Weed flora
(Shrestha et al. 2006) deduced that long-term shifts in weed species composition result from the interplay of multiple factors, including tillage practices, environmental influences, crop rotation, the presence of summer fallow periods, and the specific weed management approaches employed. (Mishra and Singh 2012) focused on studying the weed seed bank in a rice-wheat cropping system under various tillage and weed management practices, identifying four predominant weed species: Echinochloa colona, Cyperus iria, Avena ludoviciana, and Medicago hispida. The experimental plots exhibited a diverse weed flora primarily consisting of major broad-leaved weeds like Alternanthera triandra and Cyanotis axillaris, sedges such as Cyperus iria, and grasses like Echinochloa colona and Sporobolus diander. Additionally, other identified weeds included Monochoria vaginalis, Caesulia axillaris, Phyllanthus niruri, Spilanthes acmella, Ludwigia octavilis, and Brachiaria ramosa, among others. (Choudhary and Dixit 2018) discovered that in direct-seeded rice, the weed population displayed considerable diversity while maintaining a similar species composition.
Figure 1 described that at 50 days after sowing/transplanting (DAS/T), the highest percentage composition was recorded for Alternanthera triandra (32% in 2019 and 26% in 2020), followed by Echinochloa colona (20%) during both years. At a later stage, specifically at 75 DAS/T, Alternanthera triandra remained prominent, comprising 28% (2019) and 25% (2020) of the total, followed by Echinochloa colona at 16% and 15% in 2019 and 2020, respectively. Cyanotis axillaris consistently exhibited the lowest percentage composition during both observation periods. Over the four-year experimental periods, the relative dominance of weeds followed a sequence in the rainy season, with Echinochloa colona > Cyperus iria > Caesulia axillaris > Alternanthera philoxeroides > Ammannia baccifera (Nandan et al. 2018).
Relative weed density (RWD) (%) at 50 and 75 DAS/T
Throughout the observation periods of 50 and 75 days after sowing/transplanting (DAS/T), the periodic relative weed density (RWD) for various weed species was computed and visualized in Fig. 2.
Consistently, Alternanthera triandra exhibited the highest relative weed density across all tillage practices in both years. Notably, CT (TPR)-ZT-ZT displayed the highest relative weed density for Alternanthera triandra. Grassy weeds exhibited greater presence under ZT, while CT demonstrated a higher proportion of broad-leaved weeds in terms of density and biomass (Matloob et al. 2015).(Demjanová et al. 2009) revealed that among the weed groups, annual broadleaf weeds (17 species) predominantly outpaced perennial weeds (6 species) and annual grassy weeds (4 species) across all soil tillage treatments. A significant benefit of conventional tillage was observed in reducing perennial weed density. The total weed density showed significant variation, with mouldboard primary soil tillage significantly reducing the density to 16.3 plants m− 2 compared to reduced tillage systems ranging from 36.7 to 39.2 plants m− 2. In comparison to mouldboard ploughing, disk ploughing and shallow loosening treatments notably increased weed density by 240% and 225%, respectively. Additionally, conventional tillage notably decreased the density of perennial weeds from 7.5-9.0 plants m− 2 in reduced treatments to 2.6 plants m− 2.
(Nandan et. al., 2018) concluded that in first year of trial, the highest total weed density was recorded in ZTDSR-ZT, substantially exceeding those in CTTPR-CT, UPTPR(Unpuddled TPR)-ZT and ZTTPR-ZT by 43.8%, 56.8%, and 46.7%, respectively unlike in next year in which ZTTPR-ZT registered highest. The treatment with residue notably reduced the total density of narrow-leaved weeds by 7.3% compared to the without residue. Tillage systems exhibited variable influence on the density and biomass of weed species. CT favored broad-leaved weeds like Trianthema portulacastrum, while ZT plots had more growth of grasses, particularly Dactyloctenium aegyptium. These differences between tillage systems were more pronounced at the early stages of weed competition (Matloob et al. 2015). (Singh et al. 2005) noted that C. rotundus and Commelina diffusa Burm. f. were dominant weeds in drill-sown rice under ZT. Although perennial weeds that propagate through underground vegetative parts usually thrive in undisturbed soils, in this study, the density and biomass of sedges were either similar between the two tillage systems or higher in CT than ZT (Matloob et al. 2015). Adoption of ZT might prompt shifts in weed flora toward annual grasses (Tuong et al. 2005). Continuous ZT notably harbored a higher proportion of viable weed seeds in the upper 0–5 cm soil layer than CT (Mishra and Singh 2012). Under ZT, 77% of the seeds were found in the top soil surface (2 cm) or tend to reside within the 2–5 cm soil depth, leading to their early emergence (Chauhan and Johnson 2011), while CT buried 62% of seeds to a depth of 2–5 cm (Chauhan and Johnson 2009). This difference might be due to the prevention of germination of small-seeded weed species, usually occurring under CT systems, indicating an implication of light requirement for germination (Matloob et al. 2015). (Bàrberi and Lo Cascio 2001) highlighted that in the 0–15 cm soil layer, tillage had a greater influence on weed composition than crop rotation. In tilled soil under CT, weed seeds are believed to be uniformly distributed (Ball 1992). (Mishra and Singh 2012) reported that the densities of weeds like E. colona and C. iria were significantly increased in rice under ZT. (Matloob et al. 2015) added that despite a higher increase in density and biomass in CT plots in 2011 compared to 2010, it was similar to that observed for ZT. This increase might be attributed to the buildup of the soil seed bank in 2010. Grasses like D. aegyptium, E. colona, and E. crus-galli were more abundant in ZT plots than CT during 2010. However, in the second year of the study, the emergence of these weeds was similar between the two tillage systems. These inconsistent results, especially during the second year, contradict predictions made in previous studies [(Chauhan 2011); (Chauhan and Johnson 2011); (Chauhan 2012)], which assumed that weed species with smaller photoblastic seeds cannot emerge from deeper soil depths that usually occur following CT operations. The variations in results between these studies may be attributed to several factors. The impact of tillage systems on vertical seed distribution is influenced by the type and frequency of tillage implements used. Soil cultivation can alter the vertical distribution of weed seeds in the soil profile, subsequently impacting seedling establishment due to factors like seed dormancy, longevity, predation, and the potential for a seedling to emerge from a specific depth (Chauhan and Johnson 2010). Differences in the qualitative and quantitative aspects of weed seed banks might also contribute to variations among studies. Irrigation can also modify weed emergence and growth in rice (Bhager et al. 1999). Most studies assessed weed growth within a specific timeframe, such as up to 30 DAS and flowering (Mahajan et al. 2014).
Regarding weed management practices, a similar trend was observed in the weedy check, except for the highest relative density of Echinochloa colona observed in other treatments. Notably, in CT (TPR)-CT-CT, the highest relative weed density of other weeds occurred at 25 DAS/T in both years. Treatments involving ZT + R-ZT + R-ZT and ZT-ZT + R-ZT exhibited the highest relative weed density of Echinochloa colona at 25 DAS/T across both years. At harvest, Sporobolus diander dominated, followed by Alternanthera triandra, in both tillage and weed management practices during both years. Cyanotis axillaris consistently exhibited the lowest relative weed density across all observation periods and treatments throughout both years.