Burning and 2, 4, 5-T Application on Mortality and Carbohydrate Reserves in Saw-Palmetto

Abstract

On the flatwoods of the southeastern United States control of saw-palmetto (Serenoa repetrs (Bartr.) Small) is an important step in the improvement of native pastures. This study, conducted at the Ona Agricultural Research Center in south Florida, measured fluctuations in total available carbohydrates (TAC) in rhizomes of burned and unburned saw-palmetto which received a June or October application of 8.9 kg/ha (acid equiv.) of 2,4,5-T. Palmetto kill, change in palmetto cover, and grass canopy cover were evaluated. Burning reduced TAC concentration in rhizomes from 48.8% in March to 14.3% in July as compared to a drop from 47.2% to 37.4% for unburned plants. Applying 2,4,5-T caused a further significant decline in TAC concentration. Both burning and 2,4,5T resulted in lower rhizome percent dry matter indicating that treatment stress caused metabolism of carbohydrate which was replaced by water. After 1 year there was higher mortality on palmetto receiving 2,4,5-T in June, but after 2 years there was no difference in mortality between June (48%) and October sprayed (39%) plants. Burning was not found to have a significant effect on mortality of sprayed plants. Burning and 2,4,5-T decreased palmetto cover, and burned plants treated with 2,4,5-T in June had less cover than burned plants treated in October with 2,4,5-T. Burning followed by 2,4,5-T application in June increased grass cover from 29.4% at the beginning of the study to 67.5% at the end. Saw-palmetto (Serenou repens (Bartr. Small)) is one of the most abundant shrubs on the sandy flatwoods from southern South Carolina to southern Mississippi and throughout all of Florida. The primary method of reproduction is by sprouting from partially buried stems or rhizomes which are capable of elongation. However, palmettos flower in late April and produce some fruit in September or October. In Florida palmettos are relatively dormant in the winter and produce 80% of their annual growth during the rainy May to October period (Hilmon 1968). Carbohydrate reserves of saw palmetto are principally starch which is stored in the rhizomes (Hough 1968). These reserves follow seasonal cycles which are influenced by periods of dormancy, frond growth, and flowering. In rhizomes of unburned plants grown in Georgia, starch was highest in winter (37%) and lowest in summer (27%). Florida cattlemen burn range every 3 to 4 years after winter grazing in order to remove old forage and litter, but palmettos regain 80% of their crown coverage the first year after burning (Hilmon 1968). Burning significantly lowered plant reserves, and three repeated summer burnings after a winter burn lowered starch to 12% (Hough 1968). Interest in reducing palmetto cover has increased with demands for forage production. McCaleb et al. (1960) tested 25 chemicals Authors are associate agronomist, Ona Agricultural Research Center. Ona, Florida 33865; and associate plant physiologist and associate statistician, University of Florida, Gainesville 326 I I. This paper is Florida Agriculture Experiment Station Paper No. 3042. Manuscript received April 30, 1981, JOURNAL OF RANGE MANAGEMENT 36(l), January 1983 and reported 2,4,5-T to be among the best for killing saw palmetto. Recent preliminary experimentation (unpublished) conducted at the Ona Agricultural Research Center (ARC) supports 2,4,5-T (2,4,5-trichloro-phenoxyacetic acid) as being the best of 10 compounds. McCaleb et al. (1961) applied herbicides to unburned palmettos in September and October and reported better mortality (75%) with 4.48 kg acid equivalent/ha (AE/ha). Burton and Hughes (1961) evaluated mortality at 20 months after the application of 3 rates of 2,4,5-T and 2 carriers applied at 3 dates to burned and unburned palmettos. They reported that main effects due to burning and rates were not significant, and date of application was significant only on burned palmettos where the best treatment (burn on March 7 and spray August 26 with 2,4,5-T and water) resulted in 76% mortality. Altobellis and Hough (1968) experimented with 7 combinations of burning and spraying of 2 rates of 2,4,5-T. Best control (61%) was obtained with 6.16 kg/ha AE applied in July 1963, burning 7 months later, respraying in July 1964 followed by reburning in July 1965. The success of this treatment was evaluated with reference to the carbohydrate reserve work of Hough (1968), although there was no report of starch analyses in this study. Because burned palmettos have a weaker energy status than unburned plants (Hough 1968) burning may predispose plants to greater mortality (Burton and Hughes 1961, Altobellis and Hough 1968) when sprayed with 2,4,5-T, which is effective in killing palmettos (McCaleb et al. 1960, McCaleb 1961, Grelen 1960, Burton and Hughes 1961, Altobellis and Hough 1968). However, time of application after burn still remains on a trial and error basis. Hough (1968) suggested that based on this carbohydrate reserve work, October application of herbicide would probably result in the best kill of winter burned palmettos. However, since carbohydrates in unburned palmettos are at their lowest level in late June and July (Hough 1968) this would be an appropriate time for herbicide application which would likely result in maximum control of unburned palmettos. The objective of this study was to measure the total available carbohydrate and relate this to mortality of winter burned and unburned palmettos which were sprayed with 2,4,5-T in June and October. Materials and Methods Work was conducted at the University of Florida’s Ona ARC (27’26’N, 8 1’55’W) in south-central Florida. Averageannual rainfall is about 140 cm, 75% of which falls from May to October. Average temperatures from May to October are maximum/minimum 31.6/ 17.8”C while the remaining months average 25.311 I .6” C. Soil on the experimental site was a Eau Gallie fine sand (Arenic haplaquod), which supported a uniform stand of saw-palmettos that had not been burned in the past 10 years... The treatments were: ( 1) burn (March 1, 1977) and no burn; and (2) time of 2,4,5-T (propylene glycol, butyl ether esters) application at 8.9 kg AE/ha applied on June 30,1977, or October 20,1977, in