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Extra help had been drafted in to double the number of people looking for the plants, believed to have been introduced to New Zealand via fodder beet seed, Environment Southland biosecurity manager Richard Bowman said on Tuesday.
The search teams had been boosted to more than 70, including staff from Asure Quality, which contracts to the Ministry for Primary Indstries (MPI), staff from the Invercargill city, Southland district and Gore district councils, and forestry and agricultural contractors.
An assessment would be made later in the week on how many searchers would be required next week, Mr Bowman said.
Searching began two weeks ago, targeting about 1400ha of land where the seed was known to be planted.
Most of that area would be covered by the end of the week, but Mr Bowman said he expected MPI to ask for additional farms to be searched. That was because its records had uncovered more farmers thought to have planted the suspect seeds.
It was also possible MPI would request a further inspection of some farms already searched, he said.
As of Tuesday evening, 807ha had been searched with 66 plants found, including 39 collected from three properties in a single day last week.
It was ‘‘very pleasing'' only a few of the plants were close to reproductive stage, he said.
‘‘That is an advantage of our slower growing season down here.''
All the plants were in secure storage and would either be buried many metres down in a landfill or incinerated, he said.
Mr Bowman urged farmers to check their crops and report any signs of velvetleaf to MPI on 0800 809-966.
It was important farmers did not pull the plants out and move them, as that could cause seeds to drop and the plants to spread.