Weed Dynamics, Growth, Yield and Correlation Study as Affected by Weed Control Methods in Soybean [Glycine max (L.) Merril.]

Soybean [Glycine max (L.) Merril.] is one of the important oilseed crops of the leguminaceae family having subfamily papilionaceae and genus Glycine. It has revolutionized the agricultural economy with its immense potential for oil, fuel and numerous industrial products (Patel et al., 2016) and it is also rich and is cheapest source of high quality protein (40-42%), therefore called as “poor man’s meat”. Besides, it is second only to groundnut in terms of oil content among food legumes. It is also rich in other nutrients like calcium and iron. Presently, globally soybean is cultivated in 127.60 million ha with annual production of 364.07 million tons (Bagale, 2021). India ranks fourth in area with 11.40 million hectares accounting for 9.12% of the world area with production of 13.78 million tons (Anonymous 2021). So far as productivity of soybean in Jharkhand is concerned, it is very low i.e. (931 kg/ha (Anonymous 2018) as compared to other states like Himachal Pradesh (1533 kg/ha), Madhya Pradesh (1062 kg/ha), Maharashtra (1012 kg/ha) Rajasthan (1207 kg/ha) and Telengana (1625 kg/ha). Among various constraints in soybean production, weeds are considered as most serious problem. Being a rainy season crop, it is heavily infested with diversified population of weeds. The weeds compete for light, nutrient, moisture and space against the soybean crop, thus affecting the crop yield. The loss of soybean yield due to weed ranges from 40 to 85 percent (Jha et al., 1993). Uncontrolled weeds not only reduce soybean yields through their competition for light, nutrients and moisture, but they can also severely reduce harvest efficiency. Among various methods, the chemical method of weed control is considered to be cheap and effective provided suitable herbicides with judicious application techniques are followed. Success of chemical method of weed control depends on type of weeds present as well as compatibility of herbicides with crops. Several factors need to be considered including weed species, rotational crops and cost before implementing a weed management plan for soybeans. Imazathapyr is generally applied as pre plant incorporation, pre emergence or as post-emergence herbicide. The compound controls weeds by inhibiting the synthesis of branched-chain aliphatic amino acids resulting in disruption in protein synthesis. While quizalafop ethyl acts by inhibiting the enzyme Acetyl Coenzyme Carboxylase (ACCase) in susceptible species (Suzuki et al., 1991). Sodium acifluorfen is a selective post emergence herbicide. The primary target site appears to be protoporphyrinogen oxidase (protox), an enzyme involved with the biosynthesis of chlorophyll that is necessary for plants to carry out photosynthesis. The leaves of susceptible plants become chlorotic and then desiccated. Pendimithalin controls weeds by inhibiting the microtubule formation in cells causing interruption of cell division. Clodinafop-p-propargyl is absorbed by grassy leaves and rapidly move to the growing points of leaves and stems interfering with the production of fatty acids, needed for plant growth and development in susceptible grassy weeds. Considering the above facts, a field experiment was conducted with objectives to find out the effect of weed control methods on growth, yield attributes and yield of soybean.

Field experiment was conducted to study the efficacy of weed control methods on weed dynamics and productivity of soybean at Birsa Agricultural University, Ranchi (Jharkhand) during Kharif season of 2019 and 2020 on sandy loam soil, moderately acidic in nature (pH 5.4), EC (0.17 dSm^-1) having organic carbon (4.2 g/kg), available nitrogen (160 kg/ha), phosphorus (19 kg/ha) and potassium (146 kg/ha). The experiment was laid out in randomized block design with eight treatments replicated thrice. The treatments comprised of  imazathapyr 75 g /ha at 20 DAS (T₁), imazathapyr 35% + imazamox 35% Ready Mix (RM) 75 g/ha at 20 DAS  (T₂), quizalofop-p- ethyl 50 g/ha at 20 DAS (T₃), sodium acifluorfen 16.5% + clodinafop propargyl 8% (RM) 125 g/ha at 20 DAS (T₄), imazathapyr 2% + pendimethalin 30% (RM) 1000 g/ha at 20 DAS (T₅), hand weeding at 20 and 40 DAS (T₆), weed free (three hand weeding at 20, 40 and 60 DAS) (T₇) and  weedy check (T₈). The experiment was sown on 27^th and 25^th June and was harvested on 30^th October and 3^rd November during 2019 and 2020, respectively. The density and dry matter of weeds in each plot were studied at 30 and 60 DAS. To observe the weed density, a rectangular iron frame, the quadrate, measuring 25 × 25 cm was placed randomly at two places inside of each plot. Grassy, broad leaved and sedges weeds within the quadrate were counted, cut from the base of stem separately and sun dried to remove any excess moisture present on the surface of weeds then oven dried at 60oC±5oC. As the sedges weeds were not found at 60 DAS, they were not considered for that period.  After complete oven drying the dry weight was recorded on electronic balance and converted into g/m². Density and dry matter of weeds was subjected to square root transformation i.e. √(X + 0.5) prior to statistical analysis for test of significance. Different weed indices were calculated by using following formula as suggested by Barla and Upasani, (2022).
 
Weed management index (WMI)
   
Where,
YT= Yield of treated plot.
YC= Yield of control (weedy check) plot.
WC= Weed dry weight in control (weedy check) plot.
WT= Weed dry weight in treated plot.
 
Agronomic management index (AMI)
   
Where,
YT = Yield of treated plot.
YC= Yield of control (weedy check) plot.
WC= Weed dry weight in control (weedy check) plot.
WT= Weed dry weight in treated plot.
 
Weed persistence index (WPI)
 
WPI = (WT/WC) × (WPC/WPT)
Where,
WT= Weed dry weight in treated plot.
WC= Weed dry weight in control (weedy check) plot.
WPC= Weed population in control (weedy check) plot.
WPT= Weed dry weight in treated plot.

Weed flora
 
The experimental field was infested with all three categories of weed species in weedy check throughout the crop growth period in soybean crop. Altogether there were eight species of weeds namely Dactyloctenium aegyptium, Echinochloa colona, Eleusin indica and Digitaria sanguinalis among grassy weeds, Stellaria media, Commelina benghalensis and Phyllanthus niuri among broad leaved and Cyperus rotundus among sedges weeds were dominant.
 
Weed density and weed dry weight
 
Post emergence application of quizalofop 50 g /ha ) at  (T₃) recorded 100 per cent control of grassy weeds as well as total weeds density to the extent of  60.12 and 73.54 per cent, compared to weedy check at 30 and 60 DAS respectively (Table 1). It appeared that application of quizalofop-p-ethyl had an edge over other herbicides in controlling grassy weeds effectively. The efficacy of quizalofop-p-ethyl in controlling grassy weeds can be understood with its mode of action as its post emergence application inhibited amino acid synthesis in grassy weeds which ultimately withered them although latter on grassy weeds appeared but they may not be competitive enough to affect crop growth as it crossed the critical period of weed competition. Yadav et al., (2017) also suggested effective control of grassy weeds due to post emergence application of quizalofop-p- ethyl in soybean crop.
 

 
Plant dry matter accumulation
 
Plant dry matter increased as the growth progressed and the maximum dry matter was attained at harvest (Table 2). The trend of mean dry matter accumulation by leaves was 21.66 and 50.09 g/m² at 30 and 60 DAS respectively thereafter the leaves started drying and remained inactive in photosynthesizing. Similarly, the mean dry matter accumulation by stem was 4.73, 72.36, 153.85 and 212.66 g/m2 at 30, 60, 90 and 120 DAS respectively; pod was 230.72, 370.22 and 472.36 at 60, 90 and 120 DAS, while, dry matter accumulation by total plants were 26.39, 353.17, 524.07 and 685.03 g/m2 at 30, 60, 90 and 120 DAS respectively. This indicated that the photosynthates synthesized in leaves were gradually translocated to the stem and then stored in pods as the days progressed towards maturity.
 

         
The dry matter accumulation by different plant parts (Table 2) like leaf, stem and pod remained unaffected by different weed control methods, however, application of imazathapyr + pendimethalin (1000 g/ha) at 20 DAS (T₅) being similar to rest of the treatments except imazathapyr (75 g/ha) at 20 DAS (T₁) recorded significantly higher total plant dry matter to the extent of 92.56, 93.32 per cent compared to weedy check at 90 and 120 DAS, respectively. Weedy check (T₈) recorded the lowest total plant dry matter accumulation at all the stages of growth. This might be due to severe competition of weeds which restricted the crop for proper growth and development, limiting the dry matter accumulation. Similar findings were also confirmed by Singh et al., (2020) who suggested that crop dry matter accumulation was increased appreciably due to the different treatments as compared to the weedy check as all the growth stages of crop. Dry matter recorded at 30th day stage of the crop growth was not affected significantly due to different weed control treatments as at  early stage there was not much competition between crop and weed plants. But plant dry matter accumulation was influenced to a greater extent at 90 and 120 DAS as imazathapyr+ pendimethalin (1000 g/ha) at 20 DAS (T₅) being similar to rest of the treatments recorded 93.12 and 93.32 per cent higher plant dry matter  respectively  compared to that recorded under weedy check (T₈).
 
Yield attributes
 
Soybean yield attributes were greatly reduced in the untreated control (T₈) where full-season weed interference occurred. Application of quizalofop-p-ethyl  (50 g /ha) at 20 DAS remaining similar to rest of the herbicide application recorded 91.67 per cent higher number of pods per plant as compared to that recorded under weedy check. This might be due to the effective growth and development of soybean crops under a reduced weed environment during the critical period of crop growth which might have resulted in a maximum number of pods per plant. Yadav et al., (2017) have also reported higher number of pods per plant owing to application of quizalofop-p-ethyl .
 
Seed yield
 
The season-long weed infestation in the untreated control (T₈) reduced soybean seed yield by 57.27 compared with the weed-free control. Yadav et al., (2017) have also reported 37-54 per cent reduction in soybean yield in weedy plots compared to weed free. Hand weeding at 20 and 40 DAS recorded significantly higher soybean seed yield to the extent of 118.94 per cent compared to weedy check. Among herbicides, application of quizalofop-p-ethyl (50 g/ha) at 20 DAS recorded significantly higher soybean seed yield over rest of the herbicide treatments. The reason for higher soybean yield achieved by application of  quizalofop-p-ethyl may be due to reduced weed density and weed biomass particularly grassy weeds compounded with significantly higher number of pods per plant and also associated with satisfactory other yield attributing parameters.
 
Weed indices
 
Hand weeding performed at 20 and 40 DAS, recorded significantly higher weed control efficiency at 30 and 60 DAS (.,). However, among herbicides, application of imazathapyr+ pendimethalin (1000 g /ha) at 20 DAS (T₅), quizalofop-p-ethyl (50 g/ha) at 20 DAS (T₃) and sodium acifluorfen 16.5% + clodinafop propargyl 8% (RM) 125 g/ha at 20 DAS (T₄) were most effective in recording maximum weed control efficiency at 30 and 60 DAS. Maximum weed management index was recorded under hand weeding (T₆), imazathapyr + pendimethalin (1000 g/ha) at 20 DAS (T₅) and sodium acifluorfen 16.5% + clodinafop propargyl 8% (RM) 125 g/ha at 20 DAS (T₄). The maximum agronomic management index was found under hand weeding (T₆) at 60 DAS. Similar findings were also found by Kumar et al., (2019) who observed that better weed management indices in treatment was due to lowest weed infestation.
 

 
Correlation study
 
The correlation studies of various weed dry matter and periodic plant dry matter with yield of soybean (Table 4) revealed that soybean yield  was highly negatively correlated with weed dry matter more particularly grassy as well as total weed dry matter at 30 and 60 DAS respectively. While, plant dry weight was positively correlated with soybean yield.
 

 
Economics
 
Hand weeding at 20 and 40 DAS recorded 206.76 percent higher net return compared to weedy check (Table 5). However, among herbicides, application of quizalofop-p-ethyl 50 g /ha at 20DAS (T₃) similar to rest of the herbicides recorded maximum net return to the tune of 175.24 per cent compared to weedy check and also registered maximum B:C ratio (2.50). This result is due to the efficacy of herbicides as well as the reduced cost of cultivation in proportion to increased soybean seed yield. Similar findings were also confirmed by Samant and Mishra (2014) and Pratrap et al., (2019).
 

It was concluded that among herbicides, quizalofop-p-ethyl (50 g /ha) at 20 DAS or imazathapyr + imazamox  (75 g a.i /ha) at 20 DAS or imazathapyr (75 g /ha) at 20 DAS can be opted for effective weed control in soybean crop for higher yield and  monetary return.

None.