The Efficacy of Megafol in Combination with ALS and ACCase Inhibiting Herbicides in Oryza Sativa

 A.J. Anderson, D. Cheetham

Abstract

The addition of foliar fertilizers to herbicide sprays may have an increased benefit towards relieving stressful conditions to rice plants due to the herbicide application.  It may also have an increase in efficacy of the herbicide sprays.  Using multiple herbicides with different modes of action in the same mixture may also provide greater efficacy of herbicide sprays.  All of these combinations may equate to higher yields.  This study looks at the addition of MegaFol, an amino acid based foliar fertilizer, to herbicide mixtures including Clincher, Regiment, Granite SC, and Sandea.  The results of this study are overall inconclusive, since, despite poor weed control, the Sandea mixtures provided adequate yields while the Regiment and Granite SC both provided good yields. 

Introduction

The addition of amino acid based foliar fertilizers, such as MegaFol, may be able to improve the efficacy of herbicides.  The concept of using foliar fertilizer on rice is relatively new, and minimal research has been published.  The addition of foliar fertilizers to herbicides may improve harvest yields because of the reduced pressure on the rice plant from competition with weeds and by helping the rice plant overcome phytotoxic damage due to the application of the herbicides.  MegaFol contains many free amino acids as well as highly mobile and soluble potassium and nitrogen.  MegaFol 4-0-2 is a concentrated liquid foliar fertilizer solution containing highly soluble and mobile nitrogen (N) (4%) and potassium oxide (K₂0) (2%).  It is an amino acid based complex in a spray solution and contains natural plant compounds extracted from a vegetable matrix by enzymatic hydrolysis. Amino acids are important as growth factors because they provide a ready reserve for the biological process.  Regular application of MegaFol assures a balanced development of the plant. It is also very useful to overcome stress conditions (Valagro S.p.A, 2003).

Weeds are a significant problem in rice farming, specifically grasses.  Weeds in rice fields use resources and inputs that would otherwise be available to the rice plants, thereby decreasing overall performance of the rice.  This will result in lower yields per acre and less income for growers.  By using specific herbicides, growers can diminish weed populations. The problem arises in that many weeds have become resistant to herbicides due to their overuse for many years. To combat this, one of the synthesis strategies of chemists in their search for a new herbicide is to use the structure of a known herbicide as a template. Modification of such a structure often results in compounds with desirable properties not shared by the template (Shaner 1991). 

One form of herbicides is acetolactate synthase (ALS) inhibitors, which are used primarily to limit and stop the growth of grasses. These herbicides inhibit the ALS enzyme, which is the first enzyme that is common to the biosynthesis of the branched-chain amino acids isoleucine, valine, and leucine (Umbarger, 1978). Inhibition of ALS leads to the starvation of the plant for these amino acids, and it is this starvation that is thought to be the primary mechanism by which ALS-inhibiting herbicides cause plant death (Tranel and Wright, 2002). ALS-inhibitors act slowly, so the death of the whole plant can take several weeks (Shaner, 1991).  The inhibition of this enzymatic activity is the mechanism of action of four main classes of herbicides, which account for approximately 20% of the herbicide market (Singh, 1999). These are the sulfonylureas (SUs), the imidazolinones (IMIs), the triazolophyimidine sulfonanilides (TPs) and the pyrimidinylthiobenzoates (PTBs) (Shaner, 1999).

Another common form of herbicides used in rice production are acetyl-coenzyme A carboxylase (ACCase) inhibitors.  These herbicides have a similar effect on target plants as ALS inhibitors, but inhibit the ACCase enzyme instead of the ALS enzyme.  This leads to inhibition of acyl lipid biosynthesis (Burton et al., 1987) which leads to plant death (Devine and Shukla, 2000).

In particular, ALS-inhibitors have been plagued by the development of herbicide-resistant weeds (Tranel and Wright, 2002).  Most cases of ALS-inhibitor resistance have resulted from selection of an altered target site (Trucco et al., 2005). This means that the herbicide inhibits a site other than that of the ALS enzyme.  Resistance to bensulfuron-methyl (ALS inhibitor) is widespread among Cyperus difformis (smallflower umbrella sedge) populations (Osuna e. al., 2002).  Echinochloa phyllopogon (watergrass) has evolved resistance to several herbicides, including bispyribac-sodium (ALS inhibitor) (Osuna et al., 2002). The addition of these herbicides strongly enhanced phytotoxicity toward resistant (R) plants, suggesting that metabolic degradation of bispyribac-sodium contributed significantly to the observed resistance (Fischer, 2000).  Other studies have shown that some grass species are naturally tolerant to ALS- and ACCase-inhibiting herbicides.  Vulpia bromoides naturally has an enhanced metabolism for ALS herbicides and an insensitive ACCase enzyme (Yu et al., 2004).  This may help to show how other species of grasses can easily build resistance to these herbicides.   

These herbicides also have detrimental effects on non-target plants as well, in this case being the crop.  When 15 new benzenesulfonylureas were tested in rice, in vitro ALS activities of both rice and barnyard grass were significantly inhibited by the compounds (Hwang, 2000).  Bensulfuron-methyl has also been shown to inhibit growth and reduce ALS activity in soybeans (Nprom et al., 2005).

The purpose of this study is to determine if the addition of MegaFol to ALS and/or ACCase inhibiting herbicides will affect the phytotoxicity to the rice plant, efficacy of weed control, and harvest yield.

Materials and Methods

This project was conducted during the 2006 rice-growing season.  It was conducted near Butte City, CA, which is normal of Northern Sacramento Valley rice growing conditions.  The variety of rice was M202.  The trial site is known to contain ALS-resistant smallflower and arrowhead weeds.  The study used a randomized complete block design under ANOVA.  Fifteen treatments containing herbicides and/or MegaFol and/or Kinetic were applied while a 16^th treatment received no herbicide and served as the control.  Four replications of the 16 treatments were conducted.  All treatments were applied to 10 foot by 20 foot plots.  All treatments were applied at 15 gallons per acre using a CO₂-pressured backpack sprayer.  Treatments were applied when the rice was at the 5-leaf stage.

Four different herbicides were used in this experiment:  Bispyribac-sodium (Regiment, ALS inhibitor), cyhalofop-butyl (Clincher, ACCase inhibitor), penoxsulam (Granite SC, ALS inhibitor), and halosulfuron-methyl (Sandea, ALS inhibitor).  MegaFol and Kinetic were added to some treatments.  Kinetic is a wetter/spreader/penetrant adjuvant.  The treatments include one control where no herbicides were applied.  Cyhalfop-butyl was used in every herbicide treatment.  See Table 1 for treatments.

Table 1: Treatments applied to experimental units.

Phytotoxicity ratings were taken visually in percent of injury to the rice plant at both 9 and 18 days after treatment (DAT).  Phytotoxicity ratings were taken for stunting, stand reduction and chlorosis.  Herbicide efficacy ratings were taken visually in percentage of control of weeds at 18, 30, 42 and 57 DAT.  Weeds studied were Echinochloa spp. (watergrass) (18-57 DAT), Cyperus difformis (smallflower umbrella sedge) (30-57 DAT), Ammannia spp. (redstem) (42 DAT) and Sagittaria montevidensis (California arrowhead) (18 DAT).  Lodging ratings were taken in percentage of lodged rice plants at harvest.  Moisture content was taken at harvest.  Amount of yield ratings were taken by weight at harvest then converted to pounds per acre.

Results

Overall, it does not appear that the addition of MegaFol to ALS- and/or ACCase-inhibiting herbicides affected the efficacy nor did it affect the yield due to inconclusiveness.  However, there were results that had a significant difference at the P=0.05 level, but were inconsistent and did not decrease yields, and therefore inconclusive.  The following results show significant differences with the addition of MegaFol to the ALS and/or ACCase herbicides.  The significant differences in the yield (lbs/ac) were a result of the Sandea mixtures (Figure 1).  The significant differences in the herbicide efficacy were a result of the Sandea mixture’s effect on watergrass at 18 and 42 DAT (Figure 2), the Regiment mixture’s effect on smallflower at 57 DAT (Figure 3) and California arrowhead at 18 DAT (Figure 4),and the Granite mixture’s effect on smallflower at 57 DAT (Figure 5).  Significant differences in the phytotoxicity to rice plants were a result of MegaFol in addition to Kinetic added to the Regiment mixtures (Figure 6 & 7).  Total yields are shown in Figure 8.

Figure 1: Significant Differences in Sandea Mixture’s Yields

Figure 2: Significant Differences at 18 and 42 DAT

Figure 3: Significant Difference at 57 DAT

Figure 4: Significant Difference at18 DAT

Figure 5: Significant Difference at 57 DAT

Figure 6: Significant Differences at 9 and 18 DAT

Figure 7: Significant Differences at 9 and 18 DAT

Figure 8: Mean Yield among All Treatments

 From an herbicide efficacy standpoint, no Sandea mixture provided adequate weed control, other than on redstem.  In addition, all herbicide mixtures provided adequate control of redstem.  Regiment and Granite mixtures had excellent control of early watergrass.  No herbicide mixtures could provide adequate residual control of smallflower.  However, Regiment and Granite mixtures provided good control up to approximately 30 DAT.  Granite mixtures also provided good control of California arrowhead.

Discussion

Even though significant differences were seen, the only one that does this experiment justice is the yields of the Sandea mixtures.  Because the addition of MegaFol to the Sandea increased yield, this may prove that the addition of MegaFol is beneficial.  However, since it seemed to have no effect on the other herbicide mixture’s yields and did not improve the herbicide efficacy, further study is needed.  In addition, even though Sandea mixtures had a significant difference in yield, due to how poor the weed control was by this product, Sandea is not recommended for use in controlling weeds in rice.  The Regiment mixtures provided quality control over watergrass, which is in contrast to Osuna et al.’s 2002 study, which found watergrass to be resistant to Regiment.  The control of the watergrass in this experiment may be due to the addition of Clincher to the mixture.  Even though there was minimal residual smallflower control by all herbicide mixtures, this weed did not decrease yield.  Smallflower does show resistance, which is what Osuna et al. found in their study.  However, the weed was not a problem because it was heavily suppressed from the herbicide application during the 5-leaf stage of the rice plant, which is when the rice plant starts to grow off its own root system, switching from the supplies provided from the seed.  This allowed the rice plant to outcompete the smallflower for nutrients and sunlight.  The addition of MegaFol (at either 1 or 2 pt/acre) and Kinetic to Regiment and Clincher proved to provide a significant amount of phytotoxicity in the forms of stunting and stand reduction.  This is similar to what Hwang and Nprom et al. found in their studies.  However, this did not hinder yield, so no implications can be made.  Another notable point in this study was the combination of Sandea and Clincher.  Despite the lack of efficacy from the Sandea, the Clincher should have provided adequate control of early watergrass.  This may lead to some evidence of antagonism between Sandea and Clincher.

Further studies should include using less than the recommended rates of herbicides.  This will allow the weeds to not be hindered as much, magnifying the effect that the addition of MegaFol may have.  Another suggestion is to separate the herbicides, most notably Clincher to see how well these herbicides act on their own as well as if there is any antagonism.

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