As Chancellor Angela Merkel's Christian Democrats (CDU), their Bavarian CSU sister party and the Social Democrats (SPD) mull over whether to continue the governing "grand coalition," a decision about the possibly cancer-causing herbicide glyphosate is sowing mistrust between the two sides.

On Monday, German Agriculture Minister Christian Schmidt of the CSU voted in Brussels to extend glyphosate's approval for use in the European Union by five years. German Environment Minister Barbara Hendricks immediately cried foul, saying that her ministry had not agreed to the extension and accusing Schmidt of overstepping his authority in acting alone.

The issue has quickly become one of trust between the two political rivals, who are once again being called upon to cooperate to form a government.

"I told [Schmidt] clearly by telephone that I did not agree with the extension of the approval of glyphosate, no matter what the conditions," Hendricks said when the EU's decision was announced. "No one who's interested in building trust between two partners in talks can behave like that."

Germany had previously abstained from voting on the herbicide in the EU to reflect the disagreement between conservatives and Social Democrats on the issue. Caretaker governments such as the one in charge of Germany at present traditionally do not reverse policies of their predecessors.

Schmidt defended his decision, saying that the extension would have come about anyway and that he was able to win important restrictions on the herbicide's use in return for a yes vote. He also said he had acted in his capacity as minister. But that hasn't prevented the conflict from escalating into a discussion about the conservatives and Social Democrats' ability to work together and about Merkel's leadership.

Merkel acknowledges minister's insubordination

In a statement on Tuesday, the chancellor said that she had no prior knowledge of how Schmidt intended to vote and that the minister had, in essence, gone it alone.

Environmental activists say that the weed-killer is harmful to humans

"That was not in accordance with the instructions that the government had worked out," Merkel said.

Merkel's statement came after a number of senior SPD leaders said Schmidt's action had undermined their confidence in the conservatives as partners.

SPD parliamentary group leader Andrea Nahles described Schmidt's vote as "a serious breach of trust." SPD vice-chairman Ralf Stegner told German public television that the decision was a "slap in the face."

Hendricks, who saw both Merkel and Schmidt at Tuesday's Dieselgate summit in Berlin, indicated that agriculture minister had tried to apologize.

"I'm not one to reject an apology," Hendricks told news agency DPA. "But I did tell him that it was pretty stupid."

Hendricks added that she still thought some sort of "trust-building measure" was necessary. The row comes as SPD chairman Martin Schulz is scheduled to meet with Merkel, CSU chairman Horst Seehofer and German President Frank-Walter Steinmeier on Thursday to explore possibilities for forming a new grand coalition.

No consensus on whether glyphosate is harmful

The row pits Schmidt and Hendricks, two caretaker ministers, against one another

Further complicating the situation are the ongoing efforts of German pharmaceuticals and chemicals giant Bayer to take over Monsanto, one of the most prominent producers of glyphosate weed killers.

There is no consensus about whether glyphosate, which has been described as the word's most widely used herbicide, is a carcinogen. The Independent National Pesticide Information Center writes: "Studies on cancer rates in people have provided conflicting results on whether the use of glyphosate containing products is associated with cancer."

Nonetheless, criticism of the herbicide abounds, with 10 out of 28 EU member states, including France, voting on Monday not to extend its approval within the bloc.

A spokesperson for the German Environment Ministry told the news agency EPD on Tuesday that that body would be looking into ways "to ban restrict and restrict" the use of glyphosate pesticides on the national level and would be working closely together with France on the issue.