Effects of Machine-cut Stubble Height on Morphological Development and Grain Yield of Subsequent Ratoon Rice Crop

In a mechanized farming experimentation, effects of stubble height of ratoon rice on the tiller growth and development of the subsequent generation of the crop were investigated. The following results were observed. (1) The ratoon tillers grew from germinated cauline axillary buds on the stubbles of the original rice plants. Therefore, the machine-cut, 12 cm-high stubbles lost the axillary buds in the 2^nd and 3^rd nodes from top. The effective ratooning tiller panicles in the new crop had to derive from the tillers of germinated axillary buds in the 4^th and 5^th nodes from top and their filial tillers. This resulted in a more effective formation of panicles and larger spikelet with late development and delayed maturity. (2) The machine-cutting that produced tall stubbles with a height of 35 cm caused the rice to grow into two distinctively different types. One of them had 70% of plant-to-row, whose stubbles retained the entire cauline axillary buds while the ratooning tillers had to depend mostly on the germinated axillary buds in the 2^nd and 3^rd nodes from top with few filial tillers.Thus, the new plants had less effective panicles, smaller spikelet, as well as earlier development and maturity than control. The other type had 30% of plant-to-row, whose stubbles suffered from mechanical damages in harvesting showing fractures on the slender parts about 15 cm above the ground. The devitalization of axillary buds in the 2^nd and 3^rd nodes from top appeared to drive the ratooning tiller panicles from tillers of the germinated axillary buds in the 4^th and 5^th nodes from top and their filial tillers. As a result, the new plants grew similarly to those of low stubbles with large spikelets, late development, and delayed maturity. The ripening of these two types of rice differed in 15 days that hindered synchronized harvesting. And, (3) the new ratoon rice shoots had 6 internodes each.Two basal internodes developed from axillary buds in the 4^th and 5^th nodes from top exhibiting the characteristics of being short, thick, sturdy, and(5±2)cm from ground up in height. On the other hand, the middle internodes from the axillary buds in the 2^nd and 3^rd nodes from top were slender and fragile with large bending moments, which made the plant susceptible to damages in harvest or by storm. It appeared that too shorta stubble would impair the axillary buds in the 4^th and 5^th nodes from top, while too tall would end up with plants with separate ripening stages.The optimal height of stubbles for mechanized farming was, consequently, determined to be 12-15 cm that allowed the retention of two basal internodes with a 5-8 cm protection section on each stubble.