Introduction
Georgia, Mississippi, South Carolina, and Tennessee are southeastern US states that collectively produce over 1 billion kg of forage annually (NASS 2018). The Georgia cattle industry had nearly 1.1 million cattle to feed in 2018 (NASS 2018). In 2017, Georgia produced hay and haylage worth over $187 million (NASS 2018). Supplying forages in the form of hay to animals is required during late autumn and winter months to maintain animal health and productivity. During spring and summer months, producers will harvest forage grass species for hay. In the southeastern United States, a common forage species for hay production is bermudagrass. Many weed species are common in bermudagrass hay fields, and if left uncontrolled, will reduce both the palatability of the forage (Masters et al. Reference Masters, Nissen, Gaussoin, Beran and Stougaard1996) and performance of the animal. Common and troublesome weeds in forages include various species of crabgrass (Digitaria spp.), pigweeds (Amaranthus sp.), and thistles (Cirsium sp.) (Webster Reference Webster2012). When present in forages, weeds reduce hay value, nutritional quality, and animal productivity.
Many of the common weed control systems in Georgia and southeastern forage production include herbicides that share the same mechanism of action. Among these are WSSA Group 4’s 2,4-D, aminopyralid, clopyralid, dicamba, fluroxypyr, and triclopyr, and the Group 2 acetolactate synthase inhibitors chlorosulfuron, halosulfuron, imazapic, metsulfuron, nicosulfuron, and sulfosulfuron (McCullough Reference McCullough2018). Multiple herbicides are registered for bermudagrass forage production, but many have been shown to cause injury (Bhowmik and Bingham Reference Bhowmik and Bingham1990; Butler and Muir Reference Butler and Muir2006; Walker et al. Reference Walker, Wehtje and Richburg1998). The production of a perennial forage crop, utilizing the same herbicides over several years, can result in weed resistance (McCullough et al. Reference McCullough, Yu and Gómez de Barreda2013; Simmons Reference Simmons2017). Expanding the number of herbicides with different sites of action available in perennial forage crop production can reduce the selection pressure on currently available herbicides, reducing the chances of weeds becoming resistant.
Indaziflam is a WSSA Group 29 alkylazine herbicide that inhibits cellulose biosynthesis of susceptible plants (Brabham et al. Reference Brabham, Lei, Gu, Stork, Barrett and DeBolt2014), allowing it to control numerous weed species with a soil residual half-life of >150 d (Shaner Reference Shaner2014). It is registered in multiple perennial crops (Grey et al. Reference Grey, Luo, Rucker and Webster2016; Jhala and Singh Reference Jhala and Singh2012; Marble et al. Reference Marble, Chandler and Archer2016). It is also registered for use in multiple warm-season turfgrasses (Anonymous 2017). Indaziflam has pending registration for bermudagrass forage production, with a maximum usage rate of 94 g ha−1 over a 12-mo period. Brosnan and Breeden (Reference Brosnan and Breeden2012) noted that indaziflam provided similar control of nontillered smooth crabgrass to dithiopyr (560 g ha−1) and quinclorac (840 g ha−1). Control of annual bluegrass (Poa annua L.) was 88% to 100% in Tennessee when applied PRE and 4 and 8 wk after planting at 35 and 54 g ha−1 (Brosnan et al. Reference Brosnan, Breeden, McCullough and Henry2012). In Georgia, investigators noted that annual bluegrass control was 97% to 100%, as compared to the nontreated control, 30 wk after treatment (Brosnan et al. Reference Brosnan, Breeden, McCullough and Henry2012). In view of the traditional use of herbicides with the same mechanism of action in forage production, the potential use of indaziflam as an alternative for weed control could provide more sustainable weed management systems for bermudagrass forage production in the Southeast, while proactively avoiding the development of herbicide-resistant weeds.
If registered for forage bermudagrass production, indaziflam as a WSSA Group 29 could provide broad-spectrum control of many weed species and offer alternatives to the WSSA Groups 2 and 4 herbicides. The objective of this research was to determine the tolerance to PRE and POST applications of indaziflam on bermudagrass at multiple rates and timings.
Materials and Methods
Field experiments were conducted in separate areas of the same fields in 2017 and 2018 at the Coastal Plains Experiment Station in Tift County, GA (31.50333°N, 83.53222°W) at the UGA Research Dairy, and the Sunbelt Agriculture Exposition Center in Colquitt County, GA (31.14500°N, 83.71194°W) (four tests in all). ‘Alicia’ bermudagrass (Hancock et al. Reference Hancock and Harris2017) established stands (>10 yr) were utilized at both locations. The experimental bermudagrass areas were established and maintained using best management practices. At establishment and throughout the study period, bermudagrass was close to weed free. The few weeds that were observed were removed by hand. The soil at the Sunbelt Ag Expo consisted of a Dothan loamy sand (Fine-loamy, kaolinitic, thermic Plinthic Kandiudults), 88% sand, 6.2% silt, 6.0% clay, pH 6.0 to 6.2, 2.9% to 3.5% organic matter content, whereas at the UGA Research Dairy the soil was Cowarts loamy sand (Fine-loamy, kaolinitic, thermic Typic Kanhapludults), 86% sand, 8.2% silt, 5.9% clay, pH 6.9 to 7.0, 3.3% to 3.4% organic matter content.
Experimental design consisted of a randomized complete block with four replications. Plot size was 1.5 m wide by 6.1 m long. Treatments applied at spring greenup to established ‘Alicia’ bermudagrass were indaziflam at 47, 77, 155, or 234 g ai ha−1 PRE, pendimethalin at 4,480 g ha−1 PRE, a split application of indaziflam at 47 g ha−1 PRE followed by the same rate applied POST after the first forage harvest, and a nontreated control (seven treatments in all). Initial herbicide applications were made at the Tifton UGA Research Dairy on February 24, 2017 and February 28, 2018, and at the Sunbelt Agriculture Exposition Center in Colquitt County on February 27, 2017 and March 2, 2018. For the POST treatment after the first forage harvest, applications were made at the Tifton UGA Research Dairy on April 20, 2017 and May 11, 2018, and at the Sunbelt Exposition Center in Colquitt County on June 21, 2017 and May 16, 2018. No adjuvants were applied for the POST applications. Herbicides were applied with a CO2-pressurized backpack sprayer at 187 L ha−1 at 165 kPa using Teejet AIXR11002 nozzles (Teejet Technologies LLC, Springfield, IL).
Bermudagrass injury was visually estimated 20 to 30 d after PRE applications, and 15 to 20 d after POST applications on a scale of 0 (no injury) to 100% (crop death). Bermudagrass forage yield was measured by harvesting each plot with a modified mower with a chute attachment. The chute was placed on the open port of the cutting area to vacuum the clippings. The sample bag was placed at the opposite end of the chute to receive the grass clippings. When wet conditions necessitated a hand harvest, a sample size of 1 m long by 0.5 m wide was measured and grass clipped using a hand-held trimer. All samples were cut to within 5 cm of the soil surface. Forage samples were weighed for fresh weight and dried to less than 20% moisture (Hancock et al. Reference Hancock and Harris2017) with heated, forced air (50 C) for 3 d before dry weight was determined. Harvests at the Tifton Research Dairy were 47, 110, and 168 d after treatment (DAT) in 2017, and 72, 107, and 154 DAT in 2018. Harvests at the Sunbelt Agriculture Exposition Center were 79, 107, and 154 DAT in 2017, and 76, 113, and 154 DAT in 2018. Production and pest management practices other than specific treatments were standard for forage bermudagrass production in Georgia to optimize growth and development (Hancock et al. Reference Hancock and Harris2017).
Data were subjected to an ANOVA using PROC Mixed in SAS 9.4 (SAS Institute Inc., Cary, NC) combined across experiments, location, and year to test for interactions. Treatments were separated with a Tukey-Kramer least squares means test (P ≤ 0.05). Herbicide treatment was considered a fixed effect, and locations (nested within year) and replications were regarded as random factors.
Results and Discussion
Because there was no significant interaction of experiment by location by year for any variable, data were combined for analysis. Data for bermudagrass injury, harvest data for first, second, third, and total biomass for fresh weight, and dry weight were combined. Data for the main effects of herbicide treatment are presented. Average rainfall and temperatures were recorded for both years and locations (data not shown) and resulted in normal forage bermudagrass growth.
Injury
Bermudagrass stunting in response to the PRE treatments 20 to 40 DAT was not observed for 47 and 77 g ha−1 and was less than 30% for indaziflam at 155 or 234 g ha−1 (data not shown). All injury was transient, with no stunting after the initial harvest, even at the highest rates (data not shown). No injury was observed 20 to 30 DAT when indaziflam was applied at 47 g ha−1 POST after the first harvest. Brosnan et al. (Reference Brosnan, Breeden, McCullough and Henry2012) reported no bermudagrass injury in response to indaziflam applied up to 60 g ha−1 from PRE or POST applications.
Harvest Data
There was no indaziflam rate effect at the first harvest based on fresh or dry weight of bermudagrass compared with the nontreated control at 47 to 79 DAT (Table 1). The nontreated control fresh weight was 5,050 kg ha−1, with indaziflam treatments resulting in 4,770 to 5,970 kg ha−1. Pendimethalin had no effect on bermudagrass growth based on fresh and dry biomass. Pendimethalin has been previously shown to be nondetrimental to bermudagrass quality (McCullough et al. Reference McCullough, Yu and Gómez de Barreda2013). After repeated applications of dinitroaniline herbicides, some weed species, such as goosegrass [Eleusine indica (L.) Gaertn.], have become resistant (McCullough et al. Reference McCullough, Yu and Gómez de Barreda2013). Although pendimethalin provided excellent dry-matter yield, the use of other herbicides should be considered to prevent resistance development and/or the spread of a resistant population in bermudagrass pastures.
Table 1. Fresh and dry biomass for forage ‘Alicia’ bermudagrass comparing herbicide treatments in Georgia, 2017 and 2018. a
a First herbicide applications were made in Tift County on February 24, 2017 and February 28, 2018, and in Colquitt County on February 27, 2017 and March 2, 2018.
b Second herbicide applications were made in Tift County on April 20, 2017 and May 11, 2018, and in Colquitt County on June 21, 2017 and May 16, 2018.
c Means within a column followed by the same letter are not significantly different from each other according to Tukey-Kramer test at P ≤ 0.05.
Harvest 2 differed from harvest 1 in that treatment effects were significant only for fresh weight (Table 1). Bermudagrass treated with indaziflam at 234 g ha−1 had statistically the greatest fresh-weight yields. Although not different, this rate of indaziflam also provided the highest dry-weight biomass. The high rate of indaziflam-treated dry-weight biomass was significantly only greater than pendimethalin. Except for the aforementioned fresh-biomass differences at harvest 2, dry-weight biomass reflected the same as the fresh weights. Additionally, the split application of indaziflam at 47 g ha−1 PRE followed by POST yield was similar to all other treatments for fresh- and dry-weight biomass. This indicates the crop safety if a POST application of indaziflam after an initial harvest is needed to extend residual weed control. This is similar to other crop scenarios where indaziflam has been applied as split applications during the season (Grey et al. Reference Grey, Luo, Rucker and Webster2016, Reference Grey, Rucker, Wells and Luo2018). At harvest 3, 154 to 167 DAT, there were no differences in yield for any herbicide treatments for fresh- or dry-weight biomass.
When combined over the entire season, there were no differences in bermudagrass fresh- or dry-weight biomass for any herbicide treatment compared to the nontreated control (Table 1). Similar dry-weight yields of 8,000 to 10,000 kg ha−1 have been noted in other forage bermudagrass research on herbicides (Butler and Muir Reference Butler and Muir2006; Matocha et al. Reference Matocha, Grichar and Grymes2010; Walker et al. Reference Walker, Wehtje and Richburg1998). These data support the conclusion that indaziflam at suggested registered rates should provide crop safety and additional herbicides for weed control in bermudagrass.
Preventing resistant weed populations is a major concern for all growers, especially those with limited herbicide choices. The repeated usage of the same herbicide only exacerbates this problem. The introduction of indaziflam in bermudagrass forage production has the potential to reduce the reliance on limited herbicide options, adding a different mechanism of action for growers to utilize. Though there was some stunting noted, it was at rates exceeding the potential registered rates, was ephemeral, and did not affect fresh- or dry-weight yields. This study supports the use of indaziflam for safe weed management in actively growing bermudagrass with potential short-lived stunting but no long-term crop damage.
Acknowledgments
The authors would like to thank the Sunbelt Agriculture and Exposition Center and the University of Georgia Dairy Research Farm for providing forage production areas to conduct this research, the University of Georgia College of Agriculture and Bayer Crop Science for partial funding, technical support by Sidney Cromer, Owen Kimbrel, and Colby Lynn, and statistical support by Xuelin Luo. No conflicts of interest have been declared.
