种子批选择：发芽率、活力与播种量
A farmer has two lots of the same wheat variety. Lot A gives 86, 82, 90 and 78 normal seedlings in four replicates of 100 seeds, with abnormal seedlings averaging 7%; it costs ₹45/kg and has 11.8% moisture after eight months in cloth bags. Lot B germinates 92%, costs ₹52/kg and comes from a sealed certified bag at 10.2% moisture. The recommended seed rate is 100 kg/ha, a figure that assumes a seed lot of 90% germination, and the farmer has 1.5 ha to sow. The field is dry, soil temperature at sowing depth is 15 °C, and the farmer normally sows 6 cm deep. A tetrazolium test on lot A shows 91% viable embryos.
1. Calculate the mean germination of lot A and the seed rate needed for 1.5 ha from each lot. Which lot is cheaper per hectare actually sown?
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Lot A mean germination = (86 + 82 + 90 + 78) ÷ 4 = 336 ÷ 4 = 84%.
Adjusting the seed rate (the 100 kg/ha recommendation is written for 90% germination):
- Required rate = 100 × (90 ÷ actual germination)
- Lot A: 100 × 90/84 = 107.1 kg/ha → 1.5 ha = 160.7 kg → 160.7 × ₹45 = ₹7,232
- Lot B: 100 × 90/92 = 97.8 kg/ha → 1.5 ha = 146.7 kg → 146.7 × ₹52 = ₹7,630
Cost per percentage point of germination tells the same story: lot A = 45 ÷ 0.84 = ₹53.6 per kg of germinable seed; lot B = 52 ÷ 0.92 = ₹56.5.
Lot A is therefore about ₹400 cheaper for the whole 1.5 ha, and it needs 14 kg less seed to be handled. The price difference, not the germination figure alone, decides the purchase - a cheaper lot with lower germination can still be the better buy.
2. Lot A germinates 84% but the tetrazolium test shows 91% viable embryos. Explain the gap and what it means for this farmer's field.
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The germination test counts only normal seedlings; the tetrazolium test stains living tissue and counts viability, whether or not the seed can push out a healthy seedling. A 7-point gap means roughly 7% of the seeds are alive but did not produce a normal seedling - typically through mechanical injury during threshing (broken coleoptile or cracked scutellum), ageing or storage damage, or dormancy in freshly harvested seed. The 7% abnormal seedlings recorded in the test support injury or ageing rather than dead seed.
For the field this is a warning about vigour, not just count. A seed that only just germinates in the favourable conditions of a laboratory germination test will struggle in a dry seedbed at 15 °C. Field emergence is typically 80–85% of laboratory germination under good conditions and can fall to 70% or lower with deep sowing, dry soil and cold temperature. So lot A's 84% could translate to roughly 60–70% field emergence - which the seed-rate correction above does not capture.
3. Given the dry soil, 15 °C temperature and 6 cm sowing depth, what sowing adjustments do you recommend, and why would simply raising the seed rate be the wrong first response?
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Recommendation:
- Sow shallower - 4 to 5 cm, not 6 cm. At 15 °C the coleoptile grows slowly; a seed placed 6 cm deep uses a large part of its reserves before the shoot reaches the surface, and weak seedlings from the 7% abnormal fraction never emerge. Shallow sowing also warms faster.
- Sow into moisture, or give a light pre-sowing irrigation rather than sowing dry and waiting. Dry soil plus deep placement is the combination that produces patchy emergence.
- Treat the seed with a recommended fungicide against soil-borne seedling disease, because slow emergence at low temperature lengthens the vulnerable period.
- Use a modest seed-rate buffer - sow lot A at about 110–115 kg/ha (rather than the theoretical 107 kg/ha) to cover the vigour gap, and re-check emergence at 10–12 days to decide whether patching is needed.
Why not simply raise the seed rate: extra seed cannot compensate for seedlings that die before emergence, and a dense, deep-sown stand produces weak, etiolated plants that compete poorly and tiller less. Raising the rate also increases cost and disease pressure without fixing the cause. The first response should be to correct depth and moisture - the factors that determine whether the viable seed in lot A can actually become a plant. If sowing must be delayed or the seedbed cannot be improved, then lot B, with its higher vigour and lower moisture, becomes the better choice despite the higher price.