Transplanted rice, Herbicides, weed dry weight, WCE, Residual effects.

Rice (Oryza sativa L.) is the staple food for over 60% of the global population. Rice is indeed a vital staple food crop, especially in tropical regions and particularly in India. The phrase "Rice is Life" highlights its critical role in ensuring food and nutritional security, particularly in Asia, where a large portion of the population depends on it (Basu et al., 2021).

India ranks as the second-largest producer of rice globally, with cultivation spanning 43.8 million hectares, yielding 118.4 million tonnes of rice, and achieving a productivity rate of 2.7 tonnes per hectare (GOI, 2021). This underscores the importance of rice in India's agricultural economy and its contribution to feeding millions of people.

To meet the growing demand, it is estimated that an additional 114 million tons of milled rice will be needed by 2035, representing a 26% increase in production over the next 25 years (Kumar and Ladha 2011). Maintaining food self-sufficiency and addressing future needs will require India to boost its rice productivity by 3% annually, though the expansion of rice-growing areas remains limited (Sathya Priya et al., 2017). Currently, the average yield of rice in India is hindered by several constraints, with weeds being a major biotic challenge to increasing productivity.

Weeds are a significant threat to rice cultivation worldwide, particularly in transplanted rice systems, where about 60% of weeds emerge between one week and one month after transplanting. These weeds compete with rice during the critical tillering stage, reducing panicle formation and subsequently lowering grain yield (Soe Thura, 2010). In transplanted rice, weeds account for a 45-51% yield reduction (Veeraputhiran and Balasubramanian 2013), as they compete with rice for essential resources like moisture, nutrients, light, temperature, and space. If left unchecked, weeds can reduce rice yields by 33-45% (Manhas et al., 2012).

Effective weed control is crucial for maximizing rice production. Currently, herbicides like butachlor, anilofos, oxadiargyl, and pretilachlor are used in transplanted rice to control annual grasses. However, while these herbicides effectively target grasses, they are less effective against annual sedges and broad-leaved weeds. When grasses are well-controlled, sedges and broad-leaved weeds often proliferate, competing with the crop and causing significant yield losses (Singh et al., 2004).

On the other hand, rising labor costs discourage farmers from choosing hand weeding, which is no longer economical in the current scenario (Ahmed et al., 2021). Instead, herbicide-based weed control is regarded as the most cost-effective solution (Saha et al., 2021).The recent trend in weed management emphasizes the use of low-dose, high-efficiency (LDHE) herbicides. These herbicides not only reduce the total volume of chemicals applied but are also easier and more economical to use (Pal and Banerjee 2007). Sulfonylurea herbicides, such as bensulfuron-methyl, metsulfuron-methyl, chlorimuron-ethyl, pyrazosulfuron-ethyl, and bispyribac sodium, are particularly effective at low application rates, making them excellent LDHE options.

Therefore, the current study aims to evaluate different LDHE herbicides as part of an integrated weed management strategy while also focusing on improving yield attributes in transplanted rice.

Experimental Site and Soil Characteristics. The experiment was conducted during the Samba season (Rabi) of 2016-17 at the Anbil Dharmalingam Agricultural College and Research Institute, Tamil Nadu Agricultural University, Tiruchirapalli. During the cropping season, the total rainfall recorded was 161.1 mm, spread over 12 rainy days. The mean maximum and minimum temperatures were 37.8°C and 20.7°C, respectively. The relative humidity was 52.7% in the forenoon and 73.7% in the afternoon. The site experienced 6.3 hours of bright sunshine per day, with an average evaporation rate of 3.3 mm/day and a wind velocity of 3.8 km/hr.

The soil at the experimental site was alkaline with a pH of 8.8, and had a sandy clay loam texture, classified as Typic Udic Haplustalf. It was moderately drained. In terms of nutrient content, the soil was low in available nitrogen (238.3 kg/ha), medium in available phosphorus (17.2 kg/ha), and high in available potassium (303.4 kg/ha).

Experimental design with treatment details. The field experiment was laid out in the completely randomized block design with three replications with ten treatments were T1- pre-emergence application of pyrazosulfuron ethyl at 25 g a.i.ha-1 on 3 DAT fb hand weeding on 40 DAT, T2- pre-emergence application of pyrazosulfuron ethyl at 25 g a.i. ha-1 on 3 DAT fb early postemergence application of bispyribac sodium at 25 g a.i. ha-1 on 8-12 DAT, T3- pre-emergence application of bensulfuron methyl + pretilachlor at 60 + 600 g a.i. ha-1 on 3 DAT fb hand weeding on 40 DAT, T4- pre-emergence application of bensulfuron methyl + pretilachlor at 60 + 600 g a.i. ha-1 on 3 DAT fb early postemergence application of bispyribac sodium at 25 g a.i. ha-1 on 8-12 DAT, T5- pre-emergence application of pyrazosulfuron ethyl at 25 g a.i. ha-1 on 3 DAT fb post-emergence application of 2,4-D sodium salt at 80 g a.i. ha-1 on 25-30 DAT, T6- pre-emergence application of bensulfuron methyl + pretilachlor at 60 + 600 g a.i. ha-1 on 3 DAT fb post-emergence application of 2,4 D sodium salt at 80 g a.i.ha-1 on 25-30 DAT, T7- early postemergence application of bispyribac sodium at 25 g a.i.ha-1 on 8-12 DAT fb post-emergence application of 2,4-D sodium salt at 80 g a.i.ha-1 on 25-30 DAT, T8- early post-emergence application of oxadiargyl at 80 g a.i. ha-1 on 8-12 DAT fb post-emergence application of 2,4-D sodium salt at 80 g a.i. ha-1 on 25-30 DAT, T9- hand weeding twice at 20 fb 40 DAT, T10- unweeded check, and replicated thrice. 

Cultivar Selection and Biometric Observation. The field experiments were carried out during the Samba season (Rabi) 2016-17 using the medium-duration variety TNAU Rice TRY 3. Biometric observations were recorded during the study, including measurements of plant height, weed dry weight, yield, and leaf area index at the time of panicle initiation. Weed control efficiency and weed index were calculated following standard procedures.

Succeeding Green Gram. After the rice crop was harvested, the succeeding crop of green gram (VBN 2) was grown without altering the original layout of the rice experiment. The green gram was dibbled into rice stubbles, utilizing the residual soil moisture. A seed rate of 20 kg/ha was used, with a spacing of 30 × 10 cm. Germination occurred at 10 days after sowing (DAS), and plant height was measured at 30 DAS.

Statistical Analysis. The data collected from the various treatments were statistically analyzed according to the methods proposed by Gomez and Gomez (1984). Data on weed density and weed dry weight were subjected to square root transformation (√x + 0.5) to normalize the distribution. Treatment differences were evaluated at a 5% probability level, with non-significant comparisons indicated as ‘NS’.

During the Samba season (Rabi) 2016-17, the experimental field exhibited a diverse weed population comprising grasses, sedges, and broad-leaved weeds. Grasses were the most dominant, followed by sedges and broad-leaved species. The predominant grasses were Cynodondactylon, Echinochloa colona, and Panicumrepens. Among the sedges, Cyperusdifformis and Cyperus rotundus were common. The prominent broad-leaved weeds observed included Ammannia baccifera, Ipomoea aquatica, and Marsilea quadrifoliata. 

The weed control treatments significantly reduced weed dry weight during the investigation (Table 1). The lowest dry weight of 1.13 g/m² was observed with the pre-emergence application of bensulfuron methyl + pretilachlor at 60 + 600 g a.i. ha⁻¹ on 3 DAT, followed by hand weeding on 40 DAT. This aligns with the findings of Sureshkumar et al. (2016). Similarly, Singh et al. (2012) reported that two hand weeding in transplanted rice greatly reduced weed dry weight, which was also confirmed in this study. The enhanced expression of yield-related traits in rice may be attributed to the reduced frequency and growth of weeds, as shown in studies on weed dry matter in herbicide-treated plots. This finding is consistent with the research of Rani et al. (2021).

In contrast, Parthipan et al. (2013) noted that the highest weed dry matter was recorded in the unweeded check, indicating that without proper weed control, weed dry weight continuously increases, negatively impacting crop growth. This reduction in weed density and dry weight may be attributed to the effective removal of the first and second flushes of weeds, reducing competition for resources. The use of early post-emergence herbicide in combination with one hand weeding also effectively minimized weed biomass.

Higher weed control efficiency (88.1) was recorded with the pre-emergence application of bensulfuron methyl + pretilachlor at 60 + 600 g a.i. ha–1 on 3 DAT, followed by hand weeding on 40 DAT. Rawat et al. (2021) also confirmed the above findings in their research programme.

Growth and Yield of Rice. The highest plant height (102.1 cm) and leaf area index (4.6) were observed in the treatment with pre-emergence application of bensulfuron methyl + pretilachlor at 60 + 600 g a.i. ha⁻¹ on 3 DAT, followed by hand weeding on 40 DAT (Table 1). The effective weed control in this treatment likely promoted better growth by ensuring more availability of growth resources. Weed control treatments had a significant positive impact on yield attributes compared to the unweeded check. This treatment produced a greater number of grains per panicle and panicles per square meter, likely due to reduced competition for nutrients, space, and light, which led to a higher number of productive tillers per square meter.

Table 1: Effect of LDHE on Weed dry weight, Weed control efficiency, Growth and yield attributes of Transplanted rice under sodic soil.

( ) - √ x + 0.5   transformation   fb- followed by

Note : PSE- Pyrazosulfuron ethyl  ; Bensulf- Bensulfuron methyl; Bispyribac-Bispyribac sodium; Pretila-Pretilachlor; PE- Pre emergence ; EPOE- Early Post Emergence ; POE-Post Emergence ; HW-Hand weeding

The pre-emergence herbicide effectively controlled weeds at an early stage, while supplemental hand weeding managed weed growth at later stages, resulting in superior weed control efficiency. These findings corroborate the results of CharanTeja (2015).

The highest grain yield (4987 kg/ha) and straw yield were recorded in the same treatment. The increase in grain yield by 69.45% in herbicide-treated plots compared to the unweeded check was attributed to several factors, including: (i) reduced crop-weed competition, (ii) improved light transmission for photosynthesis (iii) reduced nutrient depletion by weeds, and (iv) enhanced nutrient uptake by the crop. Similar findings were reported by Abdhullah (2011); Uma et al. (2013); Charan Teja (2015); Nivetha et al. (2017), who also observed the highest grain yield with pre-emergence application of bensulfuron methyl + pretilachlor. The lower grain yield was recorded with unweeded check. 

Succeeding Greengram. The growth and establishment of green gram were not significantly impacted by the herbicide application in rice. No significant differences were observed in terms of germination percentage at 10 DAS or plant height at 30 DAS among the treatments. However, the highest germination percentage (81%) was recorded under the two hand weeding treatment (Table 2), consistent with findings from Upendra Rao et al. (2009); Parthipan et al. (2013) for succeeding black gram, as well as Ezhilarasi et al. (2012) for succeeding green gram. Basu et al. (2021) also reported that herbicides used in their research experiments in rice field recorded with no residual effect on succeeding greengram.

Table 2: Influence of herbicide and theirResidual effects on succeeding Green gram.

( ) – Ascn transformation                fb- followed by

Note : PSE- Pyrazosulfuron ethyl  ; Bensulf- Bensulfuron methyl ; Bispyribac-Bispyribac sodium ;  Pretila-Pretilachlor ; PE- Pre emergence ; EPOE- Early Post Emergence  ;  POE-Post Emergence ;  HW-Hand weeding

The future scope lies in understanding the delicate balance between optimizing herbicide use for weed control in rice, maintaining soil health, ensuring the success of subsequent crops like greengram, and minimizing environmental impacts. Research focusing on soil microbial activity, residue dynamics, and long-term sustainability will be key in making these herbicides a viable option in sodic soil management systems.

C. Nivetha, G. Srinivasan, P.M. Shanmugam and R. Mohan  (2024). Effect of Low Dose High Efficacy Herbicides on Growth and Yield of Transplanted Rice under Sodic Soil and their Residual Effects on Succeeding Greengram. Biological Forum – An International Journal, 16(9): 160-164.