(AP Photo/Charlie Neibergall)
Donald Trump.

Republican presidential front-runner Donald Trump shared an unusual tweet on Thursday asking if Iowans' had suffered some sort of brain issue due to controversial agricultural practices.

He subsequently deleted his tweet sharing the message and said an intern apologized for sending it.

"The young intern who accidentally did a Retweet apologizes," Trump wrote on Twitter.

The original tweet he shared was about a new poll released earlier in the day that found retired neurosurgeon Ben Carson overtaking Trump in a survey of likely Iowa caucus-goers.

"#BenCarson is now leading in the #polls in #Iowa. Too much #Monsanto in the #corn creates issues in the brain?" the tweet asked, possibly tongue-in-cheek.

Monsanto Company is a massive multinational agricultural company that was among the first to embrace genetically engineered foods, a hot-button issue that has generated criticism and some conspiracy theories.

The poll, from Quinnipiac University, gave Carson a 28% to 20% lead over Trump among likely Iowa caucus-goers.

Outside of that poll, Trump has led almost every other national and state-based survey in the last 100 days.

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