R.M. JONES, G.A. BUNCH and C.K. McDONALD

CSIRO Tropical Agriculture, Brisbane, Australia

Abstract

This paper reports the results of a series of studies on aspects of the persistence of the tropical pasture legume Chamaecrista rotundifolia (round-leafed cassia) cv. Wynn. The results are contributing to formulating a quantitative model of cassia persistence.
The main study, at Samford in coastal south-east Queensland, was on the fate of cassia seed broadcast into cassia-free pastures in 1990 and 1991 to simulate seed set in an established cassia pasture. The study was carried out on a heavily defoliated and a lightly defoliated site. Seedling emergence and soil seed reserves were monitored for 3 (1991 sowing) or 4 (1990 sowing) years. There was a much higher recovery of viable oversown seed as seedlings or soil seed in the second (50%) than in the first (13%) sowing. Approximately 40% of the second sowing was recovered as soil seed after 3 years compared with 6% from the first sowing. The difference is attributed to poorer quality of the 1990 seed sample. The run-down of seed banks was similar under both defoliation intensities, but there was greater loss of seed and fewer seeds recovered as seedlings on the lightly defoliated site.
Other studies showed that: in early autumn at Samford, a cassia flower bud could develop into a mature pod and dehisce in 39 days, but a longer time was required in late autumn; there was a linear relationship between herbage yield and seed set but, for the same cassia presentation yield, ungrazed pasture set more seed than grazed pasture; seed quality of cassia varied between sites and years of collection, with quality lower in years with wet conditions during pod set; there was little predation of cassia seed placed on the soil surface at Samford, whereas, at Narayen Research Station in subcoastal southern Queensland, there was considerable predation during the hotter months; and hard seed breakdown of cassia occurred at lower temperatures than is the case for some Stylosanthes spp. Equations were developed which relate rooting depth of cassia seedlings and plants, and age of plants, to taproot diameter. The implications of these measurements for management of cassia pastures and for development of a model of cassia persistence are discussed.
Further quantification of factors affecting seed set and the fate of seed will be required before a model of cassia persistence can be developed that will be reliably applicable to all time/site situations.