Management of Isoproturon-Resistant Phalaris minor in Wheat by Alternate Herbicides under Tarai Region Conditions

Phalaris minor Retz. is a major weed of wheat crop across many continents. It is highly competitive in nature and mimics the wheat morphology. Consideration is given to the eradication of isoproturon resistance Phalaris minor and management approaches designated to minimize the impact of resistance. For management of isoproturon-resistant Phalaris minor in wheat crop in the tarai region by alternate herbicides, a field experiment was carried out during the winter season of 2016-17 and 2017-18. The broadcasting of isoproturon resistant Phalaris minor seeds was done before sowing of wheat in field. Treatments included (T1) Pendimethalin @ 750 g a.i./ha, (T2) Pendimethalin+Metribuzin @ 750+210 g a.i./ha, (T3) Pendimethalin+Metribuzin fb mesosulfuron+Idosulfuron (RM) @ 750+210 fb 12+2.4 g a.i./ha, (T4) Pendimethalin+Metribuzin fb ‘Clodinafop+Metsulfuron-methyl’(RM) @ 750+210 fb 60+4 g a.i./ha, (T5) Pendimethalin fb Clodinafop propargyl @ 750 fb 60 g a.i./ha, (T6) Pendimethalin fb ‘Clodinafop+Metsulfuronmethyl’(RM) @ 750 fb 60+4 g a.i./ha, (T7) Pendimethalin fb Mesosulfuron+Idosulfuron (RM) @ 750 fb 12+2.4 g a.i./ha, (T8) ‘Clodinafop+Metsulfuron-methyl’(RM) @ 60+4 g a.i./ha, (T9) Mesosulfuron+Idosulfuron (RM) @ 12+2.4 g a.i./ha, (T10) Weedy. There was no phytotoxicity of any of the herbicide treatments on crop during both the years. The tank-mix or sequential application of herbicides would be a better option than their applications alone to manage the serious problem of herbicide-resistant P. minor in wheat.


INTRODUCTION
Undoubtedly the development of isoproturon-resistant Phalaris minor is an epidemic in wheat production mainly. In late 1970s, isoproturon herbicide was recommended for controlling Phalaris minor and continued to work properly for nearly 20 years. The resistance case was first reported by Malik in 1995 [1] and it was the most serious incident of herbicide resistance in the world ever, resulting in entire crop failure from serious weed infestations (2000-3000 plants/m 2 ) [2].
Wheat (Triticum aestivum L.) is leading food grain crop being a staple diet, and contributes about 21% to the daily dietary protein intake of humans [3]. Reduction in wheat production by means of various biotic or abiotic factors may affect global food security adversely. Weeds are the most detrimental pest of wheat crop causing in total 24% losses in wheat grain yield [4] and one of the principal limiting biological factor in global food production [5]. The isoproturon resistant affected area is ranged between 0.8 and 1.0 million ha in north-western India, mostly in the states of Punjab, Haryana, Uttarakhand, and other foothill plains areas which accounts for 3 million ha of the rice-wheat cropping system out of India's 10 million ha in this cropping system and about 35% of wheat production [6][7][8]. The Phalaris minor trouble has further aggravated in the North-western Indian plains owing to the evolution of isoproturon resistance. Infestation of isoproturon resistance population caused more than 65% reduction in wheat grain yield with the recommended dose of isoproturon (1000 g ha-1) application [9].
Chlorophyll is a pigment molecule that plays very important role in photosynthesis and light harvesting. Higher leaf chlorophyll content is the indication of higher photosynthetic efficiency of plants resulting in higher yield. The higher value of chlorophyll content was observed in some herbicide treated plants suggested that the herbicide application does not create negative impact on leaf chlorophyll content and photosynthesis of rice crop [10]. Level of chlorophyll content of wheat leaves subjected to two sulfonylurea herbicides were significantly lower compared to untreated plants [11]. Nitrate reductase catalyzes the reduction of nitrate to nitrite in plants. Nitrate reductase is very sensitive to nitrogen supply levels. It is also known to be a positively regulated by nitrate availability and limiting factor for nitrogen assimilation [12]. NR is also a rate limiting enzyme and regulated by nitrate concentration, light, growth hormones, reduced nitrogen metabolites and phosphorylation [13]. Vegetative productivity of wheat was closely related to nitrogen assimilatory capacity at post-germination and post-tillering stages. The relation between nitrate reductase activity and grain yield was less definite [14]. For managing isoproturon resistant Phalaris minor, several alternate herbicides with different modes and mechanisms of action have been screened [15,16]. Herbicides are applied alone, combinations or sequentially have been regarded as essential tools in the weed management. However information on their bioefficacy as well as their effect on growth and yield of wheat in the state of Uttarakhand has not been evaluated in a systematic manner. Keeping these points in view, the present investigation was undertaken.

MATERIALS AND METHODS
The isoproturon resistance seeds were collected from the Crop Research Centre, GBPUA&T Pantnagar and they were subjected to grow along with wheat crop in Ravi season of 2016-17 and 2017-18 as well. One genotype of wheat (DBW-17) was sown in field with ten treatments and three replications including control. The broadcasting of isoproturon resistant Phalaris minor seeds was done before sowing of wheat in field. Each plot size was 5m*2m. Wheat was sown at 20 cm row spacing. The pre-emergent herbicide pendimethalin and metribuzin was applied immediately after sowing in moist soil, and other post-emergent herbicides were applied at 30 DAS with sprayer. The crop was managed according to the standard agronomic practices. Weed samples from different randomly selected spots for each plot were taken with the help of a quadrate method. The weeds were separated as P. minor and other weeds including grasses, sedges and broadleaf weeds from each sample. The samples were oven dried at 70˚C until constant weights were achieved. Then dried weed samples were weighed and the weight taken was expressed in terms of g m -2 before subjecting to statistical analysis. The weed control efficiency was computed as a percent reduction in total weed biomass under different treatments in comparison to weedy check at different stages. The other physiological and biochemical parameters regarding wheat crop were estimated after spraying of post emergence herbicides. The crop was harvested at full physiological maturity. The grain weight was recorded after threshing and cleaning. Grain yield was recorded from a harvested area of 1 m -2 in each plot. Threshing was accomplished with a plot thresher followed by cleaning. Chlorophyll content was estimated by the procedure of Hiscox and Israelstam [17] and NR-Activity in freshly harvested wheat leaves was estimated by using the method given by the Hageman and Hucklesby [18]. Data were subjected to analysis using the STPR statistical software.

Effect of herbicides on biomass of wheat crop
The mean data on biomass of wheat crop is presented in   chlorophyll a (1.21 mg/g FW), chlorophyll b (0.26 mg/g FW) and total chlorophyll content (1.45 mg/g FW). Chlorophyll b content was comparable in T2 Pendimethalin+Metribuzin, T9 Mesosulfuron+Idosulfuron (RM)* and T10 Weedy plot ( Table 6).

Effect of herbicides on NR-Activity of wheat leaves (µmol of NO 2 g -1 FW)
Effect of herbicides on nitrate reductase activity was estimated at 45 days after sowing. The following Table 7 showing the mean data on nitrate reductase activity of wheat crop. T8 Clodinafop+Metsulfuron-methyl (RM)* treatment had maximum NR-activity at 10 min followed by T9 Mesosulfuron+Idosulfuron (RM)* treatment whereas T2 Pendimethalin+Metribuzin treatment showed maximum NR-activity at 40min. The mean data on nitrate reductase activity was recorded maximum in T10 Weedy plot followed by T3 Pendimethalin+Metribuzin fb**  Mesosulfuron+Idosulfuron (RM)* treatment while minimum activity was found in T2 Pendimethalin+Metribuzin treatment.

DISCUSSION
The herbicides use in combination or sequential application was observed to be most economical by the study under research. The dynamic nature of weeds necessitates continuous redesigning of strategies from time to time for their successful management. Furthermore, research has consistently proved that herbicides provide more effective and economical weed control leading to higher crop yields [19,20].
The degree of reduction in the weed biomass does not necessarily correspond to the degree of reduction in the weed population. When weed population decreases due to the herbicide application, more space and thus resources will be available for the remaining weeds which may result in higher growth of each plant and thus lower reduction in the weed biomass [21]. Baghestani et al. [22] found that meosulfuron-methyl plus iodosulfuron-methyl-sodium (WG) was a good herbicide option for weed control in wheat. Good performance of sulfosulfuron plus metsulfuron-methyl is also supported by the results of Zand et al. [23]. To avoid antagonism of different herbicides, sequential applications are suggested for the effective control of weeds throughout the growing season [24]. Herbicide efficacy can be increased by tank mixing, if compatible [25] or their sequential applications for the effective control of weeds in wheat [26]. Mesosulfuron methyl plus iodosulfuron-methylsodium, metsulfuron-methyl, sulfosulfuron and chlorsulfuron are used for weed control in wheat [27]. Ready mix of clodinafop -propargyl+MSM @ 64 g/ha and clodinafop-propargyl @ 60 g/ha recorded complete control on Phalaris minor population at 60, 90 DAS and at maturity stage [28].
This study suggests that Phalaris minor caused more competitive pressure on wheat than others so that their effective control by this herbicide resulted in high grain yield of wheat. Thus, the application of these herbicides is suggested to have the lowest grain yield loss. The lowest wheat yield was obtained in untreated/ weedy plot whereas herbicide T4 Pendimethalin+Metribuzin fb** Clodinafop+Metsulfuron-methyl (RM)* was the only treatment which resulted in high grain yield that was not significantly  Pendimethalin+Metribuzin fb** Mesosulfuron+Idosulfuron(RM)*, T4 Pendimethalin+Metribuzin fb** Clodinafop+Metsulfuronmethyl (RM)*,T5 Pendimethalin fb** Clodinafoppropargyl, T6 Pendimethalin fb** Clodinafop+Metsulfuron-methyl (RM)*, T7 Pendimethalin fb** Mesosulfuron+Idosulfuron (RM)* and T8 Clodinafop+Metsulfuron-methyl (RM)* were found effective to control this weed and their biomass was gradually decreased after application of herbicides. The sequential application of pendimethalin with post-emergence herbicides can improve weed control when post-emergence herbicides have a slightly poor efficacy [29]. It was found maximum in (T4 Pendimethalin+Metribuzin fb** Clodinafop+Metsulfuron-methyl (RM)* treatment; 1.300 kg/m 2 and 1.483 kg/m 2 in first and second year trial respectively.During present investigation it was found that chlorophyll 'a', chlorophyll 'b' and total chlorophyll content were reduced after application of herbicides with a maximum fluctuation in chlorophyll b. It was also reported earlier that chlorophyll b is more sensitive than chlorophyll a and carotenoids under stress conditions [30]. Since reports of P. minor resistance to some of the herbicides used in the present study such as sulfosulfuron and clodinafop are already there, it will be necessary to continue monitoring the bio-efficacy of these herbicides in the rice-wheat cropping system. Resistance management will always be challenging till herbicides dominate the weed management strategies.

CONCLUSION
As evaluated from the study, Phalaris minor (grass weed) causes more competitive pressure on wheat so that their effective control by these herbicides resulted in high grain yield of wheat. It may be concluded from the trials conducted during 2016-17 and 2017-18 that applications of alternative herbicides were found to be very effective in controlling the isoproturon-resistant P. minor. The ready-mix and sequential application of herbicides was superior to the application of single herbicides. The ready-mix of mesosulfuron+iodosulfuron at 14.4 (12+2.4) and clodinafop+metsulfuron-mehtyl at 64.0 (60+4) g/ha was effective in controlling weeds and producing wheat yield. This study shows that isoproturon-resistant P. minor could be successfully controlled by using herbicides or their combination with different mechanism of action such as PSII, ACCase and ALS inhibitors. There was no adverse effect on wheat crop at recommended dose of the herbicides.