One of the most common subjects that Noble Research Institute consultants 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, so we conducted experiments in 2008 and 2009 to evaluate its performance under our southern Oklahoma conditions. The focus of these trials was to compare the products containing aminopyralid with current industry standards.
These trials were conducted on the Noble Research Institute'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 also reinforce the importance of proper application 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.