Pasture regeneration and gorse seedling control in the Port Hills following fire

Abstract

This dissertation examined the regeneration of pasture after a fire and the control of gorse seedlings using Italian ryegrass competition and herbicides. The experiments were run at a property on the Port Hills, Canterbury which had areas of gorse and brown top dominant pasture burnt in the fires of February 2017. The experiment ran from 17 March 2017 to 3 October 2017. Pasture growth rates were higher in burnt pasture (6.3 kg DM/ha/d) compared with unburnt pasture (4.4 kg DM/ha/d). Subterranean clover seedling establishment ranged from 37.5 plants/m2 in the unburnt pasture to 648 plants/m2 in the burnt pasture. This was due to the heat from fire breaking the subterranean clover hard seed. The fire also removed resident pasture material allowing greater light interception by the seedlings. The burnt gorse area was oversown with an Italian ryegrass pasture mix in March. Gorse seedling numbers were reduced by 52% in plots sown with Italian ryegrass compared with unsown plots. On the 3 October 2017 there were >400 gorse seedlings/m2 in areas with Italian ryegrass. Other control methods, such as grazing, will need to be used to further reduce the gorse population. There were more gorse and grass seedlings and fewer subterranean clover seedlings on the south slope of the gully compared with the north slope. This was due to greater soil moisture on the south slope favouring the growth of Italian ryegrass and gorse. Thus, subterranean clover was outcompeted due to shading. The effect of eight herbicide treatments on gorse seedlings were visually assessed using the EWRS rating system. Herbicides were applied in August or September. Treatments including saflufenacil gave initially gave faster results but at the end of the experiment there was no difference between saflufenacil treatments and the single herbicide treatments. Glyphosate was the most effective herbicide at both application dates but also damaged the grass seedlings. Terbuthylazine was only effective when applied in September as it is root absorbed. Faster results were seen in all treatments when applied in September compared with August. This is due to higher growth of the plants in spring. The removal of mature gorse bushes by the February 2017 fire created on opportunity to control the gorse population on the Port Hill, Canterbury. This research shows that Italian ryegrass competition will not eliminate the gorse population and other methods, such as grazing, should be used in conjunction. Pasture that was burnt has regenerated well and should also be grazed to reduce the fire risk and to retain subterranean clover.