It depends. If duckweed is clogging your pump of filter, then yes - some treatment or pre-filter is needed to keep the floating plants away from your pump intakes. But wholesale treatment of your pond with a herbicide labeled for use over water is not the best solution. As this will not control the duckweed population long term (only changing fertility practices will) - and has potential to introduce herbicide residues to your crops when pond water is used for irrigation later.
If control is not based on actual pump related problems - but rather to the appearance of your pond (i.e. it looks bad) - think about these plants as free labor - helping to limit algal growth (shading), cool the water, and cycle nitrogen and phosphorus - keeping your pond healthy.
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