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Spray application parameters that influence the growth inhibiting effects of trinexapac-ethyl.

Foliar absorbed herbicides provide effective post-emergence control of a broad range of agronomic weeds. Uptake by the shoots speeds the arrival of active ingredient at the target tissue, as compared with root absorbed materials. However, efficacy of foliar absorbed herbicides is dependent on a greater number of application parameters than for herbicides applied to the soil. Examples of such parameters are adjuvants, chemical rainfastness, carrier water quality, chemical photolability, and spray carrier volume.

Trinexapac-ethyl is a foliar absorbed turfgrass growth regulator that can cause growth inhibition, with maximum efficacy at 14 to 21 d after treatment, in numerous turfgrass species (Johnson, 1993, 1994). Trinexapacethyl is in the cyclohexanedione class of herbicide chemistry, similar structurally to both sethoxydim and clethodim, two common graminicides. Spray application parameter effects on trinexapac-ethyl efficacy may therefore follow the same trends that exist for sethoxydim and/or clethodim. The label for trinexapac-ethyl identifies a broad range of effective spray carrier volumes (187-1683 L [ha.sup.-1]), achievement of rainfastness within 1 h after application, and no need for an adjuvant to enhance efficacy. There is little evidence available to either support or discount these stipulations.

Adjuvants may enhance spray droplet coverage on plant leaves and chemical absorption (Wanamarta et al., 1989a; Bridges et al., 1991, 1992). These effects may be due to a number of possible mechanisms, including cuticle solubilization and physical interaction with the herbicide in question (Wanamarta et al., 1989a). Adjuvants are used to increase herbicide efficacy enhance activity of photolabile herbicides, or overcome problems with antagonism from other chemicals, solution salts, or both (Campbell and Penner, 1985; Penner, 1989; Hazen and Krebs, 1992; McInnes et al., 1992). Many adjuvants have been tested for enhancement of either sethoxydim or clethodim with varying results (Wanamarta et al., 1989a; Bridges et al., 1992; Hazen and Krebs, 1992; Jordan et al., 1996). Therefore, the choice of adjuvant can be very critical. Herbicide applications on grasses may show a favorable response to additions of organosilicone surfactants, offering potential for these adjuvants to enhance herbicide efficacy (Field and Bishop, 1988; Roggenbuck et al., 1990; Sun et al., 1996).

Chemical rainfastness is the time required after a chemical application for enough absorption to occur for activity not to be diminished by subsequent rainfall, which could remove the chemical from the leaf surface. Postemergence herbicides with slow rates of foliar absorption are often susceptible to losses of activity via this mechanism. Thus, the influence of adjuvants in enhancing foliar absorption has been a research area of interest. Organosilicone surfactants have the potential for increasing the rainfastness, efficacy of some herbicides, or both (Jansen, 1973). However, the mechanism by which such improvements occur has been disputed. Field and Bishop (1988) documented reduced spray solution surface tension as a result of organosilicone surfactant action. Accelerated absorption would enhance rainfastness. The authors assumed increased absorption was related to herbicide penetration through the stomata. However, this mechanism does not explain variable efficacy responses between organosilicone surfactants that similarly reduce spray solution surface tension. Enhanced cuticular penetration appears a more likely explanation for the activity enhancement (Roggenbuck et al., 1990). Sun et al. (1996) documented rainfastness as soon as 15 min after application of primisulfuron on velvetleaf (Abutilon theophrasti Medikus) with an organosilicone surfactant. The overall potential of these surfactants to increase rainfastness is clear and warrants investigation with any chemical where rainfastness may need to be increased.

Carrier water quality can affect absorption and/or activity of herbicides due to the presence of [Na.sup.+], [Ca.sup.2+], [Mg.sup.2+], and[ F.sup.3+]. Glyphosate activity is reduced in the presence of many soluble cations because it forms a conjugate salt with the inorganic cation (Stahlman and Phillips, 1979; Nalewaja and Matysiak, 1993; Thelen et al., 1995). This antagonism was increased with high spray carrier volumes (Sandberg et al., 1978). Antagonism of clethodim has been seen in the presence of sodium bicarbonate but not with quizalofop (McMullan, 1994). Antagonism between sethoxydim and Na-bentazon [3-(-1-methylethyl)-1H-2,1,3-benzothiadiazin-4(3H)one 2,2-dioxide], through a similar formation of a Nasethoxydim conjugate salt has also been well documented (Jordan and York, 1989; Wanamarta et al., 1989b; Wanamarta et al., 1993; Nalewaja et al., 1994; Thelen et al., 1995).

Antagonisms can be overcome with adjuvants. Addition of ammonium salts effectively restores herbicide activity by replacing the metal cation portion of the inactive conjugate salt with ammonium, forming a new conjugate that has higher solubility and thus is more readily absorbed (Jordan and York, 1989; Wanamarta et al., 1989; Nalewaja and Matysiak, 1993; Wanamarta et al., 1993; Thelen et al., 1995). Other methods such as replacing tank mixed applications of Na-bentazon and sethoxydim with sequential applications have also overcome this antagonistic effect (Rhodes and Coble, 1984). The specific susceptibility of some herbicides, especially the cyclohexanediones, to antagonism with inorganic metal cations provides a basis for research with other products showing similar chemistry.

Chemical photolability, most commonly due to the effects of ultraviolet (UV) light, can reduce the efficacy of some herbicides. Cyclohexanedione grass herbicides are particularly susceptible to photolability (Zorner et al., 1989; Bridges et al., 1992; Hazen and Krebs, 1992; McInnes et al., 1992; McMullan, 1994, 1996; Nalewaja et al., 1994). The same researchers suggest a slower rate of foliar uptake than photodegradation as the explanation for reduced herbicide efficacy, a mechanism that can be counteracted with an adjuvant. [Na.sup.+] in the spray solution inhibits cyclohexanedione foliar absorption and amplified photolability-induced efficacy reductions (McMullan, 1994; Nalewaja et al., 1994). Trinexapacethyl is a cyclohexanedione but is labeled as having rapid foliar absorption and, as such, should be less susceptible to photolability.

The objectives of this research were to test whether spray application parameters that influence cyclohexanedione herbicide efficacy also influence the efficacy of trinexapac-ethyl. These parameters included adjuvants, carrier water quality, rainfastness, chemical photolability, and spray carrier volume. Successfully identifying whether trinexapac-ethyl applications should follow the same guidelines as those for other cyclohexanedione herbicides may provide valuable information necessary in maximizing efficacy of this turfgrass growth regulator.

MATERIALS AND METHODS

Turfgrass studies involving spray application parameters that impact efficacy of cyclohexanedione herbicides were initiated with trinexapac-ethyl and, in some cases, sethoxydim. Studies in the greenhouse were at 25 [+ or -] 2 [degrees] C with supplemental lighting from high-pressure sodium lights providing 1200 [micro]mol photons [m.sup.-2] [s.sup.-1] during 18 h of daylight. All pots were irrigated daily or as needed and received 5 kg nitrogen [ha.sup.-1] in the form of Peters (Grace-Sierra Horticultural Products Co., Milpitas, CA) 20-20-20 fertilizer on a weekly basis. Unless otherwise indicated, treatments were applied with a continuous link-belt sprayer at 170 kPa and 230 L [ha.sup.-1] spray pressure and carrier volume, respectively, with a nozzle height of 30 cm above the canopy. Rates of the 1E formulation of trinexapac-ethyl and the 1.53E formulation of sethoxydim were 0.191 kg [ha.sup.-1] and 0.114 kg [ha.sup.-1], respectively.

Adjuvant Impact on Trinexapac-Ethyl Efficacy

Studies were initiated in September 1995 to evaluate the impact of an activator organosilicone adjuvant on the efficacy of trinexapac-ethyl, applied to four cool-season turfgrass species. Plant material studied was `Georgetown' Kentucky bluegrass, `Mondial' perennial ryegrass, `Putter' creeping bentgrass, and `Triathlon', a blend of three varieties of tall fescue. Plugs of 1-yr-old tall fescue were imported as sod from the Hancock Turfgrass Research Center in East Lansing, MI, to the greenhouse, where they were placed into 946-mL pots containing Baccto potting media (Michigan Peat Co., Houston, TX) and allowed to acclimate over a 2-wk period before being sprayed. Kentucky bluegrass, perennial ryegrass, and creeping bentgrass were established from seed at 74 kg [ha.sup.-1], 293 kg [ha.sup.-1], and 49 kg [ha.sup.-1], respectively, 90 d prior to the studies.

The study was done twice in September 1995 with Kentucky bluegrass, creeping bentgrass, and tall fescue and twice in January 1996 with perennial ryegrass. Treatments for each species included a control and trinexapac-ethyl at 0.048 (tall fescue only), 0.095, 0.191, 0.382 and 0.763 kg [ha.sup.-1] (all species but tall fescue). All treatments were sprayed with or without Sylgard 309 (Dow Corning, Midland, MI) +28% (w/v) UAN adjuvant (5.0 mL [L.sup.-1] and 10.0 mL [L.sup.-1], respectively).

Carrier Water Quality and Chemical Rainfastness

Studies were initiated in the fall of 1995 to evaluate adjuvants for enhancing both carrier water quality and chemical rainfastness, with respect to sethoxydim and trinexapac-ethyl efficacy. Applications were made to 60-d-old Mondial perennial ryegrass, originally seeded into Baccto potting media in 946-mL pots at 293 kg seed [ha.sup.-1]. Pilot studies were conducted to determine rates for sethoxydim and trinexapac-ethyl which would be less than fully effective, such that effects of adjuvants would be detectable.

Carrier water quality studies were sprayed in September and October 1995 for both sethoxydim and trinexapac-ethyl treatments. Three carrier solutions were selected: deionized water, 0.5 mg [L.sup.-1] calcium acetate, and 0.5 mg [L.sup.-1] magnesium acetate. Trinexapac-ethyl and sethoxydim treatments were applied in each carrier solution, with or without ammonium sulfate (AMS) at 5.0 g [L.sup.-1].

For rainfastness studies, trinexapac-ethyl and sethoxydim treatments were applied with each of the following adjuvant combinations: no adjuvant, AMS at 5.0 g [L.sup.-1], Sylgard 309 at 5.0 g [L.sup.-1], and Sylgard 309 plus AMS. A simulated 1.25 cm of rainfall was applied at 303 kPa, 20 cm above the canopy surface in 1.25 min. Chemical treatments were given each of the following four rainfall events: no rainfall event, a rainfall event immediately after chemical treatment, and rainfall events either 15 or 30 min after treatment. The experiment had three replications and was repeated.

Chemical Photolability

A study was initiated in April 1996 to evaluate the potential for high intensity UV light to reduce the efficacy of both sethoxydim and trinexapac-ethyl. The plant material used was 8-mo-old Mondial perennial ryegrass, originally seeded into Baccto potting media at 293 g seed [ha.sup.-1]

Chemical treatments included sethoxydim and trinexapacethyl, with or without Sylgard 309 adjuvant at 5.0 mL [L.sup.-1] UV light exposure for each treatment was for 0, 20, or 40 rain after spray application in a Rayonet photochemical reactor (Southern New England Ultraviolet Co., Middleton, CT) containing 12 high intensity UV bulbs around the interior perimeter. Irradiance in the chamber was 15 W [m.sup.-2].

Spray Carrier Volume

Studies were initiated in September 1996 to determine the effects of adjuvants and spray carrier volume on trinexapacethyl efficacy in the greenhouse and to determine the effects of spray carrier volume and mowing height on trinexapacethyl efficacy in the field. Plant material was 1-yr-old Mondial perennial ryegrass, originally seeded into Baccto potting media at 293 g seed [ha.sup.-1], for the greenhouse study and 3-yr-old `Blacksburg' Kentucky bluegrass, established in a native sandy clay loam soil at the Hancock Turfgrass Research Center in East Lansing, MI.

Greenhouse treatments included four adjuvant combinations: no adjuvant, Sylgard 309 at 5.0 mL [L.sup.-1], AMS at 5.0 g [L.sup.-1], and Sylgard 309 plus AMS; each was applied with or without trinexapac-ethyl. All treatments including trinexapacethyl, adjuvant, or both were sprayed at each of five spray carrier volumes (187 L [ha.sup.-1], 561 L [ha.sup.-1], 935 L [ha.sup.-1], 1309 L [ha.sup.-1], and 1683 L [ha.sup.-1]) which encompassed the range indicated on the trinexapac-ethyl label.

A 7.32- by 14.64-m field plot of Kentucky bluegrass was staked out and split into three equal-sized 4.88- by 7.32-m areas. Each of these areas was mowed at a different cutting height (5, 7.5, and 10 cm). Single treatment plot area was 1.22 by 2.44 m. All plots were irrigated daily or as needed and received 25 kg N [ha.sup.-1] in the form of urea (46-0-0) on a biweekly basis.

The study was initiated in September 1996 in the early morning. Air temperature was 19 [degrees] C and wind speed was negligible. Each mowing height area received three chemical treatments plus an untreated control treatment. Trinexapac-ethyl as the 1E formulation was applied with a backpack sprayer at a rate of 0.287 kg [ha.sup.-1] for each of the three spray carrier volumes (187 L [ha.sup.-1], 561 L [ha.sup.-1], and 1683 L [ha.sup.-1]). Clippings were collected at 7, 14, 21, and 28 d after treatment. Clippings were oven-dried for 48 h and then weighed. The study was a completely randomized design for each mowing height area and all treatments had three replications. This study was not repeated.

The turfgrasses in greenhouse pots were maintained at a 4-cm cutting height (2 cm for creeping bentgrass) before they were sprayed and mowed back to this height when data were collected. Evaluation of trinexapac-ethyl growth inhibition was determined by production of clipping fresh weight. Clipping weights were determined at 7, 14, and 21 d after treatment. The multiple species study had an additional clipping harvest at 28 DAT. Efficacy of sethoxydim treatments was based on visual injury ratings (0-10 scale: 0 = uninjured, 10 = complete burndown) taken at 12, 16, and 20 d after treatment. All greenhouse studies were completely randomized designs, had four replications per treatment (except rainfastness studies, which had three replications), and were repeated. Data reflect combined means from both runs of repeated studies. Statistical analyses were based on factorial analysis of variance and regeression, with significance set at the 5% level. In the carrier volume studies, the percent of control data were transformed to the arcsine for analysis of variance and mean separation.

RESULTS AND DISCUSSION

Adjuvant Impact on Trinexapac-Ethyl Efficacy

The magnitude of growth inhibition by trinexapacethyl differed for the different species in this study. Maximum growth inhibition of trinexapac-ethyl occurred at either two or three weeks after treatment with all of the species, supporting specifications given on the chemical label. Regression analysis indicated that linear models could adequately depict significant differences between effects of different rates of trinexapac-ethyl over time for all species but perennial ryegrass (Table 1). Similar trends were found for perennial ryegrass with quadratic models. However, quadratic models detailing the effects of the Sylgard 309 plus 28% (w/v) UAN adjuvant were only significant over time for perennial ryegrass and creeping bentgrass (Table 2). Neither linear nor polynomial models showed significant interactions between rate of trinexapac-ethyl and the effects of the adjuvant. Factorial analysis of variance mean separation was used to determine whether rate or time interactions occurred.

[TABULAR DATA 1 & 2 NOT REPRODUCIBLE IN ASCII]

All trinexapac-ethyl treatments, with or without Sylgard 309 plus 28% (w/v) UAN, significantly reduced clipping production in Kentucky bluegrass 7 d after treatment (DAT). The adjuvant significantly enhanced the performance of trinexapac-ethyl at 0.095 kg [ha.sup.-1] 7 DAT (Table 3). Trinexapac-ethyl efficacy, at the lower two rates, was enhanced by the adjuvant 14, 21, and 28 DAT. Trinexapac-ethyl at 0.382 kg [ha.sup.-1] and at 0.763 kg [ha.sup.-1] performed equally well for the first two weeks of the study but trinexapac-ethyl at the higher rate caused greater growth inhibition over the last 2 wk. Trinexapacethyl at all rates, with the adjuvant, was still inhibiting growth 28 DAT.

Table 3. Trinexapac-ethyl efficacy on `Georgetown' Kentucky bluegrass: the effect of application rates and Sylgard 309 plus 28% urea ammonium nitrate.
                          Time after application (days)

                          7           14          21          28
Trinexapac-ethyl
rate                -adj. +adj. -adj. +adj. -adj. +adj. -adj. +adj.

kg [ha.sup.-1]      -- Clipping production (% of control) --
0                    100    89 100   107 100   111 100   106
0.095                 76    53  88    60  90    78 101    78
0.191                 60    45  56    31  70    47  75    66
0.382                 49    36  36    23  51    19  70    45
0.763                 31    28  21    15  16    10  37    24
LSD 0.05                18        18        17        18


Trinexapac-ethyl, at all rates, inhibited growth of perennial ryegrass 7 DAT. However, among trinexapacethyl treatments, little significance was observed (Table 4). A notable exception at 7 DAT was the enhancement of growth inhibition caused by the adjuvant at the 0.095 kg [ha.sup.-1] rate. A similar enhancement was observed 14 DAT. High variation between treatment replicates at 28 DAT resulted in a lack of significant growth inhibition in almost all treatments.

Table 4. Trinexapac-ethyl efficacy on `Mondial' perennial ryegrass: the effect of application rates and Sylgard 309 plus 28% urea ammonium nitrate.
                          Time after application (days)
                            7           14            21
Trinexapac-ethyl
rate                - adj. + adj. - adj. + adj. - adj. + adj.
kg [ha.sup.-1]      -- Clipping production (% of control) --
0                   100       89    100     88    100      82
0.095                62       38     51     27     85      57
0.191                46       41     30     20     54      37
0.382                59       38     39     23     55      27
0.763                39       29     19     10     27      17
LSD 0.05                 24              23            29


Trinexapac-ethyl, at all rates, inhibited the growth of creeping bentgrass 7 DAT but no differences between treatments were observed. The adjuvant significantly enhanced trinexapac-ethyl at both 0.095 kg [ha.sup.-1] and at 0.191 kg [ha.sup.-1] 14 DAT (Table 5). A similar enhancement occurred 21 DAT.

Table 5. Trinexapac-ethyl efficacy on `Putter' creeping bentgrass: the effect of application rates and Sylgard 309 plus 28% urea ammonium nitrate.
                        Time after application (days)

                         7            14       21
Trinexapac-ethyl
rate                - adj. + adj. - adj. + adj. - adj. + adj.
kg [ha.sup.-1]      -- Clipping production (% of control) --
0                   100      104    100     95    100     119
0.095                55       38     35     17     60      41
0.191                51       38     31     13     60      22
0.382                34       38     14     12     24      14
0.763                33       32     15      6     12      10
LSD 0.05                 30              15            35


Trinexapac-ethyl at 0.048 kg [ha.sup.-1] was included exclusively with tall fescue and it did not inhibit growth throughout the study. Trinexapac-ethyl did not have a significant overall impact on tall fescue as few treatments successfully inhibited shoot growth (Table 6). The adjuvant enhanced trinexapac-ethyl at 0.095 kg [ha.sup.-1] 14 DAT but other enhancements were not observed.

Table 6. Trinexapac-ethyl efficacy on `Triathlon' tall rescue: the effect of application rates and Sylgard 309 plus 28% urea ammonium nitrate.
                          Time after application (days)

                           7         14          21          28

Trinexapac-ethyl     -adj. +adj. -adj. +adj. -adj. +adj. -adj. +adj.
rate
kg [ha.sup.-1]      -- Clipping production (% of control) --
0                    100   102 100   101 100   118 100   106
0.048                 91    76  93    79 109    93 105   103
0.095                 85    63  82    50 104    74  95    82
0.191                 75    60  65    44  90    68  78    68
0.382                 62    57  39    38  52    52  66    57
LSD 0.05                25        22        34        25


Trinexapac-ethyl at 0.763 kg [ha.sup.-1] was generally the most effective treatment applied. However, trinexapacethyl at this rate was at least twice the recommended rate for all the species and was often seen to cause both visual injury and discoloration unacceptable to turfgrass managers. The Sylgard 309 plus 28% (w/v) UAN adjuvant combination was marginally effective in enhancing,/ the efficacy of trinexapac-ethyl. Growth inhibition in tall fescue was largely unaffected by the addition of the adjuvants. However, growth of all other species was generally more inhibited by trinexapac-ethyl with the adjuvant than without it, especially at lower rates. Significant enhancement of trinexapac-ethyl efficacy at the 0.095 kg [ha.sup.-1] rate with the adjuvant occurred in all species 14 DAT. Positive impact of the adjuvant, as might be expected, occurred most frequently 1 WAT or 2 WAT, when trinexapac-ethyl was most active in the plant. The mechanism for adjuvant enhancement of efficacy was not explored in this study.

Carrier Water Quality and Chemical Rainfastness

Perennial ryegrass was sensitive to the effects of both trinexapac-ethyl and sethoxydim. Seven DAT, the potentially antagonistic impact of calcium and magnesium in the water carrier on trinexapac-ethyl efficacy was not yet evident and AMS did not impact efficacy (Table 5). However, 14 DAT, trinexapac-ethyl applied in calcium and magnesium carrier solutions without AMS had clipping production equal to that for the control. Trinexapac-ethyl applied in calcium and magnesium carrier solutions with AMS, conversely, significantly decreased clipping production. This effect diminished by 21 DAT.

Sethoxydim injury was significantly enhanced by the addition of AMS to both the calcium and magnesium carrier solutions at both 12 and 16 DAT (Table 6). At 20 DAT, the level of injury in all treatments was greater than 60% and no significant differences among treatments were observed.

Although significant differences were observed between treatments for both chemicals, the results may have reflected the low 5.0 g [L.SUP.-1] level of AMS that was used. AMS levels of 10 and 20 g [L.SUP.-1] are commonly used in many herbicide applications. The potential for AMS to offset the antagonistic effects of hard water cations is clear and AMS may have had even a greater positive impact in these studies had the level included been greater.

Clipping production was significantly reduced by all trinexapac-ethyl treatments in rainfastness studies. Rainfastness of trinexapac-ethyl was also significantly increased by the three adjuvants selected for these studies. At 7 DAT, AMS seemed to play an important role in enhancing efficacy as only treatments containing AMS produced significantly fewer clippings than treatments without adjuvants for any of the washoff times (Table 7).

Table 7. Effects of hard water cations and ammonium sulfate (AMS) on trinexapac-ethyl activity in perennial ryegrass.
                           Time after application (days)

                          7        14        21

Treatment             -AMS +AMS -AMS +AMS -AMS +AMS

                      -- Clipping production (% of control) --
Control                100   121  100  141  100  121
Trinexapac-ethyl in
  deionized water       63    66   38   50   12   37
Trinexapac-ethyl in
  5.0 g [L.sup.-]1
  calcium acetate       71    82   55   43   38   38
Trinexapac-ethyl in
  5.0 g [L.sup.-l]
  magnesium acetate     80    69   78   36   53   33
LSD 0.05                   39        47        29


At 14 DAT, all trinexapac-ethyl treatments containing Sylgard 309 and most containing AMS showed enhanced efficacy, as compared with treatments without an adjuvant, at all three washoff times (Table 7). Loss of trinexapac-ethyl activity due to washoff was observed with washes at both 0 and 15 min after application. Compared with their unwashed counterparts, only trinexapac-ethyl with Sylgard 309 plus AMS at the 0-min washoff and trinexapac-ethyl with AMS at the 15-min washoff had no loss of activity due to the washoffs. By 30 min after treatment, washoff had no significant impact on trinexapac-ethyl activity.

At 21 DAT, loss of trinexapac-ethyl activity due to washoff occurred with treatments containing AMS and Sylgard 309 plus AMS at the 0-min washoff time and with treatments containing AMS at the 30-min washoff time (Table 7). Trinexapac-ethyl with Sylgard 309 suffered no loss of activity at any of the washoff times.

All sethoxydim treatments showed significantly greater injury than untreated controls over the duration of the study. None of the adjuvants enhanced sethoxydim activity in the unwashed treatments. However, significant losses of activity were observed with all washoff treatments. Sylgard 309 was the only adjuvant that restored activity lost due to washoff (Table 8).

Table 8. Effects of hard water cations and ammonium sulfate (AMS) on sethoxydim activity in perennial ryegrass.
                           Time after application (days)

                         12        16         20
Treatment             -AMS +AMS -AMS +AMS -AMS +AMS

                        -- % injury([dagger]) --
Control                 0    0    0    0    0    0
Sethoxydim in
  deionized water      41   48   68   79   74   83
Sethoxydim in
  5.0 g [L.sup.-1]
  calcium acetate      34   49   61   77   68   83
Sethoxydim in
  5.0 g [L.sup.-1]
  magnesium acetate    34   42   59   73   64   77
LSD 0.05                  8        11         17


([dagger]) % injury values were converted from an original 0-10 rating scale where 0 = uninjured, 10 = dead.

Overall, trinexapac-ethyl seemed to be more rainfast than was sethoxydim (Tables 9 and 10). The label for trinexapac-ethyl indicates rainfastness within 1 h of application. Results from these studies suggested rainfastness was more rapid than that, especially when an adjuvant was included. AMS, with or without Sylgard 309, significantly increased the rainfastness of trinexapacethyl while it had no impact on the rainfastness of sethoxydim. Sylgard 309 increased the rainfastness of both cyclohexanediones.

[TABULAR DATA 9 & 10 NOT REPRODUCIBLE IN ASCII]

Chemical Photolability

Neither sethoxydim nor trinexapac-ethyl treatments suffered any loss of activity from the effects of UV light exposure. Sethoxydim was expected to lose activity under such conditions while the impact on trinexapacethyl was unknown (Zorner et al., 1989; Hazen and Krebs, 1992; McInnes et al., 1992; Nalewaja et al., 1994; McMullan, 1996). Newer formulations of sethoxydim contain additives inhibitory to UV photodegradation. It does not appear that trinexapac-ethyl, applied as the 1E formulation, is susceptible to loss of activity due to photolability.

Spray Carrier Volume

AMS enhanced trinexapac-ethyl efficacy in the greenhouse at 187 L [ha.sup.-1] but had no effect on efficacy at higher volumes 14 DAT (Fig. 1). The observed enhancement supported results seen in the carrier water quality study. However, the lack of enhancement seen at 561 L [ha.sup.-1] or greater was probably related to the low AMS concentration, 5.0 g [L.SUP.-1]. At high spray volumes, the ratio of ammonium to [Ca.sup.2+] and [Mg.sup.2+] in the hard water may have been insufficient to offset the negative impact of hard water cations on uptake.

[Figure 1 ILLUSTRATION OMITTED]

A significant interaction between mowing height and spray carrier volume was observed 21 DAT. Results were not conclusive at 7, 14, or 28 DAT. Carrier volume significantly impacted trinexapac-ethyl efficacy at the 5- and 10-cm mowing heights (Fig. 2). A denser canopy stimulated by lateral growth and a higher canopy with more leaf tissue may necessitate higher spray carrier volumes for applications to the 5- and 10-cm mowing heights, respectively. Treatments mowed at 7.5 cm, with predominance of neither lateral development nor excess leaf matter, responded equally well to trinexapac-ethyl at all spray carrier volumes (Fig. 2). Because a significant interaction occurred when trinexapac-ethyl exhibited maximum efficacy at 21 DAT, an interaction between cutting height and spray carrier volume may have significant implications for field applied trinexapac-ethyl.

[Figure 2 ILLUSTRATION OMITTED]

It is suggested, based on results from greenhouse studies, that adjuvants may have a beneficial impact on trinexapac-ethyl efficacy, rainfastness, and activity in hard water, especially at lower spray carrier volumes and at lower applied rates. The role of AMS in overcoming hard water problems with trinexapac-ethyl is likely a key factor in explaining increased rainfastness observed with trinexapac-ethyl treatments containing AMS. Uptake of trinexapac-ethyl seemed to be inherently more rapid than absorption of sethoxydim.

ACKNOWLEDGMENTS

The authors thank Dr. Joseph Dipaola and Ciba-Geigy for both technical and financial support behind the completion of this research project. Thanks also are extended to Frank Roggenbuck for his greenhouse expertise and advice on methodology for the research projects in this paper.

Abbreviations: AMS, ammonium sulfate; DAT, days after treatment; E formulation, emulsifiable concentrate formulation; UAN, urea ammonium nitrate; UV, ultraviolet.

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Matthew James Fagerness and Donald Penner(*)

Michigan State Univ., Dep. of Crop and Soil Sciences, East Lasing, MI 48824-1325. Received 17 June 1997. (*) Corresponding author (salemka@pilot.msu.edu).
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Author:Fagerness, Matthew James; Penner, Donald
Publication:Crop Science
Date:Jul 1, 1998
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