Weeds can be big issues for organic farms, where the broad range of herbicides available to conventional farmers cannot be used. In this post, we assign economic values to hard red spring wheat and lentil crop losses caused by creeping thistle (Cirsium arvense) using trial data collected by Kara Hettinger, Tim Seipel, and Pat Carr as part of an Organic Research and Extension Initiative grant.
We estimate that each one-pound increase in thistle biomass corresponds to a revenue loss ranging from 4-8 cents. This may not sound like a lot, but the average prevalence of thistle by location and crop in their trials corresponds to a $21-$49 loss per acre. This translates to 4-11% of estimated gross revenue using budgets created as a part of the grant.
We made these estimates using recent (2014-2019) organic prices for Montana wheat and lentils, inflated to 2021 dollars and averaged over available years. We combined these price averages with thistle biomass measurements and thistle biomass-crop yield relationships estimated in two locations, the Fort Ellis Research Farm in Bozeman, Montana and the Central Ag Research Center (CARC) in Moccasin, Montana. Among cash crops in the trials, thistle prevalence reduces yields for wheat and lentils. They did not find a statistically significant relationship between thistle and alfalfa yields, and they didn’t estimate changes in yield for cover crops due to thistle presence.
We calculate revenue loss in dollar and percent terms in the table below. Over the 114 samples from wheat plots at the Fort Ellis Research Farm, the average density of thistle was 845 lbs/acre (column 4). Hettinger et al. (2022) estimate that for each additional pound of biomass per acre, wheat yields decrease by 0.17 lbs/acre (column 5). Thistle concentrations of 845 lbs/acre correspond to a wheat yield loss of 144 lbs/acre, or roughly 2.4 bushels. With a value of $0.51/lb, we estimate revenue losses of $37/acre (column 6). Using a gross revenue estimate of $386/acre for Montana organic spring wheat from sample budgets from Tumber et al. (2022) we estimate a gross revenue loss of approximately 10%.
The same estimation process predicts similar losses for CARC wheat plots, $41 per acre, or a 11% loss in gross revenue. Differences across locations are likely caused by different growing conditions, primarily weather and soil type.
The average thistle infestation on lentil plots at Fort Ellis leads to a $49, or 10%, loss in gross revenue. The average thistle prevalence at CARC remained much lower, resulting in only a $21/acre, or 4%, loss of gross revenue.
Revenue Losses for Wheat and Lentil Plots at Fort Ellis Research Farm and CARC
|Crop||Location||N||Mean thistle density (lb/acre)||Estimated yield loss (lb)/thistle biomass (lb)||Revenue loss at mean thistle density ($/acre)||Gross revenue ($/acre)||Revenue loss (%)|
Notes: Thistle density, N, and thistle-yield relationship estimates (columns 3-5) from Hettinger et al. (2022). Revenue loss (column 6) uses NASS (2022) and calculations by the authors (see table 1, above). Gross revenue for Montana organic spring wheat and Montana organic lentil (column 7) from Tumber et al. (2022).
The substantial difference in thistle presence may be a result of management and presence and in previous years. Additional work is needed to capture the dynamic, long-term effects of thistle on yields and revenues—having thistle in one year makes its presence and possible spread much more likely in the next. The calculations here only take into account the year in which the thistle biomass was estimated. Another important issue here is that these data come from research farms and do not match likely conditions on farms that are being managed as a business.
Still, this analysis shows that creeping thistle can cause economically important losses. What to do about those losses? The OREI grant is also examining measures that can help curb weed spread, such as incorporating perennial crops like alfalfa into crop rotations.
This post was co-written with Alex Maas (University of Idaho), and Tim Seipel and Kara Hettinger (MSU LRES).
Hettinger, Kara, Timothy Seipel, and Patrick Carr. 2022. “Creep Stop Yield Data for Ft. Ellis Research Farm and Central Agricultural Research Center, 2019-2021.” Internal Report.
Tumber, Kabir, Kate Binzen Fuller, and Alex Maas. 2022. Organic sample budgets for Montana. In review for MontGuide publication.
USDA NASS QuickStats. 2022. Available from https://quickstats.nass.usda.gov/.