Abstract
Seasonal difference in the changing patterns of the top dry matter increasing rate with the growth of Italian ryegrass population was investigated with respect to the factors determining this rate, that is, the leaf area index and the net assimilation rate, and the factors controlling the seasonal productivity as well as how the optimum cutting stage at each season should be determined were discussed. The LAI-CGR curve differs with the seasons. In autumn, it is a nearly symmetric curve of the second order, and in winter a curve with a plateau. In spring, a linear relation is obtained between the LAI and the CGR until the LAI reaches 9〜12, and thereafter the CGR drops rapidly. The optimum LAI and the LAI at average productivity max. showed different values in seasons, and when these values were indicated respectively with average values of five years, they were 7.08 and 10.37 in autumn, 5.45 and 8.88 in winter, and 10.70 and 13.73 in spring. The LAI-NAR curve also differs with the seasons. During the population growth after germination in autumn, the NAR decreases linearly with the increase in LAI. During the population growth as the regrowth in winter and spring, the relation between the LAI and the NAR shows a special pattern. In the early period of the regrowth, the NAR has a negative value, and with the growth and expansion of the regrowing leaves, that is, with the increase in the LAI, the NAR increases, and when the LAI increases further, the NAR decreases. The NAR at average productivity max. showed different values in seasons, and when these values were indicated with average values of five years, they were 0.68 in autumn, 0.63 in winter and 1.04 in spring. The rapid increase in dry matter during spring flush is a result of the concurrent increase in LAI and NAR, and the maintenance of the high NAR level which is considered to be caused by the suppression of the increase in LAI after floral bud formation and by the high radiation level. For the production in autumn and spring, the values of the LAI and the NAR giving average productivity max. are obtainable by the equations a (1)〜(2) when the average productivity max. is given. in autumn LAI=0.0209NAR+0.4616 (r=0.917) (1) in spring LAI = 0.0996NAR+0.3245 (r=0.996) (2)
