Variation in quality and performance of stored seed of green panic ( Panicum maximum ) attributable to the events of the harvest period

A search for causes of low quality in commercial seed of the pasture grass green panic (Panicum maximum) was directed at the successive events of the harvest period. Seed samples were taken at points before and during the direct combine-harvest, transport and drying of 7 commercial seed crops in central Queensland. The samples were then stored and periodically subjected to laboratory, greenhouse and field tests over a period of 3 years to determine the course of change in their viability, germination and seedling emergence from soil. The changes in quality attributes were then linked to the separate stages in the harvest. An average of 34% of pure seeds in the standing crop at harvest contained immature and thus inferior caryopses. Seed was physically damaged during harvesting, an effect attributed to threshing. The damage caused some immediate death and shortened life expectancy of the seed population as a whole, but also accelerated dormancy breakdown. Accordingly, it first stimulated but later depressed the germination of seeds in laboratory tests and the emergence of seedlings from soil, and reduced numbers of seeds surviving in soil. It reduced the viability of stored seed at any one time by about 15% and shortened life expectancy under normal storage conditions by at least 1.5 years. Ill-effects of subsequent transport and drying were detectable, but were inconsistent or less severe. Viability and properties dependent on it were sometimes reduced both as a result of prolonged periods spent in the truck bin (up to 10 hours, when seed tended to over-heat) and by conventional bulk-drying, but without marked effects on dormancy. The combination of the ubiquity of immaturity and the need to accept some threshing damage restrict the scope for improvement of quality of seed entering storage.