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This is, unfortunately, a side effect of this fertilizer. This spray should be applied before you mulch, and this will help keep the smell down. If you have sprayed, you can place a mulch, like wood chips or straw or leaves, over the area that was treated and this will help.

It will fade on its own after a rainfall or two.

I would try placing it in airtight bins or a heavy duty plastic bag. That will reduce the smell some. Also, keep it as cool as possible. Higher temperatures will make the smell worse.

As far as I know, there is nothing you can add to the liquid itself to keep it from smelling.

The best kinds of vegetables to grow in nitrogen rich soil are vegetables that are grown for their leaves. Nitrogen encourages leaves to grow and suppresses root and flower growth. Vegetables like spinach, lettuce and greens are great.

If you want to plant something else there, I would recommend adding phosphorus to balance out the nitrogen.

There is no set ideal time for how long to compost horse manure, but typically it takes 2-3 months if done properly. You are better off looking at the compost itself to see if it is ready. The horse manure compost will look like soil and will have lost its "manure" smell when ready.

You do not want to use raw horse manure on your garden beds. First, because the seeds in it have not been killed by heat from composting (and will grow) and second because the nitrogen is to "hot" and needs time to mellow. You want to leave it to compost/rot for a few months in a compost bin before you put it on your beds. You may find this article helpful: http://whatcom.wsu.edu/ag/compost/horsecompost.htm

Once it has been composted and the nitrogen has mellowed, you can spread it up to 3-4 inches deep over the ground and then work it in.

If you are amending the soil in the winter with cow manure yearly, it should be good through the summer. That being said, compost never hurts when added to the garden.