With planned mergers sprouting up all over the seed genetics business, Iowa Sen. Charles Grassley last week asked the Department of Justice’s antitrust division to take a close look at the planned merger of Dow and DuPont.

“I am concerned that this transaction will decrease competition in the agricultural sector that has already been subject to a number of waves of consolidation in recent years,” Grassley wrote in a letter to the regulatory agency. “The proposed transaction could raise barriers to entry in the market for smaller companies and potentially harm innovation, and could adversely impact choice and price of products for farmers and consumers.”

The proposed Dow-DuPont merger, which was announced in late 2015 and is expected to be completed this year, would create three separate publicly-traded companies. One of those three...