6. SUMMARY AND CONCLUSIONS

Transcription

1 6. SUMMARY AND CONCLUSIONS The objectives of this research were first, to conduct greenhouse and field efficacy studies to determine if annual and perennial weed species differ in their tolerance to glufosinate and glyphosate applications. Studies were conducted to investigate the effects of the additives ammonium sulfate and pelargonic acid on glufosinate and glyphosate efficacy and potential for re-growth of these weeds. The second objective was to determine whether applications of glyphosate or glufosinate in combination with ammonium sulfate or pelargonic acid to Roundup-Ready and Liberty-Link soybeans are injurious. Studies to determine whether the effects of ammonium sulfate and pelargonic acid on Roundup-Ready and Liberty-Link soybeans are due to increased absorption, translocation, or metabolism of 14 C-glufosinate or glyphosate were also conducted. The third objective was to use 14 C-glufosinate studies to determine whether absorption, translocation, or metabolism of glufosinate vary between annual and perennial weed species and to determine whether ammonium sulfate or pelargonic acid affect glufosinate absorption, translocation, or metabolism. The fourth objective was to determine if temperature affects the sensitivity of Liberty-Link and Roundup-Ready soybeans to applications of glufosinate or glyphosate and to determine the basis of this temperature dependent sensitivity by using 14 C-glyphosate and glufosinate to monitor absorption, translocation, and metabolism at varying temperatures. The final objective was to conduct studies to determine whether the herbicide glufosinate inhibits the growth of the soybean bacterial pathogen Pseudomonas syringae in-vitro and on transgenic Liberty-Link soybeans. Addressing the first objective, differences existed between weed species in terms of tolerance to glufosinate. Weeds varied in sensitivity with the perennial weeds common milkweed and horsenettle showing the highest tolerance, followed by sicklepod and common lambsquarters, with giant foxtail showing the most sensitivity. These data suggest the need for higher rates of glufosinate for perennial weeds, while low use rates would adequately control small annual grasses such as giant foxtail. The effect of the additives AMS and PA in combinations with glufosinate and glyphosate also varied by 98

2 species. The application of AMS with glufosinate showed a synergistic interaction in common milkweed and horsenettle, an antagonistic interaction in sicklepod and common lambsquarters, and had no effect in giant foxtail. AMS plus glyphosate treatments showed a synergistic interaction in giant foxtail, but had no effect on fresh weight reduction in any of the other weeds. Treatments of pelargonic acid plus glufosinate showed a synergistic interaction with giant foxtail and common milkweed, an antagonistic interaction with common lambsquarters, and no interaction with horsenettle and sicklepod. PA had a synergistic interaction with glyphosate in common lambsquarters, sicklepod, and giant foxtail, an antagonistic interaction with common milkweed, and no interaction on horsenettle. Field studies showed some early increases in efficacy with the use of AMS or PA on weeds with low herbicide rates; however, these effects had no effect on long-term weed control. Differences in the potential for weeds to regrow following glufosinate or glyphosate treatments exist. Glyphosate treatments showed significantly less regrowth than glufosinate treatments in both common milkweed and horsenettle. The addition of the additives AMS and PA to glufosinate or glyphosate also showed some differences in the amount of re-growth versus the herbicides alone. Differences were detected only at the 0.5 kg/ha glyphosate or glufosinate rates suggesting that the effects of the synergist are overcome by an increasing herbicide rate. The addition of AMS significantly reduced the amount of regrowth of glufosinate and glyphosate treated common milkweed at 0.5 kg/ha, versus the herbicide alone. PA plus glufosinate treatments showed no significant differences from glufosinate treatments alone, in either common milkweed or horsenettle. However, at 0.5 kg/ha glyphosate, the addition of PA gave approximately a ten-fold decrease in the amount of re-growth versus glyphosate alone. PA had the opposite effect in horsenettle. At 0.5 kg/ha glyphosate, regrowth of horsenettle was equal to the untreated control indicating possible antagonism of glyphosate with the addition of PA. These results indicate that the addition of AMS to moderate rates of glufosinate or glyphosate could reduce the regrowth potential of common milkweed. PA combinations proved only minimally successful in reducing re-growth and even antagonistic in the case of glyphosate on horsenettle. 99

3 Combinations of glufosinate or glyphosate with PA or AMS showed some promise in enhanced weed control in greenhouse and field studies, therefore, the safety of these treatments was tested on Liberty-Link and Roundup-Ready soybeans for use in these cropping systems. Greenhouse and field studies indicate that treatments with the addition of AMS to glufosinate or glyphosate show injury measurements not significantly different from treatments of the herbicide alone up to 1 or 2 kg/ha rates on transgenic soybeans. PA, which has some herbicidal ability, caused considerable damage to both Liberty-Link and Roundup-Ready soybeans in both greenhouse and field studies. In greenhouse studies, up to a 40% loss in fresh weight was measured with PA plus 1 kg/ha glufosinate or PA plus 2 kg/ha glyphosate. In treatments containing 3% (v/v) PA plus differing herbicide rates, it appears that an increasing rate of either glyphosate or glufosinate increases the phytotoxicity of the treatment to soybeans. However, in field studies, the injury to transgenic soybeans was outgrown over time and caused no significant reduction in yield. Therefore, it appears that the use of these combinations on transgenic soybeans would be safe. Absorption, translocation, and metabolism studies of 14 C-glufosinate and 14 C- glyphosate in the presence of AMS or PA were conducted to attribute the higher injury in PA treatments in transgenic soybeans to either differences in absorption, translocation, or metabolism, or to the herbicidal activity of PA. Absorption of 14 C-glufosinate by Liberty-Link soybeans was significantly higher in the presence of AMS than in glufosinate alone at 12 and 24 hours after treatment. PA treatments showed 14 C- glufosinate absorption similar to that of glufosinate alone at each time period. Differences in translocation were only present at 12 hours after treatment. 14 C-glufosinate remaining in the treated leaf was significantly higher in the presence of AMS than glufosinate alone. The amount of 14 C-glufosinate translocated to the shoots and leaves above the treated leaf was significantly lower in the presence of PA than glufosinate alone at 12 hours. Metabolism of 14 C-glufosinate by Liberty-Link soybeans did not differ over time or by synergist treatments. Roundup-Ready soybean absorption of 14 C-glyphosate alone and in the presence of AMS or PA increased very rapidly in the first 24 hours after treatment and more slowly after 24 hours. Both AMS and PA decreased the absorption of 14 C- 100

4 glyphosate. Translocation of 14 C-glyphosate by Roundup-Ready soybeans out of the treated leaf and to the above and below treated leaf portions and roots, was the same in treatments with AMS or PA as those with glyphosate alone over all time periods investigated. These data suggest that the phytotoxicity observed in glufosinate and glyphosate treatments in combinations with PA on Liberty-Link and Roundup-Ready soybeans is not due to increased absorption, translocation, or metabolism of glufosinate or glyphosate by these soybeans. It is more likely that the increasing phytotoxicity as glufosinate or glyphosate rate increases to transgenic soybeans, could be due to an increasing concentration of adjuvants in the spray solution as the glufosinate or glyphosate rate increases. These adjuvants could act to increase the phytotoxicity of PA over those treatments with PA applied alone. Studies were then conducted using 14 C-glufosinate to determine if differences in weed tolerance to glufosinate, and in effects of AMS or PA in combination with glufosinate were due to differential absorption, translocation, and metabolism of 14 C- glufosinate. Horsenettle, a more glufosinate tolerant weed from greenhouse studies, showed the highest absorption of glufosinate, as well as the highest amount of translocation from the treated leaf. Another glufosinate tolerant species showed low absorption of glufosinate, where a sensitive species, giant foxtail, showed high absorption as well as high translocation to roots. Only one species, common lambsquarters, showed any metabolism of glufosinate. About 25-30% of absorbed glufosinate was metabolized to methyl-phosphinico propanoic acid in common lambsquarters. Common lambsquarters, especially at a larger size, is difficult to control in fields using glufosinate. Perhaps this is partially due to its ability to metabolize glufosinate. Ammonium sulfate and pelargonic acid can affect absorption and translocation of 14 C-glufosinate in a very weed specific way, with some synergistic and some antagonistic effects, dependent on species. In most cases, the effect of AMS or PA found in greenhouse studies was reflected in absorption and translocation studies by either greater or lesser absorption and movement. In general, from absorption, translocation, and metabolism and greenhouse 101

5 studies, it appears that the use of AMS or PA in combination with glufosinate will give a greater benefit in perennial weeds than in annuals. Although the use of AMS or PA has shown some promise for increasing weed control, the small initial benefits received may be outweighed by the extra cost, especially in the case of PA. It may be more efficacious to apply higher rates of the herbicide to increase control. However, the initial fast burn down caused by PA may make it an appealing tank mix with slower acting herbicides. AMS is a relatively cheap and common herbicide additive and may be more affordable in combinations with glufosinate or glyphosate. Addressing the fourth objective, chlorophyll measurements taken from trifoliolates of transgenic soybeans revealed a rate-dependant loss of chlorophyll in glufosinate-treated Liberty-Link soybeans which was greater at 15 C rather than 25 or 35 C. Conversely, the rate of chlorophyll loss in the terminal trifoliolate of glyphosatetreated Roundup-Ready soybeans was greater at 35 than at 15 or 25 C. The rates of absorption, translocation, and metabolism of 14 C-glufosinate in Liberty-Link soybeans at 3, 12, 24, and 48 hours after treatment, and 14 C-glyphosate in Roundup-Ready soybeans, at 1, 3, 5, 7 days after treatment, were then examined under different temperatures in order to explain the observed injury from these herbicides at 15 C and 35 C respectively. Absorption of 14 C-glufosinate was significantly greater at 25 C than 15 C at 12, 24, and 48 hours after treatment with only slight differences in translocation. 14 C-glufosinate metabolism by Liberty-Link soybeans to the 14 C-acetylglufosinate metabolite was significantly less at 3 hours in 15 C than 25 C. After 12 hours, however, metabolism was not different at the two temperatures. Roundup- Ready soybean 14 C-glyphosate absorption was not significantly different at 15 and 35 C. 35 C soybeans translocated more 14 C-glyphosate to shoots and leaves above the treated leaf, while those kept in 15 C translocated more to the shoots and leaves below the treated leaf. Since the leaves above this treated leaf were the ones showing chlorosis, and this is where the majority of translocated glyphosate is accumulating, it is possible that the high concentration of glyphosate at 35 C in the trifoliolates above the treated 102

6 leaf is causing the injury, however since the EPSPS enzyme is resistant to glyphosate, the reason for this injury remains unclear. It could be due to lower expression of the altered EPSPS enzyme in developing tissues, or at higher temperatures. It could also be due to the ability of glyphosate to chelate metals thus inhibiting chlorophyll biosynthesis. These results suggest that injury to Liberty-Link soybeans at 15 C is due to reduced or slowed 14 C-glufosinate metabolism, and in Roundup-Ready soybeans at 35 C, to increased translocation of 14 C-glyphosate to leaves above the treated leaves. While the injury to these soybeans was great at 35 C in Roundup-Ready soybeans, it was only present at high rates of glyphosate. Normal field use rates should not cause this level of injury. The injury seen at 15 C in Liberty-Link soybeans was only present on the treated trifoliolate with all subsequent trifoliolates developing normally. Therefore, these soybeans are highly resistant to applications of glufosinate or glyphosate, but temperature has the possibility to alter the level of resistance either by a decrease in metabolism rate in the case of Liberty-Link soybeans, or increased herbicide translocation to developing tissues as in Roundup-Ready soybeans. Studies investigating the effect of glufosinate for inhibition of Pseudomonas syringae pathovar glycinea showed some promise for the use of glufosinate as both a bactericide as well as a herbicide. In-vitro studies showed growth inhibition of P. syringae at 1 mm glufosinate and higher. Studies on a whole plant basis showed that a rate of 0.5 kg/ha glufosinate inhibited the number of live P. syringae by 45% and 1 kg/ha glufosinate by 60%. These results indicate that glufosinate could potentially control both soybean pathogens and weeds in Liberty-Link soybeans. In conclusion, the use of chemical additives, temperature, and pathogens all must be considered in cropping systems using glufosinate and glyphosate in Liberty-Link and Roundup-Ready soybeans. Chemical additives can effect both weed control as well as crop safety. Temperature can alter the level of resistance of a transgenic herbicide resistant crop to the herbicide, and pathogens which formerly could only be controlled by a bactericide or fungicide, can in some cases be controlled by a herbicide. With the use of transgenic herbicide resistant crops increasing in acreage every year, 103

7 research investigating the performance of these crops under a variety of conditions will be invaluable to producers. 104