Towards large-scale prediction of Lolium rigidum emergence. I. Can climate be used to predict dormancy parameters?

Lolium rigidum (annual or rigid ryegrass) is a widespread annual weed in cropping systems of southern Australia. Seeds exhibit dormancy at dispersal and require a period of dry after-ripening to release dormancy, before germination and emergence can occur. Climate, particularly temperature and rainfall, modulates dormancy level at seed maturity and dormancy release during after-ripening. This study investigated the possibility to predict seed dormancy of L. rigidum over a large scale, based on spatial and climatic factors. Mature seeds were collected from 406 populations across 15 different agronomic zones of the 14 million hectare grain belt of southern Western Australia. For each population, initial dormancy and change in dormancy over a 6-month period were measured. Logistic growth curve models were then fitted for each population, with the resulting equation used to estimate four further parameters describing dormancy status of the population. These parameters were used to determine relationships between location in the grain belt and long-term and current-year temperature and rainfall parameters for each population. Although some trends in seed dormancy patterns were found and distinct spatial clusters were clearly evident, our results indicate that climatic parameters alone are unlikely to be a useful predictor for seed dormancy in L. rigidum on a large scale, such as the Western Australian grain belt.