Sequential pre- and post-emergence herbicide premixes for weed management in dry direct-seeded rice in the mid-hills of Himachal Pradesh, India

Introduction

Rice (Oryza sativa L.) is the staple food of more than half of the world's population, and it is commonly grown by transplanting seedlings into puddled soil, a system that is labour-, water-and energy-intensive and is becoming less profitable as these resources become scarce (Kumar et al., 2011) [1] . Direct seeding has therefore attracted growing interest; published comparisons show that it can give yields similar to puddled transplanting with savings in irrigation water, labour and production cost (Kumar et al., 2011) [1] . Weeds are, however, the main biological constraint to DSR. Aerobic rice systems face much higher weed pressure than puddled transplanted rice (Rao et al., 2007) [2] because the crop and weeds are of similar size at emergence and there is no suppressive effect of standing water at establishment (Chauhan, 2012) [3] . Where weeds were not controlled, yield losses in DSR of 59.0-65.6% in eastern India (Mondal et al., 2019) [4] and 68.7% in north-west India (Kaur et al., 2023) [5] have been reported. Hand weeding is effective but is increasingly limited by labour availability at critical times and by cost, as noted for Himachal Pradesh, where rice productivity has been reported to be well below the national average (Rana et al., 2018) [6] . Herbicides have therefore become the backbone of weed management in DSR. No single herbicide controls the diverse weed flora of DSR, and continuous use of the same molecules has been associated with a shift from annual grasses towards broad-leaved weeds and sedges (Kaur et al., 2023) [5] ; a shift towards hard-to-control weeds and the evolution of herbicide resistance are recognised risks of the move to DSR (Kumar et al., 2011) [1] . Pre-emergence herbicides such as pendimethalin provide effective control only during the first 20-25 days, and later-emerging weeds need a post-emergence application, so that sequential pre-followed by post-emergence programmes are considered necessary for broad-spectrum control during the critical period (Verma et al., 2025) [7] . Ready-mix formulations combining molecules with different spectra and modes of action can widen the range of weeds controlled and have outperformed sole herbicides in DSR (Mondal et al., 2019) [4] . Pendimethalin + pyrazosulfuron-ethyl (Kaur et al., 2023) [5] and penoxsulam + pendimethalin (Kaur et al., 2025) [8] have been evaluated as PE premixes in DSR, and flor pyrauxifen-benzyl + cyhalofop-butyl as a PoE premix in dry-and wet-seeded rice (Kaur et al., 2025; Mahapatra et al., 2023) [9,10] . To our knowledge, information on how these premixes perform when used in sequence with each other, relative to programmes supplemented with hand weeding, is limited for dry DSR in the mid-hills of Himachal Pradesh. The present study was therefore undertaken to evaluate the effect of sequential ready-mix and tank-mix herbicide combinations for weed control, weed control efficiency and grain yield in dry direct seeded rice under mid-hill conditions.

Materials and Methods

Experimental site and soil

The experiment was conducted during the kharif season of 2024 at the Experimental Farm, Department of Agronomy, CSK Himachal Pradesh Krishi Vishvavidyalaya, Palampur, Himachal Pradesh, India (32°6′ N latitude, 76°3′ E longitude, 1290.8 m above mean sea level), which lies in the North-Western Himalayan region. Seasonal rainfall totalled 2481.5 mm. The soil of the experimental field was silty clay loam and acidic in reaction (pH 5.28), with 0.54% organic carbon and available N, P and K of 248.60, 23.65 and 301.30 kg/ha, respectively.

Experimental design and treatments

The experiment was laid out in a randomised block design with 12 treatments and three replications; the gross plot size was 12.88 m 2 . The treatments comprised herbicide sequences based on ready-mix and tank-mix products, two herbicide sequences supplemented with one hand weeding, a weed-free check and a partially weedy check (Table 1). In the sequential treatments, the pre-emergence (PE) herbicide was applied at 3 DAS and the post-emergence (PoE) herbicide at 20 DAS. After weed observations were taken at 60 DAS, weeds were removed by hand weeding in T9 and T10 and cut down in T12. Herbicides were applied with a power knapsack sprayer in 500 L/ha of water. The weed-free check (T11) was kept weed free by hand weeding at 20-day intervals.

Crop management

Rice cultivar HPR-2656 (Him Palam Dhan-1) was direct-seeded in dry soil on 18 June 2024 at a seed rate of 100 kg/ha in rows 20 cm apart. A fertilizer dose of 60:30:30 kg N:P 2 O 5 :K 2 O/ha was applied through urea, single super phosphate and muriate of potash as nutrient source. All other agronomic practices followed the standard package of practices for the region.

Observations

Total weed count and weed dry weight were recorded at 20, 40, 60, 80 and 100 DAS and at harvest from two quadrats of 0.5 m × 0.5 m placed randomly in each plot. Weed samples were oven-dried at 70 °C until constant weight. Grain yield was recorded from the net plot, adjusted to 14% moisture and expressed in t/ha. Harvest index (HI) was computed as the ratio of grain yield to total biological yield, expressed as a percentage. Weed control efficiency (WCE) on a dryweight basis (Mani et al., 1973) [11] and weed index (WI) (Gill et al., 1969) [12] were calculated as:

where W Dc and W Dt are the weed dry weights in the partially weedy check and the treated plot, and Y w and Y t are the grain yields in the weed-free and treated plots, respectively. The increase in grain yield over the partially weedy check was also calculated.

Statistical analysis

Data were analysed by analysis of variance appropriate to the design (Gomez et al., 1984) [13] . Weed count and weed dry weight data were transformed by the square-root transformation √𝑥 + 1 before analysis, and treatment means were compared using the critical difference (CD) at P = 0.05; values in parentheses in the tables are original means. The data are from a single season (2024), so treatment × season interaction could not be tested. The relationship between weed dry weight at 60 DAS and grain yield across the 12 treatment means was examined by simple correlation and linear regression.

Results

Effect of weed control treatments on weed density

The data on total weed count (no./m²) as influenced by different weed control treatments at periodic intervals are presented in Table 2.

The data revealed that weed density was significantly influenced by the weed control treatments at all stages of observation. The partially weedy check (T 12 ) recorded the significantly highest weed density, increasing progressively through the early and mid-growth stages to a peak at 60 DAS, thereafter declining following the cutting down of weeds at that stage, and remaining moderately high through to harvest. The weed free check (T 11 ) remained free of weeds throughout the crop growth period. Among the herbicide treatments, pendimethalin + pyrazosulfuronethyl fb florpyrauxifen-benzyl + cyhalofop-butyl at 785 fb 150 g/ha (T and was significantly superior to pyrazosulfuron-ethyl fb florpyrauxifen-benzyl + cyhalofop-butyl (RM) at 22.5 fb 150 g/ha (T 3 ), florpyrauxifen-benzyl + cyhalofop-butyl (RM) at 150 g/ha (T 6 ), bispyribac-sodium + chlorimuron-ethyl + metsulfuron-methyl (RM) at 43 g/ha (T 7 ), pendimethalin fb bispyribac-sodium along with one hand weeding at 60 DAS at 1000 fb 25 g/ha (T 9 ), and pretilachlor fb bispyribac-sodium along with one hand weeding at 60 DAS at 800 fb 25 g/ha (T 10 ). Treatments T 6 and T 7 , comprising sole PoE herbicides, were statistically at par with the partially weedy check at 20 DAS, since these products had not yet been applied; they did, however, achieve a marked reduction in weed density relative to the check by 40 DAS.

Effect of weed control treatments on weed dry weight

The data on total weed dry weight (g/m²) as influenced by different weed control treatments at periodic intervals are presented in Table 3 and Figure 1.

The partially weedy check recorded the significantly highest weed dry weight, peaking at 60 DAS and remaining high thereafter, while the weed free check recorded no weed biomass at any stage. Among the herbicide-only treatments, T 1 recorded the significantly lowest weed dry weight up to 60 DAS, representing a substantial reduction over the check at this stage. At 20 to 60 DAS, T 1 was statistically at par with T 2 , T 4 , T 5 and T 8 , and significantly lower than T 3 , T 6 , T 7 , T 9 and T 10. Values are √(x + 1) transformed; original means in parentheses.

Weed control efficiency

Data presented in Table 4 revealed that weed control efficiency varied among treatments. At 20 DAS, the highest weed control efficiency among herbicidal treatments was recorded with T 1 , followed by T 5 , T 4 and T 8 , whereas T 6 and T 7 recorded the lowest. At 40 and 60 DAS, T1 maintained the highest weed control efficiency, followed by T 8 , T 5 and T 4, while T 3 , T 6 and T 7 recorded comparatively lower effi-ciency. The weed-free treatment (T 11 ) recorded 100 per cent efficiency, whereas the partially weedy check (T 12 ) recorded zero efficiency at all stages. T 1 recorded the significantly highest WCE up to 60 DAS (93.17, 93.75 and 87.58% at 20, 40 and 60 DAS, respectively), followed by and statistically at par with the other PE fb PoE premix sequences T 2 , T 4 , T 5 and T 8 (80.3-86.7% across these stages).

Grain yield, weed index and harvest index

The data on grain yield, weed index and harvest index are presented in Table 5

Relationship between weed growth and grain yield

Across the 12 treatment means, grain yield was strongly and negatively related to weed dry weight at 60 DAS (r = -0.966; y = 3.38 -0.0188x; R 2 = 0.93; Figure 3

Discussion

Weed density and weed dry weight

The significantly lower weed density and dry weight under the PE fb PoE premix sequences reflect the complementary action of a preemergence herbicide, active mainly during the first 20-25 days (Verma et al., 2025) [7] , combined with a broad-spectrum post-emergence partner controlling later weed flushes. The parity of T 1 with T 2 , T 4 , T 5 and T 8 across all three stages confirms these premix combinations were similarly effective, consistent with reports that ready-mix combinations outperform sole herbicide applications in direct-seeded rice (Mondal et al., 2019; Kaur et al., 2023) [4,5] .

The parity of T 6 and T 7 with the check at 20 DAS simply reflects these sole post-emergence products not yet being applied; their sharp decline in both count and dry weight by 40 DAS marks the onset of herbicidal action (Verma et al., 2025) [7] . Overall, the closely matching trends in weed count and dry weight confirm that a sequential PE fb PoE programme sustains weed suppression more effectively across the season than either a single herbicide application or a late hand weeding alone.

Weed control efficiency

The higher weed control efficiency recorded with T1 throughout the observation period may be attributed to the sequential application of pendimethalin + pyrazosulfuron-ethyl as a pre-emergence mixture followed by florpyrauxifen-benzyl + cyhalofop-butyl as a post-emergence mixture. The pre-emergence application provided effective early-season suppression of emerging weeds, while the subsequent post-emergence application controlled the escaped and newly emerged weed flora, resulting in sustained weed suppression. The comparatively lower weed control efficiency of T 6 and T 7 , particularly at the early stage, may be due to their post-emergence application, which allowed weeds to emerge before herbicide application. The relatively higher efficiency of T 4 , T 5 and T 8 may be attributed to their sequential or broad-spectrum herbicide combinations, which provided effective control of diverse weed species. The 100 per cent weed control efficiency under T 11 was due to continuous maintenance of weed-free conditions, whereas zero efficiency in T 12 was expected because it represented the untreated partially weedy condition.

Grain yield, weed index and harvest index

The grain yield of T 1 , being statistically at par with the weed free check, together with its low weed index, corroborates its superior and sustained weed suppression through the critical early period, and is consistent with reports that sequential pre-followed by post-emergence herbicide programmes are among the most effective options for protecting yield in dry-seeded rice ( [14,15,16] . The significantly lower yield of T 9 and T 10 , despite their high late-season WCE, is explained by the substantially greater weed density and dry weight these treatments carried at 60 DAS relative to T 1 -indicating that yield had already been compromised by early-season weed competition before hand weeding was carried out, and that late-season control, however effective, could not fully recover the yield potential lost earlier. This finding underscores the importance of weed suppression specifically within the first 60 days, corresponding to the critical period of cropweed competition in dry DSR (Verma et al., 2025) [7] , over cumulative control achieved by the end of the season. The significantly lower yield of T 3 , T 6 and T 7 likewise reflects their comparatively poor early-season control. The absence of a significant effect of the weed control treatments on harvest index indicates that the yield losses observed were primarily a consequence of reduced overall crop growth and biomass accumulation under weed competition, rather than any alteration in the partitioning of dry matter between grain and straw.

Practical implications and limitations

Within the treatments tested, T 1 gave the best combination of early weed suppression and grain yield, with T 5 , T 8 and T 4 statistically at par with it for yield and offering alternative modes of action, which is relevant because rotation of modes of action helps limit weed shifts and herbicide resistance (Kumar et al., 2011; Kaur et al., 2023) [1,5] . The study has limitations: it covers a single season (2024); weed flora composition was not recorded here; the economics of the treatments and possible crop phytotoxicity were not evaluated; and the check was cut at 60 DAS, which restricts comparisons at later stages. The findings should therefore be confirmed over more seasons, together with a cost analysis, before a firm recommendation is made.

Conclusions

In this single-season study of dry direct-seeded rice in the mid-hills of Himachal Pradesh, pendimethalin + pyrazosulfuron-ethyl (RM) at 785 g/ha as pre-emergence, followed by florpyrauxifen-benzyl + cyhalofop-butyl (RM) at 150 g/ha at 20 DAS, gave the lowest early weed density and dry weight among the herbicide treatments, the highest weed control efficiency up to 60 DAS (up to 93.75%) and a grain yield (3.25 t/ha) at par with the weed-free check, with a weed index of 5.25%. Sequences of pendimethalin or pretilachlor fb bispyribac-sodium with hand weeding at 60 DAS controlled late weeds best but lost yield to early competition, and sole post-emergence applications at 20 DAS were inadequate. Weed dry weight at 60 DAS explained 93% of the variation in grain yield. Effective weed control in the first 60 days, using a pre-emergence premix followed by a broad-spectrum post-emergence premix, is therefore the key to high yield in dry DSR, and the alternative sequences T5, T8 and T4, which were at par with T1 for yield, may serve for herbicide rotation.

Acknowledgement

Acknowledgments The authors are grateful to the Department of Agronomy, CSK Himachal Pradesh Krishi Vishvavidyalaya, Palampur, for providing the experimental farm facilities and technical support required to carry out this study.

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