Guard Against Nitrate Toxicity from Weeds

 BROOKINGS, S.D. -- Producers should be aware that nitrate toxicity is a danger from weeds, not just from drought-stressed crops.  Under drought conditions, livestock may graze weed plants that normally would be avoided, two South Dakota State University specialists said.

SDSU Extension Weed Specialist Leon Wrage and Extension Integrated Pest Management Coordinator Darrell Deneke said weeds and certain crop plants may accumulate high levels of nitrogen.  Weeds like kochia, pigweed or lambsquarters might be the significant sources of forage harvested where there is limited normal forage production due to dry conditions.

Factors favoring nitrate accumulation in weeds are similar to those for crops:

* Site: Form and content of nitrogen in the soil is variable.  Frequently pigweed, kochia, and lambsquarters species produce heavy growth in low, poorly drained areas.  Forage from these areas may be mostly weeds.

* Drought or low light conditions (weeds in understory) may increase nitrates.

* During dry weather, nitrate concentration may be especially high for a short period after significant precipitation.

* Treatment with 2,4-D can cause livestock to consume weeds that normally are not palatable.  The herbicide changes the plant metabolism, resulting in nitrate accumulation.  Studies have attributed abortion to grazing or feeding weeds in low land areas that were treated with 2,4-D.  Several other herbicides are likely to produce similar responses.

* As with crops, the nitrates accumulate in the vegetative tissue, not the seed.

The most common high-risk weeds with nitrate problems are pigweeds, lambsquarters, kochia, Russian thistle, Canada thistle, nightshade, bindweed, and dock species.  Others include: beggar ticks, plumeless thistle, bee plant, bindweed, barnyardgrass, wild sunflower, prickly lettuce, skeletonweed, sweetclover, witchgrass, smartweed, goldenrod, sow thistle, chickweed, mustard, and blue-green algae.

Wrage and Deneke suggest to:

* Be aware of the potential problem where weeds make up a high percent of the feed source.

* Test the feed, following guidelines for nitrate testing.

* Be aware of the increased risk associated with weed response to herbicides.

* In the future, plan to use herbicide early when weeds are small to reduce the weed material that will be grazed or harvested in the forage.

More information on drought is available at the SDSU drought Web site, http://sdces.sdstate.edu/drought/.  Extension Extra 8144, "Nitrate Toxicity from Weeds Drought Conditions" is available in pdf format at http://agbiopubs.sdstate.edu/articles/ExEx8144.pdf or from any county Extension office.
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Contact: Leon Wrage, (605) 688-4591; Darrell Deneke, (605) 688-4595
** For links to other sites showcasing SDSU's work in teaching, research, and Extension, visit http://sdces.sdstate.edu.

Wendy Mohrhauser, Intern
AgBio Communications Unit
South Dakota State University
ACC, Box 2231, Rm 200
Brookings, SD 57007
Telephone: (605) 688-5795