## 5 Discussion
### Influences of Emergence Date and Weeds  
The day of emergence decides the size of the cotton plant when monitoring the germination rate. To obtain the best images to monitor the budding rate, the day of flight is the key parameter. If the flight time is too early, then the size of the cotton is too small to be observed easily; alternatively, if the flight time is too late, the opportunity to count the number of overlapping cotton plants is lost. Choosing the best window to monitor the budding rate has no fixed rule; the growth rate of the budding seed depends on different environmental parameters, such as temperature, soil moisture, and type of seed. In this study, the flight occurred when the farmer was beginning to replant the cotton, the date of the flight in this study was appropriate for the image collection.  
The weeds in the study field are another source of error that can adversely affect the germination rate. In this study, the unique processing steps during sowing in Xinjiang Province make the results unaffected by the weeds, i.e., an herbicide was used in the field before sowing. Then, the sowing machine laid drip irrigation pipes, and the soil was covered with plastic film to improve the ground temperature, and finally, cotton was sown with equal spacing. The above steps provide many advantages for this study. First of all, the herbicide killed most of the weed seeds, thus inhibiting the growth of weeds. Then, the plastic film ensures that the weeds cannot emerge at the same time as the initial stage of growth of the cotton. Those steps make sure that almost no weeds were found in the field.  
Although the plastic film provides many benefits, in practice, it also leads to many difficulties in identifying the cotton plants. In the sowing process, the soil was placed on the plastic film to prevent the plastic film from being blown away. As a result, for some of the germinated seeds, if the upper part of the seed was covered by the plastic film, it could not pass through the plastic film after germination. As the red rectangle shows in Figure 6b, due to being covered by plastic film and soil, almost no cotton plants emerged from the surface, which definitely decreased the total monitored cotton budding rate. The effect of the plastic film means that the budding rate monitored by the study is an apparent budding rate, which does not include the part covered by the plastic film. In fact, according to several years of statistical results, in which the plastic film was removed from the field, the actual budding rate of this area is greater than 75%.  
Compared to the actual budding rate for this area, the monitored apparent budding rate for the cotton of 56.26% in the study was quite low. However, monitoring the apparent budding rate is the main purpose of this study, and it is useful for the growers. Usually, sprouted cotton seedlings covered by the plastic will not penetrate and emerge from the film, and the temperature between the ground and film is higher than the outside air temperature. If the plastic film covering the cotton plant is not removed, as temperature increases, the cotton plant will be killed by the high temperature.  
Figure 7: The cotton field images at different resolutions obtain from the UAV.  
As a result, for locations with a low density of cotton plants, growers not only remove the plastic film but also replant the cotton seeds. Hence, the apparent budding rate presented in this study will provide more useful information for growers in Xinjiang Province.