Grown for cattle feed, this is the remainder of the plant after the heads have been harvested, however please note all straw will contain SOME seeds, and as such pea straw can never be 100% weed free.
Pea Straw makes a great mulch, especially good for vegie gardens or no dig gardens, as it returns valuable organic matter to the soil as it decomposes. Because peas are a nitrogen fixing plant, the straw will return this extra nitrogen to the soil as it breaks down.
How far a straw bale will spread totally depends on the thickness used. As a general rule, allow 1 bale per 5m2. 

See our fact sheet on straw mulches for more information.