Comparison of management strategies for controlling Nassella trichotoma in modified tussock grasslands in New Zealand: a spatial and economic analysis

The weedy grass Nassella trichotoma (nassella tussock), historically an economically damaging invader of modified tussock grasslands in New Zealand, currently causes little if any reduction in farm production. This is a result of successful historical regional management programmes in which plants have been removed manually (by grubbing) each year before they seed. To inform a debate about the need for ongoing regional management, we developed a stage-structured spatially explicit integrodifference equation population model and linked this to a cost analysis. We used the model to compare the weed's future population trajectories and related regional control costs over 50years under three alternative management scenarios. The total discounted (3% p.a.) costs of no management, three-yearly grubbing and continued annual grubbing were NZ $417 million, $736 million and $131 million respectively. These analyses indicate that annual grubbing of N.trichotoma returns a net benefit of $286 million ($417 - $131 million) compared with doing nothing and a net benefit of $605 million ($736 - $131 million) compared with a 3-yearly grubbing programme. These results support the continuation of annual grubbing as the long-term economically optimal management strategy for N.trichotoma on pastoral farms infested by the weed in New Zealand.