Chute N The Bull

One of the most common subjects that Extension Agents are asked about
each spring is weed control in pastures. Devising a strategy for
effective weed control depends on many factors including weed species
and growth stage, weed density, soil fertility, pH, forage type,
stocking rate or forage requirements, and proximity to sensitive crops.
If the weed density is low or is not limiting forage availability, then
it does not meet the treatment threshold and treatment is not justified
for production reasons. Often we are able to improve grazing or
fertilizer management, which can keep weed density below the treatment
threshold. However, in many cases herbicides are necessary to maintain
infestations below that threshold.
We traditionally start a pasture herbicide program using 2, 4-D amine as
the base because it controls many common broadleaf weed problems with
the lowest chemical cost. If there are enough weeds present that 2, 4-D
will not control them or if some level of residual activity is needed,
herbicides containing active ingredients like dicamba, picloram,
metsulfuron-methyl or triasulfuron may be used. In 2006, a new active
ingredient, aminopyralid, was released. It was reported to have superior
residual soil activity as well as post-emergence activity on many
difficult to control, broadleaf weeds. Very little research had been
done on this material in our area, soi found these experiments in 2008
and 2009 to evaluate its performance under our similar conditions. The
focus of these trials was to compare the products containing
aminopyralid with current industry standards.
These trials were conducted on the Noble Foundation's Pasture
Demonstration and McMillan Research farms to evaluate activity on
naturally occurring populations of western ragweed in pastures with
unfertilized, weak bermudagrass stands. Pre-emergence treatments were
Milestone™ (aminopyralid) at 3, 5 and 7 fluid ounces per acre.
Post-emergence treatments were Milestone at 3, 5 and 7 fluid ounces per
acre, GrazonNext™ or ForeFront™ (aminopyralid + 2,4-D) at 1.5, 2 and 2.5
pints per acre, Cimarron Max™ (metsulfuron-methyl + 2.4-D + dicamba) at
Rate I, Grazon™ P + D (picloram + 2,4-D) at 1.5 pints per acre, Range
Star™ (2,4-D + dicamba) at 1.5 pints per acre and 2,4-D amine at 2 pints
per acre. All the post-emergence treatments included a nonionic
surfactant at a rate of 0.25 percent volume/volume. The post-emergence
treatments were applied to western ragweed with an average height of 6
inches. An untreated control was included for comparison. Trials were
visually rated for percent of western ragweed control periodically from
12 to 161 days after the post-emergence applications.
In the 2008 trial at the Pasture Demonstration Farm, the season-long
western ragweed control from the pre-emergence treatments was poor at 46
percent or less. However, in the 2008 McMillan Farm trial, 76 percent to
100 percent control was obtained. In both 2009 trials, the pre-emergence
treatments provided excellent season-long control of 93 percent to 100
percent. The reason for the reduced control in the 2008 trial at the
Pasture Demonstration Farm is unknown, but might be due to the higher
density of residual vegetation at this site coupled with a high
intensity first rainfall event (4.8 inches of precipitation three to
four days after application). The 2008 McMillan Farm site had less
residual vegetation, and the first rainfall event was not as intense
(3.47 inches of precipitation three to five days after application).
Both 2009 trials had less residual vegetation, and the first rainfall
events were less intense (1.36 to 1.97 inches of precipitation 12 to 14
days after application).
The activity from all the post-emergence treatments was excellent in
both years, exceeding 90 percent control for the full season. The
results of the post-emergence treatments show the herbicides containing
aminopyralid perform as well on western ragweed as the current industry
standards. These results alapplication timing and confirm that season-long control of western
ragweed is possible using current industry standards.
In summary, the results of these trials indicate that the products
containing aminopyralid can provide pre-emergence control of western
ragweed. It is important to note that control may vary depending on soil
deposition and rainfall activation. This pre-emergence control may be
particularly valuable in areas where vulnerable crops are actively
growing near pastures when traditional late spring or summer
applications would be made. These products are also equal to the current
industry standards for post-emergence control of western ragweed. Note
that these trials only address western ragweed, and more research is
needed to determine efficiency on other difficult to control weed
species. You can find this and past articles on the web at
http://www.mycountrytractor.com/ for your reference. Extension
programs serve of all ages regardless of socioeconomic level, race,
color, sex, religion, disability, or national origin. The Texas A&M
University System, U.S. Department of Agriculture, and the County
Commissioners Courts of Texas Cooperating serve of all ages regardless
of socioeconomic level, race, color, sex, religion, disability, or
national origin. The Texas A&M University System, U.S. Department of
Agriculture, and the County Commissioners Courts of Texas Cooperating



Thank you,

Tommy Neyland
County Extension Agent
Texas Agrilife Extension Service
P.O. Box 188
Centerville, Texas 75833
903.536.2531 phone
903.536.3804 fax