The Ministry for Primary Industries (MPI) is cautioning farmers not to plant left-over seed from any of the six lines of fodder beet seed imported last year that were contaminated with velvetleaf.
"MPI has banned the importation of any of the affected lines, but we believe there are likely to be farmers out there who bought contaminated seed lines last year and could have left-over seed in their sheds," response incident controller David Yard said.
While velvetleaf might not have seemed a significant problem last season, individual velvetleaf plants produce up to 17,000 seeds.
Velvetleaf is one of the world’s worst cropping weeds, affecting crops by competing for nutrients, space and water, and can reduce crop yields by up to 30%.
The contaminated seed, linked to Italian imports, was sown on more than 250 properties from Southland to Waikato.
Nationwide, velvetleaf plants were recovered from 251 properties, including 45 in Otago and 55 in Southland.
The Southland search alone cost about $700,000.
Mr Yard said plants undetected last season could lead to a "major infestation" this season.
"If left unmanaged, those plants will be a significant farm and biosecurity issue."
Farmers and contractors had to realise it was an offence under the Biosecurity Act 1993 to plant the six contaminated velvetleaf seed lines.
Contaminated seed lines
Kyros DNK -16UB128
Bangor DNK-15UB079
Bangor DNK- 16UB126
Bangor DNK- 16UB114
Feldherr DNK-16UB131
Troya DNK-16UB112
• Report new velvetleaf finds ... 0800 80 99 66.
• More on velvetleaf ... www.mpi.govt.nz/alerts.