[aqh88]

The only way I can see that the wheat grass would go down in nutritional value is if it's using up the media it's growing in. Chemically fertilized potting soils and special growing mediums will often burn out quickly. Wheat grass containers probably aren't designed to grow long term. If it's grown in a good soil then it's the same as every other grass. Newer growth may be sweeter, with less fiber, and more water but the difference is hardly worth mentioning unless it makes up nearly the entire diet. Newer grass will taste sweeter and less grassy to humans. As far as guinea pigs are concerned grass should go up in nutritional value the older it gets instead of down. More fiber, more stored vitamins/minerals, less water, and less sugar in older growth. Provided it's in good soil.