Effects of irrigation timing in autumn and spring on seed production of subterranean clover, and the change in permeability and rate of germination of seed

Abstract

The effects of time of initial irrigation in late summer-autumn and final irrigation in spring on the amount of seed present, seed burial, relative proportions of each cultivar and changes in seed permeability with time were studied in mixed cultivar swards (cvv. Clare, Woogenellup and Trikkala) of subterranean clover. Irrigation treatments were 3 times of initial irrigation (1 February, 1 March and 1 April) combined with 2 times of final irrigation (mid October and mid November) from 1982 to the autumn of 1985. Seed was sampled 3 times per year; in mid December, prior to initial irrigation and in late winter. The timing of the first irrigation in autumn had no effect on the number of seed present in December which averaged 55,57 and 78 seeds/dm2 in 1982, 1983 and 1984 respectively. Time of final irrigation in spring had a large influence on the number of seeds present in mid December, with a 3-year average of 42 seeds/dm2 for the mid October treatment and 85 seeds/dm2 for the mid November treatment. Rainfall in summer and early autumn reduced the number of seed present between December and the initial irrigation. Residual seed measured in late winter accounted for 18% of the seed present in the previous December and was mostly located below the ground. Without summer rainfall, the proportion of permeable seed averaged over all cultivars increased from approximately 20% in mid December to 40, 60 and 70% in February, March and April respectively. The rate of germination of permeable seed was influenced by time of sampling, temperature of germination test (15 and 30¦C), the incidence of rainfall, and the type of cultivar. The limitations of current subterranean clover cultivars under irrigated conditions are discussed and an outline of the characteristics required by cultivars to be used under irrigation is presented, with particular reference to autumn feed production.