The first is that you want to somewhat control the amount of dried grass/fiber they eat. Too much fiber can have a negative impact on the digestion process. The Ca:P (calcium phosphorus ratio) is off so you need to offset with calcium supplements which may be difficult to gauge without knowing how much they are eating. These animals obtain their moisture from their food and if your uromastyx is eating a lot of dried material and not enough moist greens, this could lead to dehydration. Timothy is not a perfect food (considered moderate protein) and as such you probably would not want this as a free range item.
Lastly, the dried grass would absorb waste material and be a perfect habitat for bacteria, fungus....ect
Your uro will probably not avoid those bacteria soaked blades when munching on his substrate.