Late Wheat after Cotton: Variety, Seed Rate and Limited Water
Ramesh farms 3.2 ha of canal-irrigated land at Hanumangarh, Rajasthan. His cotton was picked late and the field was vacated on 20 November. The soil is a heavy loam over clay, puddled for the previous crop, with 1.55 g/cm3 bulk density in the 0-15 cm layer and 0.31% organic carbon. Canal water arrives only on 5 December; the tube well can deliver 3 cm per irrigation for at most four irrigations. He has 90 kg seed of a 145-day wheat variety and 80 kg of a 125-day variety, 150 kg DAP and 200 kg urea in the store. Normal sowing in the block is 15-25 November; night temperature is already 9 C and falling, and the long-range forecast is dry until mid-December. Last year the same field gave 3.1 t/ha with sowing on 25 November and five irrigations. Labour is available for one sowing pass this week; certified seed can still be bought locally at Rs 4,500 per 40 kg bag.
1. Fix the sowing date, variety and seed rate. Assume 40 g test weight, 85% germination, 90% field establishment and a target of 200 plants/m2. Show the calculation, state the seed quantity needed, and decide what to do about the seed stock.
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Sowing date: Sow this week, without waiting for the canal. Every day after 25 November costs roughly 30-35 kg grain/ha/day in this zone, and the field is ready now. Check moisture in the 0-15 cm layer; if the puddled layer has dried, give a light pre-sowing irrigation from the tube well and sow at the friable stage.
Variety: Use the 125-day type. A 145-day wheat sown on 25 November flowers around mid-February and fills grain into late March/April, when heat stress cuts grain weight. The shorter-duration type escapes part of that terminal heat.
Seed rate: Seeds/m2 = 200/(0.85 x 0.90) = 261. At 40 g/1000 seeds this is 261 x 0.040 = 10.45 g/m2 = 104.5 kg/ha.
Seed stock: 3.2 ha x 104.5 = 334 kg needed. He has only 170 kg. Buy at least 5 bags (200 kg, about Rs 22,500) to reach 370 kg. Do not stretch the old seed by lowering the rate below about 100 kg/ha: a late-sown crop has fewer tillers, so plant population must be kept up. If cash is tight, buy seed for 2.4 ha and put the remaining 0.8 ha to a short-duration fodder or mustard crop rather than sowing thin wheat.
2. Plan the irrigation schedule using the four 3 cm tube-well irrigations and the canal supply. Which stages get priority, which can be skipped, and why?
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Total tube-well supply is only 12 cm, plus whatever the canal gives, so this is a deficit irrigation plan, not a normal five-irrigation schedule.
Critical stages for late-sown wheat: crown root initiation (about 20-25 days after sowing, around 17-20 December), flowering (early February) and milk/grain filling (late February-March). Jointing is the stage to sacrifice; it affects tiller survival but not as directly as flowering and grain filling.
Allocation:
- CRI, 3 cm: essential for nodal roots and tiller initiation.
- Late tillering, 3 cm: supports ears/m2.
- Flowering, 3 cm: the single most important irrigation; water stress here causes floret abortion.
- Milk/grain filling, 3 cm: grains fill in rising March temperature.
Use the canal water at CRI if the rotation permits, and bank one tube-well irrigation for grain filling. On this heavy soil apply small depths and ensure drainage; waterlogging at CRI is as damaging as drought. Do not irrigate just before rain forecast in the 2-3 days, and avoid irrigation during hot windy afternoons at flowering.
3. Evaluate his fertiliser stock (150 kg DAP, 200 kg urea) against a wheat recommendation and propose a late-sowing-adjusted split application.
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Nutrients in stock: 150 kg DAP (18-46-0) = 27 kg N + 69 kg P2O5. 200 kg urea (46-0-0) = 92 kg N. Total = 119 kg N, 69 kg P2O5, 0 K2O.
A normal wheat recommendation is about 120 kg N, 60 kg P2O5 and 40 kg K2O. His P is slightly above requirement, so do not buy more DAP; K status is not given, so check the soil test rather than adding K blindly.
Late-sowing adjustment: Avoid putting all the urea at sowing. A heavy early N dose on a short-duration, late crop produces succulent growth and lodging without raising yield.
- Basal: 150 kg DAP + 50 kg urea (27 + 23 = 50 kg N), placed in the seed furrow, not broadcast.
- First top-dress, CRI: 75 kg urea (34.5 kg N).
- Second top-dress, first node: 75 kg urea (34.5 kg N).
This uses the whole 200 kg urea and delivers 119 kg N, matching N supply to the shorter growth period. Apply the top-dressings when the soil is moist, and keep urea away from the leaf whorl.