Please check these useful PDF attachments.

https://www.hindawi.com/journals/ija/2012/809587/

http://agritech.tnau.ac.in/agriculture/agri_weedmgt_sesame.html

The critical period for crop-weed competition in Sesamum indicum L. extends up to 4 weeks after sowing. During this period dominance of broadleaved weeds in the total weed composition had been reported. Thus, application of pre-emergence herbicide had been always effective in controlling weeds and enhancing grain yield. However, exceptions reported for phenyl urea herbicides, which are partly or considerably phytotoxic to the crop at early vegetative stage or germinating seeds. Early post emergence application of herbicides like quizalofop-p-ethyl was also reported to be effective for weed management in Sesamum indicum L. Integration of chemical weed control with mechanical weeding at one month after sowing had been reported to enhance crop yield in several works.

There are selective herbicides. For example, there is a possibility that you can use post emergence herbicides to kill grass weeds within grass crop. Tjis happened in Ethiopia where a herbicide known as PALCE is used in teff which is a grass crop to kill grass weeds. So it needs some kind of research to screen selective herbicides. 

hi dear

Stanislav Stamatov