety of grass hays and legume hays, like alfalfa. Note the high calcium and protein content of alfalfa and clovers. When available, information was provided for hays in various stages of growth. Note that young hays provide more protein than midbloom and mature hays.
These values were obtained from Feeds & Nutrition by M. E. Ensminger, J. E. Oldfield, and W. W. Heinemann, (Second Edition 1990). Feeds & Nutrition covers a wide range of topics, from the principles of nutrition to nutritional disorders like ketosis, anemia, aphosphorosis, azoturia, bloat, colic, iodine deficiency, rickets, and much more.
Remember: The only way to know the precise nutritional content of a hay is to have it tested (a modest fee is charged). Weather, soil fertility and minerals are only some of the factors that affect the nutritional content of hays. Your local extension service can help if you are interested in having a particular hay tested.
See also the following nutritional info on grasses and hays: www.safergrass.org