published in


February 2016

Pasture Weed Control Following a Wet and Warm Fall

Ona Report - Dr. Brent Sellers

For questions or comments regarding this publication contact: Brent Sellers, University of Florida, IFAS

Defining a “normal growing year” has definitely become difficult over the past couple of years. We typically see a decline in temperatures and rainfall beginning around October. However, this year it has been quite different has we have rarely seen temperatures below 80 degrees, and rainfall has been more than sufficient in many areas of the state. Although temperatures have been sufficient for forage growth, short days dramatically slow down warm season forage productivity. What is odd this year is that we have short days coupled with high temperatures and soil moisture. These are the conditions that are perfect for weed establishment. For example, tropical soda apple (TSA) has become very prevalent this fall/early winter in pastures throughout central and south Florida. Besides TSA, observing crabgrass competing with ryegrass is not something we see every year, and goatweed appears to be increasing in bahiagrass pastures and bermudagrass hayfields. So, the main question becomes, how do we plan weed management programs in a year when nothing is normal and weeds may become more dominant than our forage grasses before May? 

There are several things we feel that are important to consider every year for pasture weed management, but these may be more important than ever during this upcoming growing season.

  1. Know your weeds. To optimize weed management in any pasture it is important that you correctly identify the predominant weed species. Usually, pasture herbicides will provide fairly broad-spectrum weed control, but there are cases when weed control will be optimized by choosing the correct herbicide or mixtures of herbicide. For example, if goatweed were the primary target in a bahiagrass pasture, treating the pasture with 2 qt/acre 2,4-D or a tankmix equivalent of this rate would be recommended versus GrazonNext HL or GrazonNext HL + Pasturegard HL tankmixes.
  2. Know your forages. There is nothing worse than slowing the growth of forages when attempting to control weeds. While some forage injury can be tolerated, it is best to know what herbicides should not be applied to certain forages. For example, herbicides containing metsulfuron (Cimarron, Ally, Escort, Chaparral, etc.) should not be applied to Pensacola bahiagrass pastures or substantial yield and/or stand loss is likely to occur. Refer to Table 5 of “Weed Management in Pasture and Rangeland” http://www.edis.ifas.ufl/wg006 for a list of herbicides and forage grass tolerance.
  3. Control weeds prior to applying fertilizer. There is no sense in wasting money fertilizing the weeds in the pasture. Remember that weeds compete with our desirable forages for light, water, and nutrients. Removing the weeds prior to fertilization should optimize fertilizer uptake by the forage. The only soil amendments that should be applied prior to herbicide applications are those that alter soil pH. Improving soil pH to the optimum level for the forage growing in a particular pasture will help the forage compete with undesirable species.
  4. Avoid overgrazing if at all possible. In dry years most of our pastures become overgrazed due to lack of moisture and sunlight for forage growth. If we do have above average rainfall, most of our forage grasses will remain green with some early season growth (remember, warm-season forage growth is day length dependent). If pastures are overgrazed during the early growing season with sufficient moisture for weed germination, weeds will have a better chance competing with the forage since forage growth will be limited by the amount of sunlight received.
  5. Controlling weeds early in the growing season usually results in increased weed control and is typically more economical. Although some of our weeds already have a head start on the upcoming growing season, spraying early in the year should result in increased herbicide activity, and will allow for the growth of warm-season forages with little to no competition. Also, spraying early in the growing season typically means that we are treating smaller weeds, and allowing for the use of more economical herbicides than when trying to kill weeds later in the year. For example, if a bahiagrass pasture contained TSA, goatweed, and dogfennel (<36 inches), GrazonNext HL at 24 oz/A plus 2,4-D at 48 oz/A would be the best option to control all three weed species. If treatment were delayed until the dogfennel were >36 inches, the recommendation would be changed to GrazonNext HL at 24 oz/A plus Pasturegard HL at 8 oz/A to optimize dogfennel control; however, this recommendation actually results in a decrease in efficacy for goatweed control. Therefore, waiting to treat this bahiagrass pasture could eventually be detrimental to the overall weed control in this bahiagrass pasture.
  6. Many producers have asked if they can spray hexazinone (Velpar, Velossa, Hexar) to control smutgrass this time of year since rains have continued. However, smutgrass should be actively growing before any herbicide is applied. Even though temperatures and rainfall have been above normal, it appears that most smutgrass plants have entered into a state of dormancy, judging from dead leaves observed in many smutgrass clumps. Since hexazinone works by being absorbed into the roots, the smutgrass must be growing and using water for the herbicide to work. We also typically recommend hexazinone be applied during July through September. This is in our recommendations based on previous research and our observations over the past 10+ years; however, limited work on smutgrass control with hexazinone has been performed in late spring/early summer. Therefore, it is possible that a late spring application of hexazinone on smutgrass would be effective, but the smutgrass needs to be actively growing, and rainfall needs to be sufficient to move the herbicide into the soil solution (approximately ½ to 1 inch). If a wiper is being used to manage smutgrass, a 10% glyphosate solution is typically sufficient, if ground speed is relatively slow and the smutgrass clumps are wiped in two directions to enhance coverage. It is also best to graze the desirable forage prior to wiping so that is it substantially lower than the smutgrass.

While GrazonNext HL and Pasturegard HL are commonly utilized together for broad spectrum weed control in pastures, this combination is not always the best option in certain situations. This is why it is important to know what weeds are present in your pasture, forage tolerance to herbicides, and when the herbicides should be applied to optimize weed control in your pastures. Additionally, not every pasture will contain the same weed spectrum, so it is important to scout your pastures regularly and treat them accordingly to optimize forage production. 

You are invited to join us by webinar on Feb. 19th, at noon, when Dr. Sellers will be presenting the information covered in this Ona Report and he will be available to answer any questions afterwards. To participate this webinar you simply need to register online here: https://attendee.gotowebinar.com/register/6264397502250345985.