What do weeds cost?
Graeme Bourdôt with Californian thistle
It is no secret that weeds cost the productive sector a huge amount of money each year, in terms of both control and lost production. But the size of this financial burden is not easily calculated. A recent review by New Zealand scientists has had a close look at the costs of weeds to New Zealand’s pastoral, arable and forestry sectors.
“Even a superficial look at the literature shows that most studies have focused on the cost of weeds to pastureland, with few attempts to estimate the costs associated with arable crops or forestry,” said lead author John Sanders from Lincoln University. “To develop cost-effective tools for weed management and decision-making, we need to understand the cumulative costs across all three of these industries,” said John. “Also, most of the studies reported are limited to assessments of individual weeds rather than assessing their combined impact,” explained John. As a result, his review centred on 10 common pastoral weed species in New Zealand (see table).
| Weed | $million per year (2014) |
|---|---|
| Gorse (Ulex europaeus) | 72.3 |
| Californian thistle (Cirsium arvense) | 703.9 |
| Scotch broom (Cytisus scoparius) | 8.3 |
| Blackberry (Rubus fruticosus agg.) | 14.3 |
| Chilean needle grass (Nassella neesiana) | 0.013 |
| Sweet briar (Rosa rubiginosa) | 7.1 |
| Hawkweeds (Pilosella spp.) | 5.8 |
| Yellow bristle grass (Setaria pumila) | 257.7 |
| Giant buttercup (Ranunculus acris) | 209.6 |
| Nassella tussock (Nassella trichotoma) | 27.1 |
| 10 pasture weeds | 1306.1 |
| All arable weeds | 18.2 |
| All forestry weeds | 333.6 |
| Total | 1,657.9 |
The total cost estimated for the 10 pasture weeds ($1,306.1 million per year) is likely to be an underestimate, because the costs used in the analysis are largely due to lost production. “The cost to the economy from loss of production for the three biggest culprits (Californian thistle, giant buttercup and yellow bristle grass) is $1,171.20 million alone, and this doesn’t include the cost of controlling them!” said co-author Graeme Bourdôt. Also this estimate is only based on 10 of the approximately 187 plant species that occur in New Zealand pastures.
“What became apparent after reviewing the literature was that the cost of each of the pasture weeds had been calculated differently,” said John. Some of the weeds are widespread and occur in landscapes where it is not practical to control them with herbicide (e.g. hawkweeds in the high country of the South Island). For these weed species, and others such as giant buttercup in dairy pastures, the economic analysis had been based on loss of productive land and extrapolated to an assumed loss of production for the sheep, beef or dairy industries. By contrast, the economic analysis for other weeds such as broom included the cost of herbicide control as well as the cost of replacing nitrogen into pasture that had occurred because of clover displacement.
Little data was available to assess the cost of weeds in arable systems, but the majority of land is used to grow seed crops such as wheat, barley, maize and herbage seeds (approximately 196,000 ha). “There was very little information available on the costs of weed control for vegetable crops, but they don’t really take up a large area of land (about 15% of total arable area), so the cost of controlling the weeds within them may not have added much overall,” explained John.
Weed control in the forestry sector largely occurs early in the forest’s rotation, while the trees are getting established. These weeds include species such as broom, buddleia (Buddleja davidii) and pampas grass (Cortaderia spp.), and others such as blackberry, which can affect the form of young trees.
Preparing an in-depth economic analysis relies on having good data on the cost of the weed to begin with. “Many of the studies reviewed implicitly assumed that the weed had fully occupied all its possible habitat, so the potential cost of allowing the weed to spread further was not included,” commented Graeme. Also, there are usually multiple weed species growing together, which have an additive effect, and often replacement weeds move in as one is eliminated from the system so the loss of production remains. “This has inevitably led to an underestimate of the true cost of weeds in productive landscapes,” agreed John. “Our study highlighted how little was known about the total cost of weeds to the New Zealand economy and how valuable it would be to update old studies under a unified framework,” he added.
“Despite the inherent difficulties in accurately calculating the costs of weeds to New Zealand’s primary productive sector, it remains vital to have some ball-park figures to enable more cost-effective allocation of funds for weed research and management and to provide context on the extent of the weed problem to funding providers,” concluded Graeme.
This work was supported by the Ministry for Business, Innovation and Employment.
Saunders JT, Greer G, Bourdôt G, Saunders C, James T, Rolando C, Monge J, Watt MS 2017. The economic costs of weeds on productive land in New Zealand. International Journal of Agricultural Sustainability. Available at: https://doi.org/10.1080/14735903.2017.1334179.
CONTACT
John Saunders – john.saunders@lincoln.ac.nz