A population of L. rigidum, which is known to have developed resistance to one of the diphenyl-ethertype of herbicides, diclofop-methyl, was tested for cross-resistance to three other herbicides of the same type, namely fluazifop-butyl, oxyfluorfen and the experimental herbicide CGA 82725. The population was also tested for cross-resistance to two sulfonylurea-type herbicides - chlorsulfuron and the experimental herbicide DPX-T6376. A population susceptible to diclofop-methyl was used as the controls in the tests. The two populations were treated with various rates of the herbicides during germination and at the two-leaf stage. The results show that the diclofop-methyl-resistant biotype was cross-resistant to fluazifop-butyl, CGA 82725, chlorsulfuron and DPX-T6376 but not to oxyfluorfen.
Several authors have argued that causes differ in the degree to which they are ‘specific’ to their effects. Woodward has used this idea to enrich his influential interventionist theory of causal explanation. Here we propose a way to measure causal specificity using tools from information theory. We show that the specificity of a causal variable is not well defined without a probability distribution over the states of that variable. We demonstrate the tractability and interest of our proposed measure by measuring the specificity of coding DNA and other factors in a simple model of the production of mRNA.
A population of annual ryegrass (Lolium rigidurn), known to be susceptible to diclofop-methyl, and eleven populations known to be resistant to the herbicide, were evaluated for their cross-resistance to fluazifop-butyl, haloxyfop-methyl, sethoxydim, chlorsulfuron, glyphosate and propham, applied postemergence as a spray treatment. None of the populations were cross-resistant to glyphosate or propham. All the populations showed some level of cross-resistance to the other herbicides, but there was considerable variation between populations in this resistance. The variation could not be related in any simple manner to the origin of the populations in Australia, nor to their past histories of herbicide applications. The results indicate the complexity that will be faced in resolving the biochemistry and genetics of the phenomenon and the formulation of advice to farmers.
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