THE FLORIDA CATTLEMAN AND LIVESTOCK JOURNAL
Using a Weed Wiper for Perennial Grass Control in Bahiagrass Pastures
Jose Luiz C.S. Dias and Brent Sellers
For questions or comments regarding this publication contact: Brent Sellers, University of Florida, IFAS
When we think about weed management with herbicides the first method of weed control that most often comes to one’s mind is usually broadcast herbicide applications. This method consists of uniformly applying a spray solution over an entire area by forcing the herbicide solution through nozzles evenly spaced at the boom (Figure 1a). Since the introduction of synthetic herbicides in the 1940s, this method of application has remained largely unchanged with the exception of better technologies including spray nozzles, pumps, etc.
Glyphosate was introduced in the 1970s and has completely revolutionized weed management over the years through various forms of crop improvement. Glyphosate is a postemergence, nonselective herbicide (at normal use rates) with broad-spectrum activity on both grass and broadleaf plants. It is translocated in plants, so that contact with the foliar portion of a plant results in the destruction of both the aboveground and the belowground portions. These features made glyphosate highly effective against both annual and perennial weeds. These unique properties in the 1970s and 1980s prompted the search for new application techniques where the main goal was to utilize glyphosate in a selective manner and reduce application costs.
Several new application techniques started to be tested. In around 1978 rope-wick applicators first began to be used and soon became a widely adopted method for selectively applying glyphosate, especially in row crops. Many variations of the basic rope-wick (Figure 1b) concept have been later introduced such as the roller applicators (Figure 1c) also called weed-wipers. Both methods obtain selectivity when using glyphosate by mechanically placing the herbicide solution so that it will not come in contact with the crop. In these techniques the rope or roller is saturated with herbicide. The applicator is then dragged over the area, wiping the herbicide solution onto the leaf surfaces of the target plants. Plants that are contacted by the herbicide solution are destroyed, therefore, the weeds have to be taller than the desirable crop or forage.
After the introduction of the first genetically modified glyphosate-resistant crops in the 1990s, the need of glyphosate selectivity was no longer an issue and the adoption of these application techniques fell into disuse. However, glyphosate-resistant perennial warm-season grass forage have not been (nor will likely be) developed in the U.S.; therefore, the use of these “old” techniques to achieve glyphosate selectivity in forage systems has been renewed in recent years with some technological advances in these weed wipers.
The cow-calf industry relies heavily upon perennial warm-season grass forage production because they are the largest nutritional input for beef cattle operations. Being the largest component of their diet, managing the forage properly becomes very important in terms of your herd production level as well as the economic returns from the land. With that being said, weeds are very problematic in pasture systems because they compete with our desirable forage for resources such as water, light, nutrients and space, decreasing the forage production potential. In addition, weeds may shelter pests, and some can be toxic lowering animal’s performance and sometimes even leading them to death.
Among all pasture weeds present in Florida, perennial grasses are the most challenging ones to achieve effective control in perennial warm season grass pastures. Vaseygrass (Paspalum urvillei) (Figure 2a), broomsedge species (Andropogon spp.) (Figure 2b) and giant smutgrass (Sporobulus jacquemontii) (Figure 2c) are a few examples of problematic, tough to control perennial grass weeds, especially in Central/South Florida. Giant smutgrass is considered to be the most problematic grass weed in perennial grass pasture systems, with broomsedge species a very close second. Vaseygrass tends to invade wetter areas of pastures and can become problematic in some areas.
We believe that the proper use of rope-wick or weed-wiper applicators might be an effective way to apply broad-spectrum non-selective herbicides, especially glyphosate, on these tough perennial grass weeds. Controlling these grasses will increase forage production, and potentially increase profitability by not having to reduce stocking rates. Therefore, a couple of preliminary trials and experiments have been conducted in south-central Florida over the past few years with the overall goal to test the efficacy of this method in controlling perennial grass weeds.
Weed-wiper experience with broomsedge.
Broomsedge (Andropogon) species are native, warm-season, short-lived perennial bunchgrasses with an average life span of 3 to 5 years. While some species are desirable in many natural areas and native rangeland, they are becoming problematic in improved bahiagrass pastures throughout central and south Florida. Since no herbicides can selectively remove these species, the use of the weed-wiper for selective control of broomsedge was investigated.
In August, 2014 glyphosate (10% v/v) was wiped onto broomsedge in two directions. Within a 2-week period, broomsedge plants began to show signs of glyphosate activity (Figure 3a). Unfortunately, these pastures were mowed at this time for a grazing trial. However, by 1 year after treatment, the broomsedge population declined by approximately 70%. This pasture was wiped a second time in the fall of 2015 in two directions using the same concentration of glyphosate to control escapes from the first application. After investigating this pasture in June, 2016, it appeared that we controlled nearly 95% of the original population.
Weed-wiper experience with vaseygrass.
The first experiment with broomsedge prompted us to investigate the use of the weed wiper on vaseygrass at the research center in the early summer of 2015. A bahiagrass pasture with vaseygrass was mowed and wiped with a 10% v/v glyphosate solution approximately 30 days later. We split the pasture in half, with one-half receiving glyphosate in only one direction, and the other two directions. We evaluated control at 30 and 60 days after treatment. Our results from this trial indicated that wiping vaseygrass in a single direction resulted in kill of the very top portions of the plant, but the bases of the plants remained viable. Conversely, when vaseygrass was wiped in opposite directions (2 times), the entire vaseygrass plant, including the roots, was killed (Figure 3b). This demonstrated the importance of wiping vaseygrass in two directions, rather than just a single direction.
Weed-wiper experience with giant smutgrass.
There are only two herbicide active ingredients labeled for pasture systems in Florida which are active on smutgrass species: glyphosate and hexazinone (e.g. Velpar, Velossa, Hexar). Since applying hexazinone at the recommended rates can be cost-prohibitive for many ranchers, and lack of control that often occurs when rainfall is limited (< 0.25 in.) or excessive (≥ 2.5-3.0 in.) after application, the use of glyphosate through rope-wick and roller applications could be an additional method of smutgrass management.
Like vaseygrass, we also conducted a similar study in 2015 on smutgrass using a 10% v/v glyphosate solution, but wiped only in two directions. Results were very promising within the year of application, but by the summer of 2016 almost every smutgrass clump had recovered, indicating that the glyphosate concentration was not high enough to control the entire plant. We initiated a study in 2017 at two separate established bahiagrass pastures in Hardee and Highlands counties. Our main objectives were 1) to determine the activity of different glyphosate concentrations using the weed-wiper; and 2) to determine the effects of mowing on the activity of glyphosate with the weed-wiper. We evaluated two mowing treatments (mowed and un-mowed) and three herbicide treatments (glyphosate at 17.5, 35 and 70% v/v) applied in a single or two directions.
Approximately 30 days prior to applying glyphosate with the wiper, half of the treated area was mowed to a 4-inch stubble height. Glyphosate was mixed at the proper concentration and applied in either one direction or two directions (opposite) by adjusting the wiper so that it was at least 2-inches above the bahiagrass canopy. Smutgrass control was determined by assessing the number of live and dead clumps in the center of each plot at 90 d after treatment. The data from each location was combined, and all the results presented should be considered preliminary as we will continue to monitor the regrowth of smutgrass following these treatments in 2018.
At 90 DAT the treatment combinations that showed the best results was mowed + glyphosate at 70% v/v applied bi-directionally (70% of clumps dead) (Figure 3c), followed by mowed + glyphosate at 35% v/v applied bi-directionally (58% clumps dead), un-mowed + glyphosate at 70% v/v applied bi-directionally (48% of clumps dead) and un-mowed + glyphosate at 35% v/v applied bi-directionally (44% clumps dead) (Figure 4).
To date, this research on smutgrass shows that the pre-mowing treatment appears to enhance the efficacy of glyphosate applied with the weed-wiper method. However, herbicide rate and application strategy may also play a major role, since the higher rates and bi-directional application provided greater levels of initial control (Figure 4). We will continue to monitor these plots and also repeat these experiments in 2018 and hope to provide some answers in regards to the appropriate glyphosate concentration required for effective smutgrass control.
Overall, the use of a weed-wiper appears to be fairly easy, but there are definitely challenges when using this technology. We have found that it takes significant practice and patience when using the weed wiper to ensure that you have sufficient herbicide solution on the wiper as well as enough growth of the target weed to ensure proper herbicide interception while limiting the interception of non-target, desirable plants. Furthermore, some plants grow faster than others, which means that not all plants will be controlled within the same year and will likely require multiple years of wiper applications for satisfactory control. In closing, be sure to follow the herbicide labels as there are restrictions when using glyphosate in the weed wiper. For example, the Roundup PowerMax label indicates that livestock should be removed prior to application, with a 3-day grazing restriction, and that it should not be applied to more than 10% of the total field area at any one time.