Published in Agron J 100:808-812 (2008)
DOI: 10.2134/agronj2007.0160
© 2008 American Society of Agronomy
677 S. Segoe Rd., Madison, WI 53711 USA
WEED MANAGEMENT
Influence of Hexazinone on Pensacola Bahiagrass Growth and Crude Protein Content
Brent A. Sellersa,*,
Jason A. Ferrellb and
Greg E. MacDonaldb
a Univ. of Florida, Range Cattle Research and Education Center, 3401 Experiment Station, Ona, FL 33865
b Univ. of Florida, Agronomy Dep., P.O. Box 110500, Gainesville, FL 32611
* Corresponding author (sellersb{at}ufl.edu).
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ABSTRACT
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Bahiagrass (Paspalum notatum Flüggé) is the most widely used introduced forage in Florida, due to its persistence and productivity on low fertility soils. Hexazinone [3-cyclohexyl-6-(dimethylamino)-1-methyl-1,3,5-triazine-2,4(1H,3H)-dione], the only herbicide labeled for small and giant smutgrass [Sporobolus indicus (L.) R. Br. and S. indicus (L.) R. Br. var. pyramidalis (P. Beauv.) Veldkamp] control in pastures, may impact bahiagrass biomass production and crude protein (CP) content. Field experiments were conducted at two locations in 2005 and 2006 to determine the impact of hexazinone on pure stands of ‘Pensacola’ bahiagrass biomass accumulation and CP content. Hexazinone was applied at 0, 0.28, 0.56, 1.12 and 2.24 kg ai ha–1 in July of each year to actively growing bahiagrass. Hexazinone was observed to reduce bahiagrass overall biomass accumulation, but the severity of bahiagrass injury varied depending on year and location. Generally, little or no reduction in biomass accumulation occurred from hexazinone applied at 0.28 or 0.56 kg ha–1. However, hexazinone applied at the recommended use rate of 1.12 kg ha–1 resulted in 9 to 38% less dry matter accumulation relative to the untreated 84 d after treatment (DAT). At Gainesville, hexazinone did not consistently affect CP content of bahiagrass in either year. At the Ona location, CP generally increased with the application of hexazinone in both years. Crude protein was observed to increase by approximately 16 to 21% when hexazinone was applied at 1.12 kg ha–1. These data show that hexazinone application results in reduced bahiagrass production, without reducing crude protein content.
Abbreviations: CP, crude protein • DAT, days after treatment
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NOTES
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All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher.
Received for publication May 14, 2007.
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INTRODUCTION
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THE PRIMARY PASTURE FORAGE in Florida is bahiagrass, which covers approximately 1 million ha (Chambliss and Sollenberger 1991). Bahiagrass is a popular forage in Florida due to its adaptability to extreme variations in environmental conditions and fertility regimes (Ball et al., 2002, p. 26, 210). Since bahiagrass produces adequate forage for 1.2 heifers ha–1 following fertilization with 40 kg N ha–1, many ranchers grow bahiagrass under a low fertility regime (Stewart et al., 2007). Increasing the N fertility rate to 120 kg N ha–1 increases bahiagrass forage yield and typically increases the thickness of the sward (Stewart et al., 2007). Most crops are competitive with weeds when growing under optimal conditions. Considering that many ranchers in Florida use a low fertility regime for grazing bahiagrass, it is possible that bahiagrass growing under such conditions does not compete well with weeds.
Smutgrass species have been problematic bunch-type perennial grass weeds in bahiagrass pastures for many years in Florida. Small smutgrass is common in grazed pastures throughout the southeastern United States and other tropical regions of the world. Giant smutgrass, also a native to tropical regions, is primarily found in central and south Florida, but is quickly becoming the predominant smutgrass species in the state of Florida as it is increasing in prevalence in north Florida. These species have low palatability and forage quality (0.68% N concentration) and are generally avoided by foraging animals (Mears et al., 1996). Smutgrass is also an avid seed producer with reports documenting in excess of 45,000 seeds per plant (Currey et al., 1972). Therefore, the combined effects of selective grazing of desirable forage, avoidance of smutgrass, and high smutgrass seed production lead to an annual increase of smutgrass density. Popovic (1992) suggests that these processes contribute to an overall 10% increase in the smutgrass population in New South Wales and Queensland each year.
Selective control of smutgrass in desirable forage is needed, but has been difficult to obtain. McCaleb et al. (1966) reported that mechanical control practices were largely ineffective on smutgrass. These experiments varied mowing frequency from one to four times over a 4 mo period. It was shown that basal diameter of smutgrass was reduced with mowing, but overall plant population was not reduced (McCaleb et al., 1966). Moreover, not only were existing plants not controlled, it was observed that mowing facilitated smutgrass seed dispersal (McCaleb and Hodges 1971; McCaleb et al., 1966). Considering the lack of efficacy with mechanical control, forage producers have relied on herbicides for smutgrass control. The first herbicide widely used for smutgrass control was dalapon (2,2-Dichloropropionic acid). However, multiple reports have shown that the dalapon application rates required for consistent smutgrass control were highly injurious to Pensacola bahiagrass, the principle desirable forage in Florida. To illustrate, >80% smutgrass control often required dalapon applied at 3.3 kg ha–1, which caused injury to Penascola bahigrass at this application rate ranged between 30% (Brecke 1981) and 50% (Mislevy and Currey 1980). Considering the large openings in the sward caused by smutgrass removal and the slow recovery due to bahiagrass injury, the use of dalapon often resulted in an overall increase of pasture weeds.
Dalapon is no longer registered for use in pastures and hexazinone (3-cyclohexyl-6-(dimethylamino)-1-methyl-1,3,5-triazine-2,4(1H,3H)-dione) has become the standard for smutgrass control. Hexazinone has been repeatedly shown to provide excellent smutgrass control at rates as low at 0.84 kg ha–1 (Brecke 1981; Mislevy et al., 1999, 2002). Penascola bahiagrass response to hexazinone has been reported to show minimal chlorosis at 20 DAT, with full color recovery by 40 DAT, sometimes surpassing the untreated in overall color (Mislevy et al., 1999). This suggests that crude protein concentration may be altered by hexazinone application, but this has not been documented. Numerous herbicides have been shown to growth regulate (reduce biomass production) of Pensacola bahiagrass, often with no change or slight improvement in leaf color (Goatley et al., 1998; Baker et al., 1999). Considering the visual response of bahiagrass, it is likely that hexazinone reduces bahiagrass production and may affect crude protein concentration. Therefore, the objectives of this research were to determine if hexazinone reduces overall biomass accumulation of Penascola bahiagrass and if crude protein is altered.
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MATERIALS AND METHODS
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Field experiments were conducted in Gainesville and Ona, FL in 2005 and 2006 on a fully established, smutgrass-free, stand of Pensacola bahiagrass. The soils at the experimental sites were conducted on an Arredondo fine sand (loamy, siliceous, semiactive, hyperthermic Grossarenic Paleudult) with 2% organic matter and a pH of 5.4 in Gainesville and an Pomona fine sand (sandy, siliceous, hyperthermic Ultic Alaquods) with 1.5% organic matter and a pH of 5.0 in Ona. Plots were 1.6 by 2.3 m at both locations. The design was a randomized complete block with four replications. The experiment was initiated in July in both years and locations, approximately 4 to 6 wk after the experimental area was clipped to a uniform height; approximately 10 cm at Gainesville and 15 cm at Ona. At Gainesville, the experimental area was fertilized with 56 kg N ha–1 in March of both years, whereas, no fertilizer was applied at the Ona location. At the conclusion of the first year, it was realized that only Gainesville had been fertilized. Therefore, it was decided that Ona would not be fertilized in 2006, since N fertilization was not intended to be a factor in this study.
Hexazinone was applied at 0, 0.28, 0.56, 1.1, and 2.2 kg ha–1 with a CO2 pressurized plot sprayer calibrated to deliver 187 L ha–1. Nonionic surfactant (0.25% v/v) was added to each treatment. Bahiagrass height at the time of application was 18 and 15 cm in 2005 and 2006, respectively at Gainesville and 24 and 18 cm in 2005 and 2006, respectively at Ona. The plot area was clipped to a 5 cm stubble height using a self-propelled sickle bar mower 14, 28, 42, 56, 84 d after hexazinone application. Clippings from each plot were gathered into paper bags and dried at 50°C for 48 h and dry weights recorded. For N analysis, samples were ground through a 1-mm sieve and digested using a modification of the aluminum block digestion procedure of Gallaher et al. (1975). Sample weight was 0.25 g, catalyst used was 1.5 g of 9:1 K2SO4:CuSO4, and digestion was conducted for at least 4 h at 375°C using 6 mL of H2SO4 and 2 mL H2O2. Nitrogen in the digestate was determined by semi-automated colorimetry (Hambleton 1977). Crude protein was derived by multiplying N content by 6.25 and was corrected to a 100% dry matter basis.
For statistical analysis, each location and year was considered as fixed variables. Data were analyzed using PROC MIXED in SAS (SAS Institute, 1999) with replication and associated interactions as random variables. Means were separated using the LSMEANS statement at P = 0.05.
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RESULTS AND DISCUSSION
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Cumulative yield
Year x location, year x harvest, and location x harvest interactions were significant (P < 0.001), therefore, data are reported by year and location (Fig. 1A
–1D). Rainfall was different among locations (Table 1
), which is likely the cause of the year x location interaction. Application of hexazinone reduced cumulative yield in all locations with an inverse relationship between application rate and bahiagrass biomass. In 2005 at Gainesville (Fig. 1A), 0.56 kg ha–1 hexazinone did not decrease cumulative yield at any harvest compared to the untreated control. In contrast, 1.12 and 2.24 kg ha–1 hexazinone resulted in at least a 40% reduction in cumulative yield compared to the untreated control at all harvest intervals.
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Fig. 1. Cumulative yield of bahiagrass following hexazinone application at (A) Gainesville in 2005, (B) Ona in 2005, (C) Gainesville in 2006, and (D) Ona in 2006. Means are significantly different where errors bars do not overlap.
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There were no significant differences among treatments at 14 and 28 d after hexazinone treatment in 2005 at Ona (Fig. 1B). At 42, 56, and 84 DAT, 2.24 kg ha–1 hexazinone resulted in at least a 24% reduction in cumulative yield compared to the untreated control. In contrast to Gainesville, 1.12 kg ha–1 hexazinone, the recommended rate for smutgrass control, did not reduce cumulative yield at any harvest interval. Cumulative forage yield at Ona in 2005 was higher than that at Gainesville. This was due to the fact that the experimental area was clipped to a uniform height 2 to 3 wk before that in Gainesville.
There was less separation among treatments at Gainesville in 2006 (Fig. 1C). Similar to 2005, 2.24 kg ha–1 hexazinone decreased cumulative yield compared to all other treatments at all harvest intervals. Applications of 1.12 kg ha–1 resulted in at least a 19% reduction in cumulative yield compared to the untreated control. All other treatments did not differ significantly from the untreated control, except at 56 and 84 DAT with 0.56 kg ha–1 hexazinone.
Results in 2006 at Ona (Fig. 1D) were similar to those in Gainesville in 2005 and 2006. Applying hexazinone at 2.24 kg ha–1 resulted in at least 41% less biomass than the untreated control at all harvest intervals. Reductions in cumulative yield at all harvest intervals following 1.12 kg ha21 hexazinone were not >34%, but cumulative yield was significantly lower than the untreated control at all harvest intervals.
Bahiagrass is a resilient species that tolerates heavy grazing, environmental fluctuations, and low fertility (Ball et al., 2002, p. 26, 210). These are the predominant reasons why bahiagrass is the most widely used forage species in Florida. Since bahiagrass will persist under low fertility, it is often unfertilized for many growing seasons, especially for ranchers who employ low stocking rates and have less forage demand. Although we did not intend on fertilization being a factor in these experiments, it may have affected the results.
The impact of fertilization on the effects of hexazinone did not appear to impact cumulative yield differently among locations as the trends were similar. However, in conjunction with rainfall, it is possible that fertilization decreased the recovery of bahiagrass following hexazinone application. It is also possible that fertilization caused the bahiagrass to grow more actively in Gainesville, making it more susceptible to injury from hexazinone. It is widely accepted that actively growing plants are more susceptible to herbicides than those under stress (Dortenzio and Norris, 1980; Kells et al., 1984).
The effect of rainfall may have affected the recovery of bahiagrass following hexazinone applications of 1.12 kg ha–1. During the month of July in 2005 and 2006, rainfall totaled 100 and 230 mm, respectively, at Gainesville (Table 1). In 2005, 1.12 kg ha–1 of hexazinone resulted in a 72% reduction in cumulative yield, relative to the untreated, at 56 DAT. In contrast, cumulative yield was only 20% lower than the untreated control 56 DAT in 2006. At Ona, rainfall was higher in 2005 than in 2006 (Table 1). Similar to Gainesville, the reduction in cumulative yield 56 d after an application of 1.12 kg ha–1 hexazinone was 31 and 53% lower than the untreated control in 2005 and 2006, respectively, but the difference in 2005 was not significant. Hexazinone is a fairly mobile herbicide and can leach out of the root zone following rainfall (Oliveira et al., 2001). Even with adequate moisture, cumulative yield was always lower compared to the untreated control 84 d after applying the recommended rate of 1.12 kg ha–1 for smutgrass control, with the exception at Ona in 2005.
Crude Protein
There was a significant location x treatment x harvest interaction (P < 0.0001) for CP. Therefore, data for each location and harvest are shown separately (Table 2
). In all years and locations, CP values ranged from 65 to 167 g kg–1, with the lowest values commonly occurring at 14 d after hexazinone treatment. Bahiagrass nutritional value typically declines from 4 and 10 wk after grazing or harvesting (Arthington and Brown, 2005), which is likely the reason for the low CP values at the first harvest. Following the first harvest interval, CP values are well within the range reported previously (Sinclair et al., 2003; Evers et al., 2004; Stewart et al., 2005; Ezenwa et al., 2006; Stewart et al., 2007; Mislevy and Martin, 2007).
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Table 2. Crude protein (CP) of bahiagrass 14, 28, 42, 56, and 84 d following hexazinone application at Gainesville and Ona, FL, in 2005 and 2006. Within each harvest interval, year and location (location), means followed by the same letter are not significantly different at P = 0.05.
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In 2005 at Gainesville, there was no consistent trend in CP response to hexazinone application. For example, at 14 DAT, CP of untreated bahiagrass was at least 14% lower than when treated with hexazinone. At all other harvest intervals, there was no significant difference among treatments.
Hexazinone increased crude protein at Ona in 2005. At 14 and 28 DAT, CP was at least 8% greater with hexazinone compared to the untreated control. When 2.24 kg ha–1 was applied, hexazinone resulted in at least an 18% increase in CP compared to the untreated control at all harvest intervals. At the recommended rate of 1.12 kg ha–1, CP was at least 11% higher compared to the untreated at all harvest intervals.
In 2006 at Gainesville, there were no differences in CP among treatments at any harvest interval, except at 56 DAT. At this harvest interval, CP was at least 12% greater following 1.12 and 2.24 kg ha–1 hexazinone compared to all other treatments. Overall, there was no observable trend in CP content at any other harvest interval.
Crude protein concentration was similar at Ona in 2006 compared to 2005 in that hexazinone increased CP. Hexazinone at 1.12 kg ha–1 resulted in 12 to 21% higher CP than the untreated control at all harvest intervals. Similarly, CP was 9 to 32% higher when bahiagrass was treated with 2.24 kg ha–1 hexazinone compared to the untreated control.
It was expected that decreasing bahiagrass growth would result in an increase in crude protein due to a concentration gradient; slower growing plants would concentrate more crude protein than those that are adequately growing. However, from the cumulative forage data from Gainesville in 2005 and 2006, CP did not increase as a result of decreased forage accumulation. Conversely, at Ona in 2005 and 2006, CP was always higher following at least 0.56 kg ha–1 hexazinone compared to the untreated control. Although cumulative yield was not significantly lower than the untreated control at any harvest interval following 1.12 kg ha–1 hexazinone, CP was almost always higher at Ona in 2005. Crude protein was also higher than the untreated control at Ona in 2006 following an application of 1.12 kg ha–1, which also resulted in lower cumulative yield compared to the untreated control. The difference in CP content between Gainesville and Ona is not easily explained, but could be due to differences in soil type, rainfall, or the fact that the Gainesville experimental sites were fertilized at the beginning of the experiment and the experimental sites at Ona were not fertilized in either year.
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CONCLUSION
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Dry matter accumulation of bahiagrass is impacted by hexazinone, but it appears to be dependent on several factors as the results of this study were different among years and locations. However, the trends in the data for cumulative forage yield were similar among locations and years suggesting that the effect of hexazinone can be dramatically affected by environmental conditions or soil fertility. Therefore, it can be expected that hexazinone will reduce bahiagrass biomass production at the recommended hexazinone rate of 1.12 kg ha21, but recovery may be dependent on several unknown factors. Crude protein content of bahiagrass following hexazinone increased only at the Ona location. It is unknown why this occurred, but it may be related to the soil type at each location, differences in rainfall, or soil fertility levels during the experiment. Recovery of the bahiagrass may be dependent on environmental conditions or soil fertility levels at the time of hexazinone application. Therefore, fertility of the bahiagrass sward before or after hexazinone application should be investigated.
All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher.
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ABSTRACT
INTRODUCTION
