[Stock & Land 16 Dec 2010, p. 20] -- DURING the past season, a number of growers in the Western District have commented on poor levels of ryegrass control, even including Group A herbicides such as Select (Clethodim). Reduced chemical uptake during the colder winter months and some application issues will certainly explain some of the poor levels of control, but with others herbicide resistance will almost certainly be the main cause.
As our years of constantly using the same ,roups of herbicides increases, so does the pressure applied to select for weeds resistant to the herbicides being used. A test for resistance is the only way to be sure a weed is resistant to one or more herbicides and ideally seeds need to be collected from surviving plants before harvest.
Tests require 10-12 weeks to complete and therefore collection and submission of samples are required by mid-January to ensure results are available for pre-season planning.
Sampling processes depend on the resistance situation of the paddock. Seed tester Dr Peter Boutsalis says: "If resistance is widespread, collect seeds, following a 'W' shaped area every 10-20 metres across the suspected area." "Alternatively, collect seeds from spot infestations (patches) where weeds survived. Take care to spread sampling across a large number of plants and to collect a similar number of seeds from each plant sampled. "For smaller seeded grasses such as ryegrass, about 3000 seeds (one cup equivalent) are needed for a 4 Mode of Action herbicide resistance test. Fortesting, more seed is better than less."
It is less expensive to spend a few hundred dollars on a resistance test rather than spending thousands on specific herbicides that are not effective.
Article: WeedsNews1462 (permalink) Categories: :WeedsNews:herbicide resistance, :WeedsNews:weed control Date: 16 December 2010; 12:32:49 PM AEDT