A brahmin feed trough can be crafted in a settlement, and will cause the brahmin to stay close to the trough. Having brahmin on the trough will improve productivity, hence increasing food output rate into the workshop inventory. They also produce fertilizer - a component for chemistry. One can only have one brahmin producing fertilizer per settlement, so one only needs one trough.
Kelly will sell a brahmin to the player character for 100 caps. The player character will need to choose a settlement to send the brahmin to. This can be done multiple times.
Kelly is a character appearing in Fallout 4. She can often be found near Tenpines Bluff, traveling with a Brahmin.
Kelly will offer to sell a Brahmin to you for 100 caps. It can be sent to a settlement to improve the efficiency of farming at that settlement. If asked whether the Brahmin is stolen, Kelly will become evasive, with her tone ambiguous as to whether she's feeling guilty or merely thinks it's a stupid question.