Abbreviations: AVK, the average total kernel number per ear in the whole field; CP, cross-pollination; GM, genetically modified Field experiments were conducted in Central Taiwan for 2 crop seasons to examine the effect of non-coincidence flowering on the cross-pollination (CP) rate of maize at various distances. Four local maize hybrid varieties with different flowering dates and one local maize variety were sown as the pollen sources and recipient, respectively. All varieties were sown on the same day to simulate the real situation of coexistence in which adjacent fields are sown with different genetically modified (GM) and non-GM varieties of maize. The CP rate was <0.2% at a distance of 3 m for the first crop season when the flowering time for the recipient was 5 d later than that of the pollen source variety. The CP rate was <0.02% at all distances for the second season when the flowering time for the recipient was 7 d later than that of the pollen source variety. The CP rate was <1% at a distance of 0.75 m when the flowering time was 3 d later. However, varieties with closer synchrony may result in a CP rate of >1% at a distance of 1.5 m and <1% at 2.25 m. Temporal separation and isolation distances can work together in Taiwan with fragmented landscapes to minimize the adventitious presence of one crop with another.

A field experiment was conducted in Taiwan to measure the cross-pollination (CP) rate of maize pollen recipients from pollen sources using phenotypic marker and to determine the isolation distance between the 2 maize varieties. A waxy variety (Black Pearl) with purple kernels simulated the genetically modified (GM) pollen donor, and another waxy variety (White Pearl) with white kernels simulated the non-GM recipient. For the first crop, the total area was approximately 1.5 ha with a pollen source and recipient acreage ratio of approximately 1:32. For the second crop, the total area was approximately 1.83 ha with a ratio of approximately 1:17.3. The source fields were surrounded by the recipient fields for 2 crop seasons. The results showed that the rate of CP was <0.05% beyond 15 m upwind and 84.8 m downwind in all crop seasons. The CP rate was below 5% at a distance of 10 m in the downwind direction. A sample with 0.24% CP was recorded at 107.3 m downwind; however, the CP rate was 0% at 68 m upwind. Three empirical models were used, that is, exponential, log/log and log/log, and a simplified Gaussian Plume model, to examine the relationship between the CP rates and the source-field distances. All of the models were appropriate for predicting CP rates, and the Gaussian Plume model performed better compared to the empirical models. The results show that it is possible to control CP from foreign pollen by using an appropriate isolation distance.