RICE RATOONING - IRRI books - International Rice Research Institute

250 RICE RATOONINGYield and yield-componentsGenotypic. Yielding ability, traced through a sufficient number of tillers per unitarea, is partly a varietal trait. Several authors report ratoon yields correlatedstrongly with percentage of ratooning ability (17, 19). IR22 ratoon had morepanicles than IR8 ratoon, and yielded higher (25). Ratoon crop yields also wererelated to ratoon crop durations (31).Environmental. Important management factors are plant population,fertilization, harvest time (as early as possible) (31), and cutting height (15-20 cm). Acutting height of 15 cm and a spacing of 15 cm were best (2). Phosphate fertilizer onthe main crop gave a very significant increase in ratoon crop yield, adding phosphatefertilizer to the stubble gave less yield (15).In direct seeding, ratoon crops sometimes had more tillers than the main crop(2, 3). The importance of nutrient status for the ratoon and the main crop has beenwell-documented (15,16). Ratoon yield from unshaded main crop plants was 34.2%higher than from 2/3 shaded plants (16).Insect and disease resistanceGenotypic. The most important diseases are viruses; they affect ratoon crops moreseverely than main crops (31). Grassy stunt virus infection has been mentioned (2).Environmental. In nutritional value to insects, the ratoon crop was inferior tothe main crop. Population densities of most insects did not increase; in fact,populations in the ratoon crop most often decreased (13).GENETICS OF RATOONINGRatooning ability is seen as a complex trait, dependent on many heritable andenvironmental factors. Table 1 gives selected data from a study evaluating ratooningability as a gauge of main crop physiologic traits (19).The correlation coefficients, estimated from phenotypic and error varianceand covariance components, indicate a very strong codetermination of ratooningability and main crop traits, especially when cuttings are made before harvest(forfeiting the main crop). However, genotypic correlation estimates are still high 30and 40 d after heading.In the genetic study cited earlier (11), the F 3 -on-F 2 regression coefficients forratooning percentage were low and insignificant. On the other hand, within-yearvariance components indicated significant varietal effects, with estimates of a0.26:0.43 ratio of genotypic variance to phenotypic variance. It has also beenTable 1. Genotypic correlation coefficients between percentage of ratoon tillersand agronomic characters of the mother plant (19).Cutting time Culm length Fertility Grain yield(d after heading) (%) (%) (%)10203040–0.63–0.26–0.34–0.230.940.760.380.260.550.290.1 10.22