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Hi! Can you please clarify - do you mean using weedkiller spray on garden plants around the lawn or a systemic lawn treatment?
In the latter case, please wait until you have mowed the grass 3 times as the weedkiller is absorbed by the grass and needs time to grow out. It is not a matter of rain washing it off - it is present in the grass blades.
Some weed killers or fertilizers have a very long time of effect. My cousin moved houses, lived there about a month, had not put anything on the lawn in that time and thus figured it was safe, and ended up losing her two pigs. Apparently some kind of fertilizer that was time-released had been used and it was not safe for the pigs. I'm a lot more cautious about treated lawns since that happened to her so I err on the side of being overcautious, probably.