How long after applying herbicide should it not rain to avoid affecting the efficacy?

After applying herbicide, if it rains, do you need to re-spray? Many friends have this question in mind. If you don't re-spray, you worry that the rain will wash away the solution and it won't be effective; if you do re-spray, you worry about causing herbicide damage. So how long after applying herbicide is it safe from rain?

After spraying, generally if it rains 4~5 hours later, no re-spraying is needed. If it is a contact insecticide, even 3 hours later no re-spraying is needed. If it is a systemic pesticide, the efficacy will be reduced. For fungicides, it is best if rain falls 5 hours after application for good results, but some fungicides do not need re-spraying if it drizzles, so it depends on the actual situation, and there is no uniform standard.