Monochoria vaginalis (Burm. f.) C. Presl, belonging to the family Pontederiaceae, is an aquatic herbaceous plant, native to temperate and tropical Asia. The species often occurs in paddy fields as a noxious weed in East Asia, and in the USA, and causes a significant reduction in rice production. The objective of the present research was the evaluation of the resistance levels of M. vaginalis against three chemical groups of acetolactate synthase (ALS)-inhibitor herbicides and other two different groups of herbicides, and the investigation of the mutations in the ALS gene of the resistant biotype of M. vaginalis. Herbicide dose–response experiments showed that the resistant biotype of M. vaginalis was highly resistant to bensulfuron-methyl, moderately resistant to bispyribac-sodium, had low resistance to penoxsulam and 2,4-D dimethyl ammonium, and was susceptible to sulfentrazone. The nucleotide sequences of the ALS gene of resistant and susceptible biotypes showed 14 base substitutions, which caused two amino acid substitutions: Val-143-Ile and Val-148-Ile. It is the first report of the substitutions of amino acids Val-143-Ile and Val-148-Ile in ALS protein. Those mutations may give different resistance spectra against three ALS-inhibitor herbicides: bensulfuron-methyl, bispyribac-sodium, and penoxsulam. Further research is needed to elucidate the molecular basis of target-site resistance mechanisms such as the transformation of the ALS gene of M. vaginalis. It is also necessary to evaluate herbicide mixtures and/or the rotation of herbicide sites of action to control the resistant biotype of M. vaginalis.

Eleusine indica (L.) Gaertn, commonly known as goosegrass or wiregrass, is a type of grass that is widespread in many parts of the world. The broad-spectrum herbicide glyphosate is most frequently used in Indonesian oil palm plantations to get rid of weeds and other undesirable plants. However, improper rotation of herbicide types by farmers has led to an increased risk of resistant weed emergence. This investigation tries to validate E. indica’s glyphosate resistance, investigate mutations in the EPSPS gene of the resistant biotype, and determine the type of herbicides that can control E. indica glyphosate-resistant biotypes. The whole plant pot test method was used to measure the resistance level, while DNA sequencing using the PCR method was conducted on all samples to identify mutations in the EPSPS gene of the resistant biotype. The study results showed that all biotypes of E. indica were identified as resistant to glyphosate but susceptible to propaquizafop, ametryn, and sulfentrazone herbicides. Several biotypes, such as the North Sumatra biotype, were identified as having multiple resistances to glyphosate, paraquat, and ammonium glufosinate. Thr102Iso and Pro106Ser amino acid substitutions were found in the EPSPS gene of E. indica-resistant biotypes. The findings of this study showed that E. indica was resistant to paraquat and ammonium glufosinate; further research is required to determine the mechanism.

Unexpected rainfall before herbicide absorption by plants can wash away herbicides from plant tissue surfaces, which may reduce the herbicidal efficacy and increase the adverse effects on nontarget organisms and the environment, including water networks. The objective of this study was the evaluation of the effect of chitosan on paraquat efficacy under simulated rainfall conditions. Simulated rainfall within 3 h after paraquat application decreased its herbicidal efficacy. A mixture of paraquat (280 g a.i./ha) and chitosan (0.05% w/v) significantly increased the herbicidal efficacy against Ageratum conyzoides L. (21% increase), Borreria alata Aubl. (15%) and Paspalum conjugatum Bergius (8%) under the rainfall conditions. The chemical structure of chitosan may contribute to the penetration of paraquat into plant tissues. However, a mixture of paraquat and chitosan did not affect the herbicidal efficacy against Imperata cylindrica (L.) Beauv. The morphological characteristics of I. cylindrica may interfere with the enhancement effect of chitosan. Chitosan is a degradable, nontoxic and easily available and low-cost material made from crustacean shells. These results suggest that chitosan may increase paraquat efficacy against some noxious weed species under rainfall conditions, which may reduce the risk of paraquat contamination into the environment. Therefore, the application of herbicides with chitosan may provide the economic and environmental benefits. Chitosan may enhance the efficacy of other herbicides under unexpected rainfall conditions; however, this possibility requires further investigation.

Ludwigia decurrens Walter is a dicotyledonous plant belonging to the family Onagraceae. It is native to Central Eastern USA but has been spreading quickly and has naturalized in aquatic and riparian ecosystems (including rice paddy fields) in many countries; therefore, it is now considered an invasive noxious weed. L. decurrens is highly competitive with rice and causes a significant reduction in rice production. The objective of the present study was to evaluate the efficacy of the herbicide penoxsulam for the control of L. decurrens in rice fields. The seeds of L. decurrens were collected from four villages in Indonesia, and penoxsulam was applied to L. decurrens in seven dosages (0, 2.5, 5, 10, 20, 40, and 80 g a.i. ha⁻¹) 3 weeks after seed sowing. The plant populations from Hegarmanah, Jatisari, and Joho showed complete mortality at the recommended dosage of penoxsulam (10 g a.i. ha⁻¹). However, the plants from Demakan grew, flowered, and produced seeds 56 days after treatment with 40 g a.i. ha⁻¹ of penoxsulam. The resistance index value of the population was 36.06. This is the first report of a penoxsulam-resistant weed from a dicotyledonous plant species and also the first report of a herbicide-resistant population of L. decurrens. The appearance of herbicide-resistant L. decurrens is a serious issue from both an environmental and an economic perspective, especially since protected forest and freshwater ecosystems are located at a short distance from the study area. Further research is needed to evaluate herbicide mixtures and/or the rotation of herbicide action sites. The identification of the penoxsulam-resistance mechanism in L. decurrens is also necessary to develop a herbicide resistance management strategy.

Chinese sprangletop (Leptochloa chinensis (L.) Nees) is recognized as the most disturbing weed in rice fields in Africa, Australia, and Asia due to causing a significant reduction in yields. The habit of most lowland rice farmers in Indonesia is using high doses of herbicides more than once without any rotation, leading to increased weed resistance potential. Therefore, this study aimed to confirm the resistance level of L. chinensis to acetyl-CoA carboxylase (ACCase)-inhibiting herbicides using the whole-plant pot test method. We identified other herbicides that can control the resistant biotype and performed DNA sequencing on a sample to determine mutations present in the biotype′s ACCase gene through polymerase chain reaction. The herbicide dose–response experiment showed that the L. chinensis from Bantimurung Subdistrict, Maros Regency, South Sulawesi, exhibited resistance to ACCase inhibitors (metamifop and cyhalofop-butyl), which is the first case of resistance to ACCase herbicides reported in Indonesia. An acetolactate synthase inhibitor (bispyribac-sodium) and a 1-deoxy-d-xylulose-5-phosphate synthase (DOXP) inhibitor (clomazone) were effective at controlling the resistant biotypes of L. chinensis, so could considered for use in rotation or as an ingredient in mixed herbicides. Single-nucleotide substitution of guanine for thiamine at position 6081 (TGG; susceptible, TGT; Maros) that causes a Trp₂₀₂₇Cys mutation in the target gene contributes to the resistance of the Maros biotype to ACCase inhibitors.