Effect of seed rate and row spacing on growth and yield of black gram under rainfed conditions

Introduction

According to several pulse agronomists, including Ali and Kumar [1] , plant stand establishment is the single most controllable factor in rainfed pulse production. Black gram (Vigna mungo L. Hepper) is the fourth most important pulse crop in India, occupying about 4.5 million hectares almost entirely under rainfed conditions yet its average productivity hovers near 600 kg/ha, well below its genetic potential of 1500-2000 kg/ha [2] . Part of this yield gap traces to suboptimal plant population. Farmers in central India commonly sow black gram at 15-20 kg/ha in 30-45 cm rows, a legacy of bullock-drawn seeding equipment and low seed availability [3] . Under these wide spacings, canopy closure is delayed, weeds compete aggressively during early growth, and radiation interception remains low during the critical pod-filling phase [4] . Higher seed rates or narrower rows could close the canopy sooner and intercept more light, but excessive density promotes inter-plant competition for water a critical constraint under rainfed conditions and can increase disease pressure from foliar pathogens [5] . This research aimed to identify the seed rate and row spacing combination that maximises grain yield and economic returns of black gram cv. T-9 under rainfed kharif conditions at Betul, Madhya Pradesh, across two seasons (2021 and 2022) [6] .

Material and Methods

Material

The experiment was conducted at the research farm of the Satpura Range Agricultural Institute, Betul (21.88°N, 77.91°E, 610 m elevation), Madhya Pradesh, during kharif 2021 and 2022. The soil was a medium-black Vertisol with pH 7.1, organic carbon 0.54%, available N 218 kg/ha, available P 14.3 kg/ha, and available K 242 kg/ha. Mean annual rainfall at the site is 1180 mm, with 85% falling between June and September. Certified seed of black gram cv. T-9 (maturity 65-70 days, erect growth habit) was obtained from the Jawaharlal Nehru Krishi Vishwa Vidyalaya seed store. Seeds were treated with Rhizobium inoculant (strain IC-3087) at 25 g/kg seed and trichoderma (Trichoderma viride, 4 g/kg) before sowing.

Interpretation

Leaf Area Index (LAI) increased with density, reaching 3.91 at 30 kg/ha and 20 cm rows, but per-plant dry matter was lowest there (7.8 g). The 25/25 treatment struck a balance: LAI of 3.17 provided adequate light interception while allowing individual plants to accumulate 10.8 g dry matter and set 19.4 pods. This outcome supports the compensatory yield model, where moderate stand density optimises the trade-off between plant number and per-plant productivity [4] .

Discussion

The 14.2% yield advantage of 25 kg/ha over 20 kg/ha at 25 cm spacing aligns with findings of Patel and Thaker [7] , who reported 11-18% yield increases in mung bean when seed rate was raised from 20 to 25 kg/ha in Gujarat. The absence of a further gain at 30 kg/ha is consistent with the densityyield plateau described by Holliday [8]: beyond a speciesspecific optimum, additional plants compete without adding grain. Under rainfed conditions, water is the binding constraint. Narrow rows (20 cm) at high seed rates (30 kg/ha) likely exhausted topsoil moisture during the vegetative phase, leaving insufficient reserves for pod filling a pattern reflected in the low yield (1043 kg/ha despite 39.1 plants/m²) of that treatment [9] . The 25 cm spacing appears to allow enough root-zone separation for water uptake while maintaining ground cover that reduces evaporative loss [10] . The practical implication is straightforward: farmers in central India can improve yields by increasing seed rate by just 5 kg/ha and narrowing rows from 30 to 25 cm changes that require no new equipment beyond minor drill adjustment and a modest increase in seed expenditure [11] .

Cost analysis and economic feasibility

Economics were calculated using prevailing 2022 input and output prices. Seed cost at 25 kg/ha was Rs 2,500/ha (Rs 100/kg), compared to Rs 2,000/ha at 20 kg/ha an incremental cost of Rs 500/ha. The yield gain of 140 kg/ha (from 984 to 1124 kg/ha, comparing 25/25 with 20/25) at a minimum support price of Rs 6,300/quintal translated to an additional gross return of Rs 8,820/ha. Net returns were highest with 25 kg/ha at 25 cm (Rs 34,180/ha) and lowest with 30 kg/ha at 30 cm (Rs 18,740/ha). The benefit-cost ratio was 2.38 for the optimal treatment versus 1.94 for the conventional 20/30 practice. This suggests that the incremental investment is highly profitable.

Limitations

The trial was conducted over two kharif seasons at a single location; multi-location testing across contrasting soil types and rainfall zones is needed before generalising the results. Only one cultivar (T-9) was tested; spreading-type cultivars may respond differently to spacing. Disease and pest pressure was low in both seasons, and the interaction of density with biotic stresses under epidemic conditions was not captured. The economic analysis used 2022 prices, which are subject to annual variation.

Conclusion

Sowing black gram cv. T-9 at 25 kg/ha with 25 cm row spacing produced the highest grain yield (1124 kg/ha) and net returns (Rs 34,180/ha) under rainfed kharif conditions at Betul over two seasons. This treatment outperformed the conventional 20 kg/ha at 30 cm by 14.2% in yield and Rs 9,620/ha in net returns. The response reflects an optimal balance between plant population, individual plant vigour, and soil moisture demand. Farmers in central India's rainfed pulse tracts should consider this low-cost adjustment to improve black gram productivity.

References

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