Bio-Efficacy of Pre-and Post-Emergence Herbicides on Growth, Productivity, Nodulation of Black Gram (Vigna mungo L.) under Coastal Plain of Odisha

Black gram or Urdbean (Vigna mungo L.) is a vital pulse crop in tropical and subtropical regions, promoting soil fertility through symbiotic nitrogen fixation and showing resilience to harsh weather cultivated year-round globally. Commercially and nutritionally, black gram, is a very important source of protein (12-42%) and is consumed as “dal” (Girish et al., 2012). Black gram is gaining significance in North India as it plays a crucial role in meeting the escalating food and protein demands of the expanding population (Singh and Pandey, 2017). In India, the annual production is 22.29 lakh tonnes from 41.42 lakh hectares, averaging 538 kg per hectare in 2020-21. India leads in rabi black gram production, mainly in Andhra Pradesh and Telangana, covering 8.33 lakh hectares (DES, GoI, Min. of Agri. and FW, 2020-21). The low productivity of black gram in the country can be attributed to various factors, including cultivation on marginal lands and insufficient or imbalanced fertilization and crop-weed competition. Weeds compete with plants for resources during cultivation, producing an 55.4% reduction in urd bean yield (Ghosh et al., 2023). The control of weeds during the critical period of crop-weed competition is crucial to avoid yield loss. The first 4-5 weeks are critical for black gram crop-weed competition (Patel et al., 2015).
       
However, challenges such as the unavailability of labor during critical periods and wet field conditions can hinder timely management of weeds. In rice-fallow black gram systems, farmers often avoid traditional weed management techniques like hand weeding due to concerns about crop trampling and the difficulty of removing dense rice crop stubble. Consequently, selective herbicides emerge as a practical and cost-effective solution for weed control in this system.
       
In this study, we compared the effects of different herbicides with weed-free and weedy check treatments to evaluate the reduction in weed dry weight and achieve high yields of black gram. The use of pre-emergence herbicides is a proven technique for weed management in black gram in the rainy season (Reddy et al., 2023). Pre-emergence treatments with pendimethalin inhibit plant cell division and elongation in sensitive species and stop weeds from appearing especially during the crop’s critical growth stage (Appleby and Valverde, 1988; Gilliam et al., 1993). Quizalofop ethyl 5% EC, a selective post-emergence herbicide controls annual grassy weeds effectively in black gram (Mundra and Maliwal, 2012). In legume crops, Quizalofop ethyl 5% EC as POST resulted in effective weed control and higher grain yield of lentil (Singh et al., 2018), groundnut (Tokasi et al., 2023) and horse gram (Naik et al., 2022). Herbicides alone cannot achieve total weed control due to their selectivity. However, their effectiveness ought to be enhanced by supplementing them with hand 2 weeding or hoeing (Komal et al., 2015).
       
Keeping in view the importance of weeds in black gram and the higher cost involved in weed management, the present study was undertaken to formulate suitable weed management practices for improving the growth and yield of black gram.

Site description
 
The present field experiment was conducted at the Agronomy Main Research Farm of the Orissa University of Agriculture and Technology, Bhubaneswar (20°15'N latitude and 85º52'E longitude with an elevation of 25.9 m above the mean sea level (MSL) and a distance of 64 km away from the Bay of Bengal), India. The study site falls under the East and South Eastern Coastal Plain Agro-climatic Zone of Odisha from November to February with a mean maximum temperature of 25-30°C and a mean minimum temperature of 15-20°C. There was no rainfall during the cropping season with 0 rainy days in both the years. The soil at the experimental field belongs to the order Inceptisol (Typic Ustochrept) with a sandy-loam texture. Chemical analysis of the upper 15 cm soil showed low organic carbon (3.39%), low nitrogen (186 kg ha^-1), medium P (P₂O₅: 26.9 kg ha^-1), K (K₂O: 123.6 kg ha^-1) and slightly acidic soil reaction (pH 6.44, 1:2.5 soil: water).
 
Experimental details and crop management
 
The experiment was conducted in a randomized block design with three replications consecutively in the rabi season (November to February) for 2021-2022 and 2022-23. The field experiment comprised 8 treatments. The details of the treatment adopted in the experiment are presented in Table 1. The dimension of each experimental plot was 6 m × 4 m (24 m²). Black gram was grown under conventional tillage after cultivator ploughing followed by two passes with a tractor-drawn rotavator to break up the clods. To achieve the proper leveling and pulverization, laddering proved efficient. Black gram cultivar OBG-33 (Shashi) was sown on 23^rd November for 2021 and 26^th November for 2022. The crop was sown with a seed rate of 25 kg ha^-1 with a row geometry of 30 × 10 cm. Fertilizers were applied as basal with 20 kg N as Urea and 40 kg P₂O₅ ha^-1 as DAP. The crop had two irrigations: the first at 21 DAS and the second during the blossoming stage. As plant protection chemical, chlorpyriphos (20 EC) solution was sprayed at the flowering stage to control pod borer infestation in the black gram in both the seasons. The crop was harvested on 13^th February 2022 and 15^th February 2023 respectively for both the years.
 
Herbicide description and management
 
All POST herbicides such as fenoxaprop-p-ethyl, cyhalofop-butyl, tembotrione and quizalofop-p-ethyl were applied at 21 days after sowing (DAS). Pendimethalin and oxyflourfenas PRE were applied on the next day of sowing. Required quantities of herbicides for 24 m² of each experimental plot were applied with 500 L ha^-1 water. Details of herbicide rate, formulationand trade name are given in Table 1. Herbicides were applied by a hand-operated knapsack sprayer (200 kPa pressure) of 16-liter capacity with a flat fan nozzle. In weed-free check treatment, three hand weedings at 15 DAS, 30 DAS and 45 DAS were performed to create the weed-free condition. While in the unweeded control treatment, no-weeding operation was undertaken and weeds were allowed to emerge to assess dominant weed flora and yield loss.
 

 
Crop growth and yield estimation
 
Nodule number, dry weight of the plant and nodules were estimated at 40 DAS and then after the harvest. Five plants were randomly selected from each plot and nodules were counted. Plants and nodules were sun-dried and kept in an electric oven at 70°C for 72 h for dry weight. Plant dry weight was expressed as g plant 1, while nodule dry weight was expressed as mg plant 1. Black gram was harvested manually from the net area for yield estimation (576 m²). Manual threshing of black gram was performed and seed yield was recorded from dried and cleaned produce after shelling. The seed yield of black gram was expressed as kg ha^-1.
 
Crop phytotoxicity score, density and dry weight of weeds and control index
 
Visual scoring for crop phytotoxic symptoms was undertaken after post herbicides application at 7-day intervals up to 21 days of application of herbicides on a 0-10 scale. In black gram, 0 means no phytotoxicity and 10 stands for complete death of the plantand a score of <3 is considered acceptable (Rao, 2000). Species-wise density and dry weight of weeds were assessed at 35 DAS and harvested from each plot using a quadrate size of 0.25 m² (0.5 m × 0.5 m). Two quadrants were selected randomly in each plot. Weeds collected from a 0.25 m² area were identified, counted species-wise and expressed as No. m^-2. Identified weeds were sun-dried for 3 days and then kept in an electric oven at 70°C. Dry weight was expressed as g m^­2. Weeds were categorized as broad-leaved and narrow-leaved (grasses + sedges). After the weed dry weight, the weed index (Gill and Kumar, 1969) was calculated using the following formula:
   
Here,
Ywf: Yield from weed free plot
Yt: Yield from treated plot.
 
Weed control efficiency was calculated by using the following formula as suggested by Mani et al., (1973).
   
Here,
WCE = Weed control efficiency.
WDc = Dry matter of weed in unweeded control.
WDt = Dry matter of weeds in treated plot.
 
Statistical analysis
 
Data were analyzed for randomized block design (Gomez and Gomez, 1984). Before analysis of variance estimation, all data were subjected to test for homogeneity of error variances. Given that there was a linear relationship between the means and variance, Bartlett (1947) recommended that the data on weed count be transformed by a factor to enable proper analysis of variance (√ x+0.5) for species and category-wise weed density and dry weight because of larger variability. Treatment means were compared using a protected least significant difference test at p≤0.05.

Dominant weed species and category-wise weed density
 
Dominant weed flora under different herbicides comprised 12 species of weeds including 6 grasses, 1 sedge and 5 broadleaf weeds. Among the grassy weeds Echinochloa colonum, Eleusine indica and Cynodon dactylon were the most observed species. Cyperus iria was the dominant sedge. The broadleaf weeds such as Phyllanthus niruri, Cleome viscosa, Ageratum conyzoides, Amaranthus viridis and Heliotropium indcum were more evident during different stages of growth. The lowest grass weed population at 20 DAS was recorded from the weed-free treatment which remains statistically at par with pendimethalin 30 EC 750 g/ha as PE at 2 DAS. However, cyhalofop-butyl 10 EC 75 g/ha invariably reduced the density of grassy weed species compared with the weedy check (Table 3). The category wise distribution (%) at 40 DAS showed the dominance of grassy and broadleaved weeds in the weedy check plot (Fig 1).  At 40 DAS tembotrione 34.4 SC 120 g/ha as PoE at 21 DAS recorded a lower weed population that was at par with pendimethalin 30 EC 750 g/ha as PE at 2 DAS in comparison to other treatments. At 40 DAS lowest broadleaf m^-2 was recorded in weed free plot followed by tembotrione 34.4 SC 120 g/ha as PoE at 21 DAS which was at par with oxyflourfen 23.5 EC 50 g/ha as PE at 2 DAS. The dominance of these weeds was also earlier reported in this region (Nath et al., 2022; Patel et al., 2015 and Mohanty et al., 2020).
 

 
Black gram plant height and seed yield
 
Pendimethalin 30 EC 750 g/ha as PE increased 24%-26% of black gram plant height over the unweeded control across all the crop stages (Table 2). Nodule numbers in cyhalofop-butyl 10 EC 75 g/ha as PoE were significantly higher over tembotrione 34.4 SC as PoE and weedy-check (control). It resulted in 84.92% and 12.47% higher nodule numbers per plant over tembotrione 34.4 SC as PoE and weedy-check (control) respectively (Fig 2).Pendimethalin followed by cyhalofop-butyl has the potential to enhance weed control and black gram yield with effective control of weeds throughout the cropping period resulting in better environment for crop growth with reduced crop-weed competition for various plant growth resources. Similar results were reported by (Sharma and Yadav, 2006; Ghosh et al., (2016) and Kumar et al., 2016).
 

 

       
Unweeded control resulted in seed yield loss of 53.62% in comparison with the pendimethalin 30EC 750 g/ha as PE and 35.16% in comparison with fenoxaprop-p-ethyl 9.3 EC 65 g/ha as PoE (Table 5). The highest black gram seed yield (1035.46 kg ha^-1) was recorded in the weed-free check. Cyhalofop-butyl 10 EC 75 g/ha as PoE had a seed yield advantage of 9.7% over the fenoxaprop-p-ethyl 9.3 EC 65 g/ha as PoE. Among the PoE, tembotrione 34.4 SC as PoE had the lowest seed yield (286.51 kg ha^-1).
 
The highest yield and yield attributes were obtained from weed-free plot which might be due to absence of crop-weed competition for entire crop season resulting in a better micro-climate for higher crop growth, nodule number and seed yield. The results are in conformity with the findings of (Yadav et al., (2015); Painkra et al., (2021) and Mansoori et al., (2015).
                      
Category-wise weed density, dry weight and weed index
 
All herbicides resulted in a significant reduction in weed density and weed dry weightcompared with the weedy check (Table 3 and 4). Lower weed density and higher weed index were observed with the application of pendimenthalin 30 EC 750 g/haas PE and cyhalofop-butyl 10 EC 75 g a.i./ha as PoE (p<0.05) creating a more favorable micro-climate for black gram growth with a reduction in crop-weed competition for light, space and nutrients at critical stages of crop. In herbicidal treatments, pendimethalin 30 EC 750 g/ha minimized total weed density and dry weight more than cyhalofop-butyl due to its effectiveness of controlling both narrow- and broad-leaved weeds during the critical period of crop weed competition through inhibition of shoot and root growth of weeds. Oxyflourfen 23.5 EC 50 g a.i. ha^-1 as PE significantly reduced the total dry weight of weeds (by 61.37%-64.72%) compared with weedy check across stages. In the case of PoE, cyhalofop-butyl 10 EC 75 g a.i. ha^-1 reduced the total dry weight of weeds by 2.11%- 35.41% in comparison with the weedy check plot (p<0.05) at all growth stages. Pendimethalin 30 EC 750 g/ha as PE (66.41% at harvest) and cyhalofop-butyl 10 EC 75 g a.i. ha^-1 (59.31% at harvest) as PoE resulted in a higher (p< 0.05) weed control efficiency compared with remaining herbicides (Table 4). The weed control index followed an order of pendimethalin>clodinafop-butyl>fenoxaprop-p-ethyl>tembotrione>quizalofop-p-ethyl>oxyfluorfen for different herbicides. These findings validate the results (Mansoori et al., 2015; Sashidhar et al., 2020; Kumar and Chinnamuthu, 2014 and Naidu et al., 2012). Among the post-emergence herbicidal treatments, cyhalofop-butyl 10 EC 75 g a.i. ha^-1 could lead to increased weed control and grain yield of blackgram by inhibiting the cell division and the plant enzyme ACCase which prevents the production of essential fatty acids for the construction of cell membranes (Rao, 2008 and Kumar et al., 2014).
 

 

 
Black gram phytotoxicity score
 
The PE and PoE herbicides resulted in variable black gram phytotoxicity at different crop growth stages (Table 5). In general, phytotoxicity increased at 14 days over days 7 and subsequently reduced at 21 days after the application. Herbicides such as oxyfluorfen 23.5 EC 50 g a.i. ha^-1 as PE (2 DAS) and tembotrione 34.4 SC 120 g a.i. ha^-1 as PoE (21 DAS) showed a higher black gram phytotoxicity with a score of 1-9. Fenoxaprop-p-ethyl 9.3 EC 65 g a.i. ha^-1 as PoE (21 DAS), pendimethalin 30 EC 750 g a.i. ha^-1 as PE (2 DAS) and cyhalofop-p-butyl 10 EC 75 g a.i. ha^-1 as PoE (21 DAS) did not show any phytotoxicity. The degree of phytotoxicity experienced varied throughout different stages of crop development and over time. Notably, oxyfluorfen demonstrated more pronounced initial phytotoxic effects; however, these effects diminished gradually in black gram as time progressed.Fenoxaprop-p-ethyl and cyhalofop-p-butyl are recommended for controlling narrow-leaved weeds (Nath et al., 2022). The selectivity of black gram for fenoxaprop-p-ethyl 9.3 EC, quizalofop-p-ethyl 5 EC, chalofop-butyl 10 EC and pendimethalin 30 EC was possible because of the inherent tolerance of the crop against these herbicides.For instance, tembotrione and oxyfluorfen belong to the pigment synthesis inhibitor herbicides and had chlorosis and bleaching effects on plants (Das, 2008). Among the herbicide treatments, tembotrione had the highest black gram phytotoxicity because of over-sensitivity to these herbicides and inhibiting the plant enzyme 4-hydroxyphenylpyruvate dioxygenase (HPPD). This enzyme is necessary to produce several important amino acids and carotenoid pigments. This results in the yellowing and chlorosis of plant tissues, which ultimately kills cells and restricts plant development (Dumas et al., 2017).
 

Pendimethalin 30 EC 750 g ha^-1 as PE at 2 DAS showed selectivity in black gram without any phytotoxicity on the crop. Cyhalofop-butyl 10 EC 75 g ha^-1 at 21 DAS significantly reduced weed dry weight among PoE at all growth stages. It resulted in a lower weed index. It gave a comparable yield with weed-free check without any negative impact on nodulation. Pendimethalin 30 EC 750 g ha^-1 as PE at 2 DAS followed by Cyhalofop-butyl 10 EC 75 g ha^-1as PoE at 21 DAS can be recommended for broad-spectrum weed control (reduced weed density and dry weight), as it provides a lower weed index and higher seed yield with weed-free check in black gram.

All authors declared that there is no conflict of interest.