A pastoral weed which has been ranked as bad as ragwort is spreading across the country, after being reasonably dormant for 100 years.
Yellow bristle grass is mainly found on dairy farms in Waikato.
However, a senior scientist at AgResearch says investigations are underway into why it has spread over the past five years.
Dr Trevor James says the spiky yellow weed, which can grow to one metre high, reduces the quality of pasture and can not be grazed.
He says if the weed gets to Canterbury it would really like the climate there, helping it to become established.
Dr James says yellow bristle grass is a summertime annual, so farmers should keep an eye out for it over the next few weeks.