After Herbicide: New Growth Dangers

Understanding Toxic Plant Risks Following Herbicide Application

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Why Spraying Can Make Things Worse

Herbicide application is a necessary tool for controlling toxic plants on pastures and rangelands. But the period right after treatment creates some unexpected dangers for livestock. Dying plants can become more palatable, stressed plants may concentrate toxins, and the regrowth that follows can actually be more dangerous than the original infestation. Knowing these risks helps you time grazing correctly and avoid post-treatment poisoning.

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How Herbicides Affect Plant Toxicity

Increased Palatability

Sugar Accumulation: Dying plants often accumulate sugars as metabolic processes break down
Texture Changes: Wilting reduces toughness and improves texture
Odor Reduction: Bitter or repellent volatile compounds may dissipate
Reduced Defense Compounds: Some defensive chemicals decrease initially
Salt Concentration: Herbicide-stressed plants may taste saltier
Extended risk: Up to 2-3 weeks depending on plant and herbicide
Some plants remain dangerous until completely dead and dried

Toxin Concentration Effects

|-----------|--------|----------| | Growth cessation | Toxins not diluted by new tissue | Days to weeks | | Metabolic disruption | Some toxins increase as byproducts | 24-72 hours | | Cell breakdown | Toxins released from storage tissues | 1-7 days | | Stress response | Plants may produce more defensive toxins | 24-96 hours |

Wild cherry: Cyanide release enhanced by cellular damage
Sorghum species: HCN potential increases significantly
Pigweed: Nitrate and oxalate levels may increase
Kochia: Nitrate accumulation continues in dying plants

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Herbicide-Specific Grazing Restrictions

Phenoxy Herbicides (2,4-D, MCPA, Dicamba)

|---------|----------------------------| | 2,4-D amine | 0-7 days (varies by label) | | 2,4-D ester | 7-14 days | | Dicamba | 7-30 days (rate dependent) | | MCPA | 0-7 days |

Plants taste sweeter immediately after treatment
Cattle actively seek out treated plants
Normal avoidance behavior is overridden
Bitterweed
Threadleaf groundsel
Snakeweed/broomweed
Silverleaf nightshade

Picloram-Based Herbicides (Grazon, Tordon)

|---------|---------------------|-------------| | Grazon P+D | 0-14 days | 30 days | | Grazon Next | 0 days | 7 days | | Tordon 22K | 14 days | 30 days | | Surmount | 14 days | 30 days |

Glyphosate (Roundup, Generic Products)

However, palatability concerns still apply
Johnsongrass (prussic acid)
Wild cherry (cyanide)
Any toxic broadleaf being controlled

Triclopyr (Remedy, Garlon)

|---------|---------|-----------------| | Remedy | 0 days | 14 days | | Garlon 4 Ultra | 0 days | 14 days | | PastureGard | 0 days | 14 days |

Aminopyralid (Milestone, GrazonNext)

GrazonNext: 0 days

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High-Risk Plants After Herbicide Treatment

Prussic Acid Producers

Time Post-Treatment	Risk Level	Recommendation
0-24 hours	Extreme	Absolute exclusion
24-72 hours	Very High	Continued exclusion
3-7 days	High	Test before grazing
7-14 days	Moderate	Safe if completely dead/dry

No green tissue remaining
Minimum 7 days since treatment
Test if any uncertainty

Nitrate Accumulators

|-----------|-----------------| | Growing plant before treatment | Moderate (diluted by growth) | | Day 1-3 after treatment | High (uptake continues, growth stopped) | | Day 4-7 after treatment | Very High (maximum concentration) | | After complete death | Moderate (no new uptake but retention) |

Locoweed (Astragalus and Oxytropis species)

2,4-D treatment increases palatability 5-10 fold
Cattle may preferentially seek out treated plants
Swainsonine toxin remains fully active
Risk period extends until plants are completely dead and dry
All treated plants must be dead and dry
Monitor cattle behavior when returned to pasture
Consider alternative pastures if heavy infestation was treated

Snakeweed/Broomweed (Gutierrezia species)

Late-term abortion in cattle
Retained placenta
Weak calves

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Regrowth Hazards

The Regrowth Risk

When herbicide treatment kills top growth but doesn't completely eliminate root systems, regrowth can be more dangerous than the original plants.

Rapid Juvenile Growth: Young tissue often has higher toxin content
Stress Response: Regrowth plants may produce extra defensive compounds
Reduced Dilution: Small amount of tissue contains full toxin complement

Sorghum/Johnsongrass Regrowth

|-----------------|-------------|--------| | <6 inches | Extreme | Absolutely no grazing | | 6-12 inches | Very High | No grazing | | 12-18 inches | High | Test before any grazing | | >18 inches | Moderate | Generally safe if not stressed |

Oak Sprout Regrowth

Re-treat sprouts before they become established
Keep cattle away from areas with active sprouting
Consider follow-up treatment in subsequent years

Mesquite Regrowth

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Safe Grazing Protocols After Herbicide

General Timeline Guidelines

|------------|-------------------| | Prussic acid plants (sorghum, Johnsongrass) | 21 days minimum | | Wild cherry | Until all leaves fallen and dried | | Nitrate accumulators | 14-21 days + testing | | Locoweed | 21-30 days until completely dead | | General broadleaf weeds | 7-14 days | | Woody browse | Until leaves dropped |

Pre-Grazing Assessment

Are treated plants completely dead?
Is there any green tissue remaining?
Are sprouts or regrowth present?
What is the condition of remaining forage?
Testing (When Indicated):
Nitrate test if pigweed/kochia/Johnsongrass present
Prussic acid test if sorghum/sudan treated
Consider general forage quality test
Forage Assessment:
Is there adequate desirable forage available?
Will cattle need to consume treated plant residue?
What is the body condition of cattle being returned?

Introduction Protocol

Monitor: Observe for 24-48 hours
Day 3-4: If no issues, introduce more cattle
Week 1: Continue monitoring all animals daily
Week 2+: Resume normal observation schedule
High-value breeding stock
Young, naive animals
Animals with previous plant poisoning history

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Seasonal Considerations

Spring Treatment Timing

Longer time before fall grazing pressure
Better control efficacy
Cattle attracted to green treated plants
Longer potential exposure window

Summer Treatment Timing

Drought stress enhances control
Less competition with desirable forages
Drought reduces alternative forage
Cattle more likely to eat anything available

Fall Treatment Timing

Plants moving nutrients to roots
Cattle can be excluded during winter
Treated sorghum + frost = extreme prussic acid
Limited time for plant death before cold weather

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Documentation and Record Keeping

Treatment Record Template

``` HERBICIDE APPLICATION RECORD

Date of Application: ________ Applicator: ________________ Product: ________________ Rate: ________________ Method: [ ] Spray [ ] Granular [ ] Other

Target Plants: ________________ ________________

Weather Conditions: Temperature: ______°F Wind: ______ mph Recent rain: ________________

Treatment Area: Pasture/Field: ________________ Acres treated: ______ Map reference: ________________

GRAZING RESTRICTIONS: Label requirement: ______ days Extended caution period: ______ days Cattle exclusion start date: ________ Planned return date: ________

PRE-GRAZING CHECKLIST: [ ] Visual inspection completed [ ] Target plants dead and dried [ ] No regrowth observed [ ] Testing completed (if needed) [ ] Test results: ________________

ACTUAL RETURN DATE: ________ Initial cattle count: ______ Test group used: [ ] Yes [ ] No

MONITORING NOTES: Day 1: ________________ Day 3: ________________ Day 7: ________________ Day 14: ________________

ANY INCIDENTS: ________________ ```

Annual Treatment Summary

Track all herbicide treatments and outcomes:

Pastures treated
Products used
Target plants
Efficacy results
Any livestock issues
Recommendations for next year

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Emergency Response

If Cattle Consume Treated Plants

Identify what they consumed
Estimate consumption amount
Contact veterinarian immediately
Monitor for symptoms
Time since treatment
Plants consumed
Estimated consumption amount
Number of animals potentially affected
Current symptoms observed

Symptom Watch List

Staggering, tremors
Cherry-red or blue mucous membranes
Sudden death
Brown/chocolate blood
Weakness, rapid death
Depression, incoordination
Staring, difficulty eating/drinking
Behavioral changes

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Prevention Strategies

Planning for Successful Treatment

[ ] Plan cattle exclusion period
[ ] Arrange alternative grazing
[ ] Have forage testing kit available
[ ] Review label grazing restrictions
[ ] Document pre-treatment plant density
[ ] Note weather conditions
[ ] Mark treatment boundaries
[ ] Secure gates and fencing
[ ] Begin grazing restriction clock
[ ] Scout for regrowth at 14, 30, 60 days
[ ] Test forages before grazing
[ ] Introduce cattle gradually
[ ] Continue monitoring after return

Alternative Approaches

Pregnant animals cannot be moved
Alternative forage unavailable
Previous problems with treated plants
Intensive rotational grazing
Goat grazing for brush control
Prescribed fire (where appropriate)
Fencing out problem areas

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Sources and References

Texas A&M AgriLife Extension. "Herbicide Selection and Use Guide for Pastures."
USDA NRCS. "Managing Grazing Following Herbicide Application."
Oklahoma State University Extension. "Prussic Acid Poisoning."
Dow AgroSciences. "Range and Pasture Stewardship Guidelines."
University of Nebraska Extension. "Grazing Crop Residues After Herbicide Application."
Merck Veterinary Manual. "Plants Poisonous to Animals."
Texas A&M Veterinary Medical Diagnostic Laboratory. "Common Toxicoses in Cattle."
Knight, A.P. "A Guide to Plant Poisoning of Animals in North America."

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Last Updated: January 2026