The relevance of variation in thistles to herbicidal control
J.R. Peirce, Agriculture Western Australia, 3 Baron-Hay Court, South Perth, Western Australia 6151, Australia.
Variation in thistles is the result of environmental, morphological and genetic factors. The success of any strategy for thistle control could depend on one or all of these factors and on the thistle species concerned. Seasonal rainfall and temperature patterns can have a significant bearing on the behaviour of many of the thistle species common to Australia. The intensity and frequency of rainfall events can markedly affect the emergence and establishment behaviour at the early part of the growing season and the amount and germinability of the seeds formed at the end of the season.
The ability of some thistle species to form biennial and perennial plants that flower over a long period results in the production of large numbers of seeds. When these seeds germinate over an extended period in a single season, the seedlings pose a managerial problem and cause economic constraints through the cost of repeated herbicide or cultural treatments. In addition, the ability of thistles to produce seeds with the potential to remain dormant over several seasons adds to the cost of any control strategy. The presence of genetically distinct forms or 'ecotypes' in several of the thistles is well documented, but there have been very few studies of the responses of these forms to cultural and chemical treatments. Studies in future should consider the morphological and physiological features of the various thistles and investigate methods to improve herbicidal control without reduced production caused by damage to the infested pasture or crop.
Plant Protection Quarterly (1996) 11 (Supplement 2) 277-279.