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Hieracium Control Trickier than Thought

Grazing high country land infested with the weed Hieracium hasn't necessarily helped spread the weed, as some have suggested, but has actually helped control some of the taller-growing species, according to AgResearch scientist Peter Espie.

Trials at AgResearch Tara Hills near Omarama have shown that spread of upright Hieracium forms, such as king devil hawkweed, can be limited by grazing. However, a dilemma facing farmers is that controlling the taller-growing species by grazing also encourages spread of the lower growing form, mouse-ear hawkweed.

Espie suggests farmers adopt pasture renewal and grazing strategies that encourage pasture species to grow tall, which encourages mouse-ear hawkweed to become more upright. This can then be successfully controlled by grazing.

"Infestation of taller and lower growing types would fluctuate over the year, but there would be some control, which is substantially better than none," he stresses.

The first ever modelling work on Hieracium spread, undertaken by Espie, indicates some unpleasant predictions. The model suggests the weed will reach its peak within 20-30 years, indicating its spread is likely to worsen for at least another decade in many high country areas.

Collaborative research with Dr Ross Bicknell into the different species has also highlighted the plant's genetic variation. For the first time they have shown that the tetraploid form of mouse-ear hawkweed occurs in New Zealand, and is widespread in the Mackenzie country. Tetraploids reproduce sexually, which therefore means greater genetic variation.

This knowledge has implications for biological control. Tetraploid plants have greater genetic ability to resist control and therefore will not be as susceptible to a single biological or chemical control method.