A Method for Estimating Damage by Cool Weather in Rice Production

Abstract

Until the recent time in the northern part of Japan, there used to be many disastrous years for rice production, whenever unseasonably cool weather occurred in the summer. Lately, however, especially since 1955, the rice production in that part has rapidly increased, and even in years of the cool summer it has become not so much decreased as in the past years. This is due to the recent advance in the technique of rice cultivation in cold districts. But, it can not be considered yet that the danger of cool domage has been completely swept away in cold districts. If the extremely low temperature would happen there, it is not difficult to suppose the occurrence of reduced rice production.It is very important therefore, for the improvement of research on prevention methods against cool damage more efficiently, as well as for the deciding of the food policy, to estimate beforehand where the damage will occur or how much it will be, under the level of advanced technique and similar bad weather conditions as in former years. The author has designed a new method of estimating damage by cool weather and danger grade for cool damage of rice plants, and requested the technicians of all prefectures in cold districts to try to get the estimation by using his method. As his method of estimation, however, only indicates the basic ways and means, the technicians in each prefecture must decide by themselves the most suitable values for the conditions tentatively given in this method. The results obtained in every prefecture will be published elsewhere.The most important parts of the method reported in this paper are as follows1. There are three types occurring in cool damage, i. e., delay, injury, and mixed types.When the growth of rice is considerably delayed mainly due to unseasonably cool weather during its vegetative growth period, the ripening period comes during the low temperature of autumn and results in a reduced crop. This is called the delay type of cool damage. The preiods in which rice is most sensitive to low temperature during its growth are those at the reduction division stage of reproductive cells and at the flowering stage respectively. The rice plants suffer from non-ripened grains due to a temporary low temperature during such reproductive growth periods. This is called the injury type. If the above-mentioned two types of cool damage occur simultaneously, it is called the mixed type.Therefore, the next three relations may be important indexes for estimating damage, i. e., relations between (1) the decrease rate in the production and the air temperature during the ripening period, (2) the decrease rate and the air temperature at the reduction division stage, and (3) the decrease rate and the air temperature at the flowering stage.2. The respective values of these three relations can be prepared from many past results of investigations and experimental reasearch in each prefecture.The decrease rate may be easily got by both these relations and the course of air temperature given in one of the years in which cool damage occurred in the past.3. First of all, however, it is necessary for this estimation to decide the growth stage of rice plants. The flowering stage is almost the same as the heading stage. The reduction division stage is during a period from 10 to 15 days before the heading stage, and the ripening period is the period of 40 days after heading. Then, if the heading stage can be fixed, these three growth stages may be easily decided. Since the accumulated air temperature from sowing to heading is said to be constant for each variety of rice plant, the heading day of the rice plant concerned can be fixed by summing up the daily air temperatures from sowing day till the day on which the accumulated value reaches the constant.4. The result of the estimation in a prefecture, in practice, is obtainable by summing up the results in many small areas such as towns and villages. And the