Abstract
The six cool temperate grasses had been grown under mimic grazing system for six years on field plots treated with 2 levels of fertilizer supply respectively. From the data obtained above, seasonal fluctuation of dry matter production was mainly examined in relation to occurrence of summer depression using statistical method. The results obtained were as follows: 1) It was recognized that occurrence of summer depression of each species was closely related to elevation of temperature in previous regrowth period and increase in total precipitation during rainy season. 2) Much closer relation between its occurrence and two factors was shown in all plots treated with higher supply of fertilizer and the extent to which each species was affected by two f actors decreased in the order as follows: Orchardgrass>Perenial ryegrass>Red top>Tall fescue>Kentucky bluegrass. 3) In plots treated with lower supply of fertilizer, only Perenial ryegrass and Orchardgrass which were considered to be less resistant to the summer depression showed the same relation as mentioned in 1), but the extent affected by two factors was much less than that in case of 2). 4) Above two factors did not affect to each species at the same rate. For instance, Perenial ryegrass was severely affected by both factors, but Red top was more likely to be affected by temperature than precipitation. It was concluded that precipitation during rainy season was recognized to be one of the important factors which induced summer depression, added to high temperature, drought, diseases and insects which had beeh considered to be main factors of its occurrence.
