(Bloomberg) -- A rare cargo of wheat shipped from Poland to Houston was rejected from being imported because it contained “prohibited seeds,” the US government said Monday in a statement.

The vessel Yochow, carrying about 30,000 metric tons of Polish grain, was held up last week off the coast of Texas. Years of drought in the American heartland had hit grain harvests, making some imports like wheat from Europe a relative bargain. However, US Customs and Border Protection ordered the Yochow’s cargo to be exported elsewhere after authorities determined it was contaminated. 

“Invasive species and toxic substances are among the many threats to American agriculture and natural resources,” a US Customs and Border Protection spokesman said.

Read more: US Halts Import of Rare Vessel of Polish Wheat at Houston

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