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Economic Cost of NZ Pasture Weeds Larger Than Thought

01 February 2016

NEW ZEALAND - New research shows the true cost of California thistle to be close to $700 million in lost pastoral farm revenue each year and also suggests that previous estimates of the cost of other agricultural weeds has been significantly underestimated.

The figure is the work of scientists in the Farm Systems team at AgResearch and is based on the development of a robust new method for quantifying the economic losses attributable to pasture weeds.

The scientists developed the “seasonal pattern” model primarily for Californian thistle (Cirsium arvense) but it is applicable to any weed species that exhibits a seasonal pattern in its pasture occupancy.

The work has just been accepted for publication in Weed Research, an international journal of weed biology, ecology and vegetation management.

Lead author Dr Graeme Bourdôt says researchers can now use this new model for accurate economic analyses for Californian thistle at various scales.

“We applied the model at the national scale, scaling it using farmer estimates of peak whole-farm cover to derive mean monthly covers, and then mean annual percentage covers of the weed for dairy, beef, sheep/beef, sheep and deer farms.

"The percentage of pasture lost, in combination with 2011-12 farm statistics, revealed that Californian thistle caused a national loss in pastoral farm gross revenue in 2011-12 of $685million ($446m dairy, $233m sheep/beef, $6m deer)."

Dr Bourdôt says that at the scale of a particular farm, in conjunction with a single point-in-time estimate of the weed’s cover on the farm, the expected cover of the weed on for each month of the year can be determined.

“These estimates can then be used in a farm system model such as Farmax, to explore the impact of the thistle and compare the economics of alternative management methods.”

Dr Bourdôt says that previous estimates of economic cost of California thistle (and of pasture weeds in general) have been based on either anecdote or single point-in-time estimates of the weed’s occupancy that do not account for the seasonal dynamics of the weed in a pasture nor the changing feed value of the pasture over the year.

“Since Californian thistle is not eaten by most classes of livestock, the loss in pasture production due to the weed is directly related to the pasture area that it covers. Our research uniquely provides a way of quantifying that.

"We have established, through a two-year field experiment in which we measured the within-patch cover of the thistle on many pastoral farms throughout New Zealand, that the seasonal pattern of thistle’s cover is the same on all farm system types (dairy, sheep, beef, deer) throughout New Zealand. So we have one simple model of how Californian thistle ground cover changes over the year in a pasture.”

Dr Bourdôt says the $685 million is a significant loss and confirms that the thistle is a very significant constraint to production. A financial loss of this magnitude also indicates that research leading to a long-term reduction in the coverage of the thistle has the potential to deliver a very significant return on investment and large benefits for pastoral farmers, he says.

Dr Bourdôt says the size of this estimate of the national value of lost production due to this one weed species indicates that previous aggregate estimates of pasture weed impacts in New Zealand, such as the $1.2 billion per year derived from a Monsanto study in 1984, are large underestimates.

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