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Weed & Pest

Extension Education: Does Larkspur Control Make Financial Sense?

Poisonous plants disrupt management on millions of acres in the United States. Larkspurs (Delphinium spp.) kill more cattle on high mountain rangelands in the western U.S. than any other plant or disease.  These plants dominate the tall forb community and kill cattle when consumed at a sufficient dose.  Cattle losses from poisonous plants represent the value of the animals and prior inputs into each animal. Chemical control of tall larkspur can reduce death losses incurred while grazing infested rangelands.

There are added benefits to controlling tall larkspurs.  It can increase forage productivity of the pasture and increase the flexibility of management.  Managers usually delay grazing until after larkspur plants have senesced, or dried up, and are no longer toxic to cattle.  At this point, pastures often have less nutritious forage.  Control of larkspurs would allow for use of these pastures earlier in the growing season.  Controlling larkspurs can reduce other indirect losses.  These losses can include overutilization of other pastures, cost of leasing larkspur-free pastures, fencing, feed supplements, herding, monitoring, cholinergic drugs and labor.

Recently an herbicide efficacy study was performed in the Big Horn Mountains of Wyoming.  This study used a new herbicide called aminocyclopyrachlor. Aminocyclopyrachlor (AMCP) is a new synthetic auxin herbicide that works as a growth regulator.  It has good residual characteristics that give it the ability to control perennial broadleaf weeds.  However, it may reduce grass production from both direct damage to plants and by potentially reducing subsequent seed production.  Dupont will be marketing AMCP for use in rangelands and pastures in a variety of herbicides including Streamline (AMCP + metsulfuron-methyl) and Perspective (AMCP + chlorsulfuron) in the future.  Perspective and Streamline were used in this research due to their likely labeling for rangeland and pasture use and were thought to likely have a significant impact on tall larkspur 

Simply stated, research results indicated that application of Streamline and Perspective (and also Tordon 22k and Escort XP) could control tall larkspur to a level that would provide a significant reduction in cattle deaths. However, application of these herbicides should be weighed against decreases in diversity and vegetation cover as well as shifts of plant species composition.  The next step was to evaluate the economics of using these herbicides.  

To do this, we compared spraying the recommended rates of picloram (Tordon 22K) and metsulfuron-methyl (Escort XP) and the rates of AMCP-containing herbicides resulting in 90 percent control in the previous study.  The cost of applying each of these chemicals was compared to leasing similar, larkspur-free pastures, as seen in Table One, pasture and to accepting either a 2.5 or five percent death loss.  This study assumed that death loss would be reduced by 90 percent with chemical control.  It also assumed that cost of herbicide application per acre would be  $9.50 for aerial application, $24 for boom application and $37.50 for spot application.  Herbicides would only be applied once.

The economic analysis found that, in this situation, it is always more profitable to lease non-infested pasture instead of accepting either amount of death loss.  Chemical control would require two to six years of control for Perspective and two to five years for Streamline to break even compared to leasing similar larkspur-free pasture.  Tordon 22K would require five to eight years of control and applying Escort XP would require three to five years to break even.  When chemical application is compared to accepting either amount of death loss, the break-even time is a maximum of three years for any of the herbicides used.  

Our results indicate that, if three years of control are obtained from one herbicide application, the cost of applying herbicides are justified as compared to accepting either a 2.5 percent or five percent death loss to larkspur, as seen in Table Two. Application method (aerial, boom spray or spot spray with backpacks) should also be taken into consideration because of their effects on cost.

The information given herein is supplied with the understanding that no discrimination is intended and no endorsement by UW Extension is implied. 

Brandon J. Greet is a UW Assistant Extension educator in Worland and can be reached at 307-347-3431 or This email address is being protected from spambots. You need JavaScript enabled to view it.. Brian A. Mealor is an assistant professor and Extension weed specialist in Laramie and Andrew R. Kniss is an assistant professor in Laramie.