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© 2006 Plant Management Network.
Accepted for publication 31 January 2006. Published 8 March 2006.

Evaluation of Postemergent Herbicides on Rhizoma Peanut Injury and Yield

Jason A. Ferrell, Assistant Professor, Department of Agronomy, University of Florida, Gainesville 32611-0500; Brent A. Sellers, Assistant Professor, Range Cattle REC and Department of Agronomy, University of Florida, Ona 33865; and Christopher R. Mudge, Graduate Research Assistant, Department of Agronomy, University of Florida, Gainesville 32611 and Clyde A. Smith, Regional IPM Agent, University of Florida, Marianna 32448

Corresponding author: Jason A. Ferrell. jferrell@ufl.edu

Ferrell, J. A., Sellers, B. A., and Mudge, C. R., and Smith, C. A. 2006. Evaluation of postemergent herbicides on rhizoma peanut injury and yield. Online. Forage and Grazinglands doi:10.1094/FG-2006-0308-01-RS.

Abstract

Rhizoma peanut (Arachis glabrata Benth.) is one of the few high-quality legume forages that will persist in Florida. Rhizoma peanut hay is valued by horse and cattle producers, but weeds reduce its quality and value. Herbicides are often required for weed control, but it is not known which herbicides can be applied without causing injury and yield loss. Herbicides were applied to the rhizoma peanut cultivars ‘Florigraze’ and ‘Arbrook’ in 2004 and 2005 at 3 or 21 days after clipping (DAC). Dicamba + 2,4-D was highly injurious at both application timings, while hexazinone was most injurious when applied at 21 DAC for both cultivars. However, no herbicide applied at 3 DAC resulted in yield loss for either cultivar. When applied at 21 DAC, dicamba + 2,4-D reduced yield by 41 and 22% of Florigraze and Arbrook cultivars, respectively compared to the non-treated control. Similarly, hexazinone (0.28 and 0.56 kg/ha) reduced yield of Florigraze and Arbrook cultivars by at least 50 and 36%, respectively, compared to the non-treated control. Applications of 2,4-D alone reduced Florigraze yield by 41% compared to the non-treated control, but Arbrook yield was not affected by this herbicide. Florigraze appeared to be more sensitive to all herbicides with regard to visual injury and forage yield. Applications of imazapic, imazamox, and 2,4-DB did not result in visual injury or yield loss at either application timing for either cultivar.

Introduction

The dominant forage grass grown in Florida is ‘Pensacola’ bahiagrass (Paspalum notatum Flugge). This species is highly persistent, tolerates close grazing, grows on a variety of soil conditions, and is tolerant to a number of insects as well as pathogens (11). However, Pensacola bahiagrass is not considered highly nutritious, as weight gain for steers grazing exclusively on this species was only 0.37 kg/day (8). Additionally, Rollins and Hoveland (7) have shown that bahiagrass is not satisfactory to serve as the sole nutritional source for dairy cattle. Considering the low nutritional contribution of bahiagrass, it is desirable to add legumes to the feed ration.

Cool-season legumes such as alfalfa (Medicago sativa) and clover (Trifolium sp.) are the most commonly used species in the United States to provide the necessary nutrition for livestock. However, neither alfalfa nor clover persists under the environmental conditions that prevail in Florida (2). This has led livestock producers to view rhizoma peanut as an attractive warm-season legume that can be added to their grazing or haying systems.

Rhizoma peanut is a legume that performs well in the subtropical climates (Fig. 1). A native of Brazil, rhizoma peanut is most commonly grown along the Gulf Coast region (2). Forage quality is similar to alfalfa and has been shown to support weight gains in cattle between 0.7 and 1.0 kg/day (3,9,12). Unlike other legumes, rhizoma peanut is highly persistent in Florida and has been documented to persist on drought-prone sandy soils for over 18 years (5). Rhizoma peanut also produces similar yields to alfalfa under Florida conditions, approximately 9000 to 11,000 kg/ha/year (5), but is only harvested 2 to 3 times during a growing season.

Fig. 1. Rhizoma peanut.

The largest obstacle to rhizoma peanut production is weed control (6). Although weeds are most problematic during establishment, they continue to be challenging in mature stands. With fewer cuttings per season than alfalfa, the invasion of annual broadleaf and sedge weeds are particularly troublesome. With only 2 or 3 harvests per year, weeds are not suppressed by regular clipping and generally require the use of herbicides for weed management. However, very little information has been collected to document the impact of herbicide application on established stands of rhizoma peanut. Herbicide injury, in the form of growth reduction, could be highly detrimental to yield and profits. Additionally, much of the rhizoma peanut grown in Florida is produced for the horse or dairy market. Since weeds reduce palatability and forage quality, practical weed control recommendations must be developed in order to produce high-quality, weed-free, rhizoma peanut hay. Therefore, the objectives of this experiment were to determine if selected herbicides and herbicide application timing adversely affect rhizoma peanut appearance or yield.

Field Experiments

Two separate field experiments were conducted in Citra and Myakka City, FL in 2004 and 2005 on ‘Florigraze’ and ‘Arbrook’ rhizoma peanut cultivars. In 2004, Florigraze and Arbrook were evaluated in Citra while in 2005 Arbrook was assessed at Citra and Florigraze at Myakka City. The stand for each cultivar was fully established for at least 10 years. Soil type at Citra was an Arredondo fine sand (loamy, siliceous, hyperthermic, Grossarenic Paleudult) with 1% organic matter, and a Leon fine sand (sandy, siliceous, thermic Aeric Alaquods) with 1% organic matter at Myakka City.

The experimental design was a randomized complete block with a factorial arrangement of two application timings and eight herbicides. See Table 1 for the list of herbicides and application rates used. There were four replications and the experiments for each cultivar were repeated for two consecutive years. Studies were initiated on June 21, 2004 and June 27, 2005. Prior to the initial herbicide application, the entire experimental area was mowed to a height of 5-cm and all plant residue was removed; this was performed to simulate a hay cutting. Herbicides were applied to 3-m by 5-m plots at 3 and 21 days after clipping (DAC) using a CO₂-pressurized sprayer calibrated to deliver 187 liters/ha. Spray adjuvants were added as recommended by the respective herbicide label.

Table 1. Visual herbicide injury from herbicides applied in 2004 and 2005 at Citra and Myakka City on Florigraze.

 ^x Statistically different from control at α = 0.05.

 ^y DAC = days after clipping.

 ^z DAT = days after treatment.

Rhizoma peanut injury resulting from herbicide treatments were assessed visually at 10 and 21 days after treatment (DAT) using a 0 to 100 scale where 0 = no injury and 100 = complete crop death (4). The degree of stunting, chlorosis, and necrosis caused by herbicide application was used collectively to determine the percent injury of treated plants. Plots were mechanically harvested 42 DAC and whole-plot samples were weighed in the field. A sub-sample from each whole-plot sample was collected, weighed, and then dried at 60°C for 4 days. After drying, the sub-samples were weighed to determine percent water content and establish the dry weight of the whole-plot sample.

Data collected from each cultivar were analyzed using the General Linear Model procedure from SAS (SAS Institute Inc., Cary, NC). Each cultivar was analyzed separately and when no treatment by year interaction was detected, data were combined over years. Preplanned orthogonal contrasts were used to determine if herbicide treatments were statistically different (P = 0.05) from the non-treated control within an application timing. Orthogonal contrasts were also used to determine if application timing was significant within an individual herbicide treatment.

Herbicide Injury

A treatment by location interaction was detected for Florigraze and data are presented by year (Table 1). The interaction could have been due to climatic factors or soil type since Florigraze was tested at different locations each year. However, soil type and rainfall amount and distribution were similar between locations. Isolation of variance found that the interaction was due to differences in years for 2,4-D applied alone at 21 DAC.

Less than 8% injury was observed for 2,4-D, 2,4-DB, imazamox, imazapic, and hexazinone (0.28 kg/ha) when applied 3 DAC. In 2004, however, hexazinone (0.56 kg/ha) and dicamba + 2,4-D resulted in 27 and 25% crop injury, respectively, 10 DAT. Injury from the hexazinone application was transient and declined to 6% by 21 DAT, while injury from dicamba + 2,4-D remained as high as 23%. Several legumes have previously been shown to be highly sensitive to dicamba (1). Additionally, dicamba has a soil half-life of approximately 14 days and is weakly adsorbed to soil (10). Therefore, uptake of dicamba by plant roots is likely to occur for many days after treatment. Consequently, it is likely that the prolonged injury in 2004 was due, in part, to residual activity of dicamba. In 2005, injury from all herbicide applications was generally less than that in 2004. Rhizoma peanut injury from hexazinone (0.56 kg/ha) was 10% at 10 DAT and declined to 0 by 21 DAT.

By delaying herbicide applications on Florigraze until 21 DAC, herbicide injury increased dramatically for some herbicide treatments. In 2004, 2,4-D, 2,4-D + dicamba, and hexazinone (0.28 and 0.56 kg/ha) resulted in 21, 40, 25, and 52% injury at 10 DAT, respectively (Table 1). Hexazinone was the most injurious and injury increased to 40 and 60% for the 0.28 and 0.56 kg/ha rates, respectively, by 21 DAT. Injury for 2,4-D decreased to 15% by 21 DAT, but remained significantly greater than the non-treated. In 2005, all herbicides were less injurious to Florigraze. The use of 2,4-D alone did not result in any visual injury. Hexazinone applied at 0.28 and 0.56 kg/ha resulted in 15 and 30% injury, respectively, at 10 DAT, then increased to 33 and 50% by 21 DAT. Imazamox, imazapic, and 2,4-DB were not injurious to Florigraze at either application timing.

For the Arbrook cultivar, no treatment by year interaction was observed and the data were pooled across years. Dicamba + 2,4-D applied at 3 DAC resulted in at least 30% injury at 10 and 21 DAT (Table 2). Hexazinone applied at 0.56 kg/ha resulted in 10% injury at 10 DAT, but declined to only 4% by 21 DAT. Although 10% injury was significantly greater than the non-treated at 10 DAT, this level of injury is generally acceptable to producers.

Table 2. Visual herbicide injury from herbicides applied to Arbrook in 2004 and 2005 at Citra.

 ^x Statistically different at α = 0.05. Comparisons were made between the non-treated and the treated.

 ^y DAC = days after clipping.

 ^z DAT = days after treatment.

As observed with Florigraze, delaying applications until 21 DAC increased injury for both application rates of hexazinone and 2,4-D + dicamba. For hexazinone, injury at 10 DAT was 29 and 33% for 0.28 and 0.56 kg/ha, respectively. Similarly to Florigraze, injury increased for both application rates between 10 and 21 DAT. By 21 DAT, injury had intensified to 38% (9% increase) for the 0.28 kg/ha rate, while injury rose for the 0.56 kg/ha rate to 60% (27% increase). A similar pattern of increasing injury with time was also observed with 2,4-D + dicamba. Initially, 43% injury was observed 10 DAT and increased to 53% by 21 DAT. Unlike Florigraze, delaying application of 2,4-D did not result in significant injury. For both application timings of 2,4-D, injury was 8% and not significantly different from the non-treated. For all other herbicide treatments, injury was less than or equal to 5% and was not significantly different from the non-treated.

These data indicate that it is unadvisable to apply dicamba + 2,4-D (0.39 + 0.14 kg/ha) at any time or unacceptable injury may be observed. Delayed applications of hexazinone at rates of 0.28 kg/ha (or greater) will also likely result in excessive injury. Conversely, hexazinone applied within 3 days of harvest can be a safe and effective use of this herbicide. Although herbicide injury is observable soon after application, this often declined to near zero within 3 weeks of application. It was also found that 2,4-DB, imazamox, and imazapic could be applied at any stage of rhizoma peanut growth without causing significant levels of crop injury. However, none of these herbicides are expressly labeled for use in rhizoma peanut.

Forage Yield

No treatment by location interaction was detected for the Florigraze cultivar and the data were pooled over years (Table 3). For herbicides applied at 3 DAC, no statistical differences were detected relative to the non-treated, and forage yield ranged from 3029 kg/ha to 3447 kg/ha. Although dicamba + 2,4-D applications resulted in as much as 23% injury at 21 DAT, this level of injury did not ultimately translate into lower yields. However, delaying application to 21 DAC did result in substantial yield losses for some treatments. Applications of 2,4-D and dicamba + 2,4-D applied 21 DAC reduced forage yield by 24% and 41%, respectively, as compared to the non-treated control. Hexazinone applications at 21 DAC also reduced forage yield and application rates of 0.28 and 0.56 kg/ha reduced yield by at least 50% to 1701 and 1463 kg/ha, respectively. Delaying applications of 2,4-D or dicamba + 2,4-D to 21 DAC rather than 3 DAC resulted in a yield reduction of 21 and 34%, respectively, compared to the non-treated (Table 3). The impact of delayed hexazinone application was even more dramatic since yield was reduced by at least 48% compared to the non-treated control. Applications of 2,4-DB, imazamox, and imazapic did not reduce yield at any treatment timing.

Table 3. Impact of herbicides applied in 2004 and 2005 at Citra and Myakka City on Florigraze yield.

 ^w Statistically different at α = 0.05. Comparisons were made between the non-treated and the treated.

 ^x Statistically different at α = 0.05. Comparisons made within a treatment comparing application time.

 ^y DAC = days after clipping.

 ^z Percent reduction relative to applications made at 21 DAC versus 3 DAC.

No treatment by year interaction was detected for the Arbrook cultivar and the data were pooled over years (Table 4). No yield reductions were observed for any herbicide applied 3 DAC. Furthermore, application of 2,4-D (0.56 kg/ha) did not result in a yield reduction when applied 21 DAC, as was observed with Florigraze. However, yield was 22 to 39% lower than the non-treated control when dicamba + 2,4-D or hexazinone was applied 21 DAC. Additionally, yield reductions from applying dicamba + 2,4-D and both rates of hexazinone were not as great with Arbrook as was observed with Florigraze. Delaying the application of hexazinone from 3 to 21 DAC resulted in a relative yield reduction of 28 and 31% for Arbrook and to 48 and 51% for Florigraze.

Table 4. Impact of herbicides applied in 2004 and 2005 at Citra on Arbrook yield.

 ^w Statistically different at α = 0.05. Comparisons were made between the non-treated and the treated.

 ^x Statistically different at α = 0.05. Comparisons made within a treatment comparing application time.

 ^y DAC = days after clipping.

 ^z Percent reduction relative to applications made at 21 DAC versus 3 DAC.

These data suggest that Arbrook was more tolerant to herbicides applied in this study. This was expected since Arbrook has been described as a larger and hardier plant than Florigraze that is more tolerant to adverse conditions (5). Therefore, it is likely that Arbrook was growing more rapidly at the time of herbicide application than Florigraze and able to more rapidly overcome the effect of the herbicide.

From these data it was observed that 2,4-DB, imazamox, and imazapic can be applied at 3 or 21 DAC with minimal risk of visual injury or yield loss. Conversely, hexazinone could only be applied at 3 DAC or significant yield loss is likely to result. The use of 2,4-D is not as clear since injury and yield loss with this herbicide was cultivar dependent. Regardless, it is likely that applications of dicamba + 2,4-D at either 3 or 21 DAC will not provide a significant benefit in relation to the risk of injury and yield loss associated with application of this herbicide combination.

Currently, no herbicide labels explicitly list rhizoma peanut as an approved crop for herbicide application. Therefore, it is essential to contact local state regulatory agencies to determine which herbicides are approved for use in rhizoma peanut. Additionally, as new rhizoma peanut cultivars are released, it will be important to consult with extension weed specialists to determine if a particular herbicide is safe for application.

Literature Cited

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11. Watson, V. H., and Burson, B. L. 1985. Bahiagrass, carpetgrass, and dallisgrass. Pages 255-257 in: Forages: The Science of Grassland Agriculture, 4th Ed. M. E. Heath, R. F. Barnes, and D. S. Metcalfe, eds. Iowa State Univ. Press, Ames, IA.

12. Williams, M. J., Hammond, A. C., Kunkle, W. E., and Spreen, T. H. 1991. Stocker performance on continuously grazed mixed grass-rhizoma peanut and bahiagrass pastures. J. Prod. Agric. 4:19-24.