Based on the content of your question, it seems that you are asking about ways to make farmed animals less capable of suffering, rather than (as the title suggests) to make chickens less intelligent. This has been discussed before; see e.g.
Shriver, Adam (2009) Knocking out pain in livestock: Can technology succeed where morality has stalled?, Neuroethics, vol. 2, pp. 115–124.
Shriver, Adam & Emilie McConnachie (2018) Genetically modifying livestock for improved welfare: a path forward, Journal of Agricultural and Environmental Ethics, vol. 31, pp. 161–180.
Shulman, Carl (2012) Breeding happier livestock: no futuristic tech required, Overcoming Bias, December 6.
Based on the content of your question, it seems that you are asking about ways to make farmed animals less capable of suffering, rather than (as the title suggests) to make chickens less intelligent. This has been discussed before; see e.g.
Shriver, Adam (2009) Knocking out pain in livestock: Can technology succeed where morality has stalled?, Neuroethics, vol. 2, pp. 115–124.
Shriver, Adam & Emilie McConnachie (2018) Genetically modifying livestock for improved welfare: a path forward, Journal of Agricultural and Environmental Ethics, vol. 31, pp. 161–180.
Shulman, Carl (2012) Breeding happier livestock: no futuristic tech required, Overcoming Bias, December 6.