Abstract: 【Objective】The objective of this study was to clarify relationships between seed maturity and vigor, and to determine optimum harvest of maize seed with high vigor so as to provide references for seed enterprises to avoid early autumn frost, in Hexi in China.【Method】Samples for maize seeds were obtained at 3 to 5 days intervals after pollination to test seed moisture, standard germination-SG (7 days at 25℃), and vigor, using cold test-CT (3 days at 6℃) and electrical conductivity-EC (three 50-seed samples, 24 hours inhibition at 25℃), and milk line development was classified by a five-stage system, using three genotypes (hybrid XY335, ZD958 and JX1) during the crop seasons of 2013.【Result】Seed moisture and seed moisture-loss rate and germination and vigor were highly influenced by genotypes and days after pollination (DAP). XY335 seed dehydrated early and quick, and seed moisture content was 10-15 percentage points lower in XY335 than that in ZD958 and JX1 at 32 DAP. During grain filling period from 32 to 72 DAP, seed moisture in ear decreased with time, average moisture-loss rate for XY335 was 0.70% per day that was higher than 0.62% per day in ZD958 and 0.49% per day in JX1, and 100-seed weight increased 0.73 g in XY335 and 0.50 g in ZD958 and 0.29 g in JX1 when seed moisture reduced one percentage point. Seed maturity and vigor at different days after pollination had a great difference, and seed quality values in terms of standard germination increased and electrical conductivity values decreased with seed moisture decreases from 70% to 40%, and the SG values increased by 3.37 percentage points in XY335 and 2.76 percentage points in ZD958 and 1.67 percentage points in JX1 as one percentage point reduction in seed moisture. With grain filling moving forward, the SG tended to increase and the EC to decrease but these changes were different across genotypes and DAP, and the SG ranged from 30% to 59% and the vigor at cold test from 2.5% to 36.9% at 32 DAP, and the seed germination values at standard and cold test for XY335 were comparable at 47 DAP onward, but the seed germination at standard was always lower than that at cold test for ZD958 and JX1, suggesting the order of seed low temperature resistance was XY335, ZD958, and JX1, respectively. Seed moisture during grain filling period was highly correlated with milk line development stage, and milk line moved from the seed’s upper to base as seed moisture decreased. Milk line presenting and disappearing were the earliest for XY335, came second for ZD958, and the latest for JX1. When the milk lines for three hybrids were present at the midpoint of the seed, seed moisture would be 38% to 40%, and the SG and vigor in CT would be close to high values, implying optimum harvesting can be made at half- milking stage, in which 100-seed weight of XY335 and ZD958 accounted for 76.6% and 88.0% of its maximum at physiological maturity. 【Conclusion】 Seed maturity affected greatly on seed quality, and optimum harvest for high germination values and vigor using cold test and EC can be determined at half-milk (stage 3) in which seed moisture was about 40%, and can not be made at physiological maturity of seed weight usual reaching its maximum. Suitable harvesting started from 47 DAP onwards for XY335, and 60 DAP for ZD958 and JX1, 10 to 15 days before usual harvesting, to prevent frost damage on seed quality in Wuwei, Gansu.

Key words: Hexi corridor , maize , seed maturity , days after pollination , seed vigor