Economic Thresholds in Weed Management and Demonstration of HADSS

Damage Threshold: The weed population at which a crop response can first be measured. Or, the minimum population necessary to cause a measurable response. Economic Threshold: the weed population at which cost of control is equal to value of crop yield attributable to that control. The weed population which reduces crop value to a level equal to the cost of control. Or, value of the loss equals cost of the control.

Planning a Herbicide Program 1.What crop to be grown? 2.What is the weed problem? a.Field history b.Weed maps c.In-season scouting

Weed Map A written record of species infesting a field Made in fall of year; augmented during the growing season More detail is better  Specific species  Level of infestation  General location within field Best if maintained consecutively over years  better record of all species present  will allow you to see population shifts over time

Planning a Herbicide Program 1.What crop to be grown? 2.What is the weed problem? 3. What are my herbicide options? a.What products registered for the crop will control my problems b.Of those available, which fit with my soil characteristics, my production practices, my preferences, the level of risk I am willing to take, etc. c.Which fits my rotational plans d.Costs

Economic thresholds are not static; specific values vary. Economic thresholds depend upon a number of things; primary factors include: Potential crop yield in absence of the weeds Selling price of the crop Cost of treatment, whatever the treatment is. Usually think chemical control, so it is cost of herbicides + application cost, but it could be cost of cultivation or hand-weeding. Also, thresholds vary by species (crop and weeds) due to differences in competitive ability

Hypothetical Example Assumptions: Each weed per 100 ft 2 reduces yield 1% Weed-free potential yield = 100 bu/acre Crop sells for $3/bu Cost to control the weed is $15/acre What is the economic threshold (ET)? What weed density would reduce crop value equal to the cost of control? (treatment cost)(100) density = ______________________________________________________ (weed-free yield)(value/bu)(%loss per weed) ($15/acre)(100) density = _________________________________________________ (100 bu/acre)($3/bu)(1% loss per weed) density (or threshold) = 5 weeds/100ft 2

Hypothetical Example: Effect of control cost Assumptions: Each weed per 100 ft 2 reduces yield 1% Weed-free potential yield = 100 bu/acre Crop sells for $3/bu Cost to control the weed is $15 $25/acre What is the economic threshold (ET)? What weed density would reduce crop value equal to the cost of control? (treatment cost)(100) density = ___________________________________________________ (weed-free yield)(value/bu)(loss per weed) ($15 $25/acre)(100) density = _________________________________________________ (100 bu/acre)($3/bu)(1% loss per weed) density (or threshold) = weeds/100ft 2

Hypothetical Example: Effect of crop yield potential Assumptions: Each weed per 100 ft 2 reduces yield 1% Weed-free potential yield = bu/acre Crop sells for $3/bu Cost to control the weed is $15/acre What is the economic threshold (ET)? What weed density would reduce crop value equal to the cost of control? (treatment cost)(100) density = ___________________________________________________ (weed-free yield)(value/bu)(loss per weed) ($15/acre)(100) density = _________________________________________________ ( bu/acre)($3/bu)(1% loss per weed) density (or threshold) = weeds/100ft 2

Hypothetical Example: Effect of crop selling price Assumptions: Each weed per 100 ft 2 reduces yield 1% Weed-free potential yield = 100 bu/acre Crop sells for $3 $5/bu Cost to control the weed is $15/acre What is the economic threshold (ET)? What weed density would reduce crop value equal to the cost of control? (treatment cost)(100) density = ___________________________________________________ (weed-free yield)(value/bu)(loss per weed) ($15/acre)(100) density = _____________________________________________________ (100 bu/acre)($3 $5/bu)(1% loss per weed) density (or threshold) = 5 3 weeds/100ft 2

Hypothetical Example: Effect of weed species Assumptions: Each weed per 100 ft 2 reduces yield 1% 3% Weed-free potential yield = 100 bu/acre Crop sells for $3/bu Cost to control the weed is $15/acre What is the economic threshold (ET)? What weed density would reduce crop value equal to the cost of control? (treatment cost)(100) density = ___________________________________________________ (weed-free yield)(value/bu)(loss per weed) ($15/acre)(100) density = _____________________________________________________ (100 bu/acre)($3/bu)(1% 3% loss per weed) density (or threshold) = weeds/100ft 2

Factors affecting economic thresholds: Treatment cost: As treatment cost increases, threshold increases As treatment cost decreased, threshold decreases Crop yield potential: As yield potential increases, threshold decreases As yield potential decreases, threshold increases Crop selling price: As selling price increases, threshold decreases As selling price decreases, threshold increases Differential competitive ability of weeds As competitive ability of weeds increase, threshold decreases

Hypothetical yield loss from three species

Thresholds Very few fields have a single weed species; usually have multiple species Thresholds developed for single species Need multi-species thresholds –HADSS (Herbicide Application Decision Support System) developed in NC; based on multispecies thresholds –www. Webhadss.ncsu.edu

Multispecies Thresholds NCSU’s HADSS Program Weed species are not all equally competitive. Used a competitive index (CI) to rank species. The most competitive species (cocklebur) arbitrarily given a CI of 10. Other species ranked 0 to 10, relative to most competitive species.

Competitive indices of selected weed species

Multispecies Thresholds NCSU’s HADSS Program Used a competitive index (CI) to rank species. Most competitive species arbitrarily given a CI of 10. Other species ranked 0 to 10, relative to most competitive species. Competitive load for a given species (CL) is the CI for that species times its density Total competitive load (TCL) is of CL for all species summed. Each unit TCL equates to a percent yield loss unique to a given crop.

Where might HADSS fit? Most fields have enough weeds to require treatment (above threshold). Thresholds could be useful in deciding whether or not a second herbicide application is justified. For example, a POST herbicide following a PRE, or a second POST following an earlier POST.

Limitations that prevent growers from using weed economic thresholds 1 % of growers Concern about interference with harvest64 Landlord concerns 38 Weed seed production38 General appearance of field36 Effect of growing season on weeds22 Need to improve weed id skills 7 Time required to scout fields 6 Lack of weed competition data available 6 1 Czapar, Curry, and Wax Grower acceptance of economic thresholds for weed management in Illinois. Weed Technol. 11:

Concerns with Trying to Use Economic Thresholds Decisions only as good as the input data. Scouting can be time-consuming, if you do a good job; expensive. ET’s guide an in-season decision during a single crop year. Do not account for the cost associated with increases in soil seed bank. Economically optimum thresholds (EOT) attempt to take long-term effects on seed bank into account. EOT typically much lower than ET. EOT based on modeling (with major assumptions being made); no practical guides to date. Use common sense. –Threshold of a species previous not in the field is zero. –Herbicide resistance puts a new “kink” in the system