Hybrid maize seed lot with an isolation breach, shedding tassels and a failing grow-out test
A cooperative produces hybrid maize seed on 10 ha with a 4:2 female-to-male row arrangement. The contract specifies 200 m isolation, no female plants shedding pollen at flowering, and 98% genetic purity. At the first flowering inspection, 3% of female plants are off-types and 12 female plants per 100 are shedding tassels. The nearest neighbouring maize field is 150 m away. Detasseling began two days after the first pollen shed was seen. Four roguings were done. The harvested lot is 22 t. A grow-out test of 400 plants shows 9 off-types; germination is 92% and moisture 13.5%. The cooperative asks whether to certify the lot, blend it with a cleaner lot, or reject it, and wants a corrective protocol for next season.
1. Assess whether the 22 t lot should be certified. Use the purity and field data, and state what additional information you would require.
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The lot should be provisionally rejected, not certified. The grow-out test shows 9 off-types in 400 plants = 2.25%, so apparent purity is 97.75%, below the contractual 98%. The field data reinforce the laboratory result: isolation is 150 m against a required 200 m, 12% of female plants were shedding tassels at flowering, 3% were off-types, and detasseling started only after pollen shed. These are exactly the conditions that produce selfed and foreign-pollen contamination.
Germination at 92% and moisture at 13.5% are acceptable and do not rescue the genetic result. A single 400-plant sample has sampling error, so I would re-test a larger sample and check the label's tolerance for off-types, but I would not certify on the present evidence. Blending with a cleaner lot would dilute the defect and is not an acceptable way to meet the standard. Segregate the lot, retain samples, and use it only for non-seed purposes unless a properly designed retest and a field-history review support a lower off-type estimate.
2. Rank the causes of contamination and explain how each one contributed to the failing result.
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1. Female tassels shedding pollen (12%) is the largest and most direct cause. Maize is wind-pollinated, and a shedding female tassel produces selfed seed on the same plant and pollen for nearby females. Detasseling after pollen shed began means contamination was already possible before roguing.
2. Isolation breach (150 m instead of 200 m) is the second major risk. Maize pollen can travel well beyond 150 m under wind, so the neighbouring field could have contributed foreign pollen. The effect depends on wind direction, flowering synchrony and the neighbour's pollen load, but the contract distance was not met.
3. Female off-types (3%) contributed a genetic impurity even before pollination. Some may have been taller, earlier or more vigorous plants that escaped roguing; their pollen and seed both enter the lot.
4. Late detasseling and insufficient inspection allowed the first three problems to overlap. Four roguings are not a substitute for detasseling before any tassel emerges. The male rows and border rows should also be checked, because uneven male fertility can reduce pollination and increase selfing.
3. Give a corrective protocol for next season, including inspection timing and the calculation that would define an acceptable grow-out result.
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Field establishment: Use land with at least 200 m isolation, or a border of at least 4–6 male rows where the contract permits it. Confirm the neighbour's maize planting date and flowering window. Plant the female and male rows according to the 4:2 plan, and keep separate male blocks for pollen supply.
Detasseling: Begin before any tassel emerges, not after pollen shed. Walk the female rows every 2–3 days and remove tassels before they open. Stop only when all female tassels are gone. Rogue off-types at the seedling, pre-flowering and flowering stages, and record counts.
Inspection: Inspect at pre-flowering, at 10–20% flowering, at full flowering, and before harvest. Check isolation, off-types, shedding tassels and male-row condition at each visit.
Grow-out calculation: For 400 plants at 98% purity, the maximum acceptable off-types is 2% of 400 = 8 plants. Nine off-types fails; eight or fewer would pass that sample, subject to the contract's sampling plan. Do not blend a failing lot. Retain the current lot's samples and records, and calculate the next crop's pure-seed yield as harvested weight × purity only after the lot passes-not as a way to make a failing lot acceptable.