Great question.

You probably need to keep the swans off the grass for at least 48 hours. If you can bring them in or keep them from accessing the grass for that long, it should be safe.

If the chemical being used is something like Round-up, then the chemical is actually killing the weeds from preventing photosynthesis. If this is the case, 24-48 hours should be adequate. If unsure, the best situation is to only spot treat the lawn so that you can block the section of lawn being treated for several days or longer periods. This will also provide the swans with access to some grass and not cut back on the vegetation needed in their diets. Once the spot treated area grows back new grass, open that area and treat the next section. The Regal Swan