WASHINGTON — In a landmark first ruling on genetically modified crops, the US Supreme Court overturned Monday a four-year ban on alfalfa seeds engineered by biotech giant Monsanto to resist weed killer.

A California district judge voided in 2007 the Department of Agriculture’s authorization of the seeds, finding that a proper environmental review had not been conducted. The decision was upheld on appeal in 2009.

But justices voted 7-1 Monday to reverse the ruling, saying the injunction overstepped the mark and prevented the agency from carrying out a ‘partial deregulation’ of the crop, known as Roundup Resistant Alfalfa (RRA).

‘We agree that the district court’s injunction against planting went too far,’ Justice Samuel Alito wrote. ‘In sum, the District Court abused its discretion.’

Opponents of RRA claim it could cross pollinate with conventional alfalfa seeds and other neighboring crops, promoting ‘super-weeds’ with a tolerance to the Roundup herbicide.

‘Until APHIS (the DoA’s Animal and Plant Health Inspection Service) seeks to effect a partial deregulation, any judicial review of such a decision is premature,’ the Supreme Court said.

‘The district court barred the agency from pursuing any deregulation, no matter how limited the geographic area in which planting of RRA would be allowed.’

Justices ordered APHIS to carry […]