The new U.S. tax law has placed Rick Tronson, a North Dakota grain-company operator, in a precarious position by unexpectedly bestowing big benefits on his main competitors.

A provision inserted into the tax code during Senate and House negotiations in December gave farmers more lucrative deductions when they sell agricultural products directly to the farm cooperatives he competes against rather than to businesses like his own.

Mr. Tronson, whose four storage facilities handle 17 million bushels of grain a year, said the competition could spell the end of his 76-year-old family-owned business.

“We’ve made a big investment. And this law, if they don’t change it, the scenario is that we’ll go broke,” he said.

Farm groups and agricultural cooperatives battled last year to preserve a deduction on domestic U.S. production, which manufacturers also received. That deduction went away in the tax rewrite, but lawmakers including Sen. John Hoeven (R., N.D.) won the inclusion of a new deduction.