Herbicide management and thistle control—to avoid resistance
K.C. Harrington, Department of Plant Science, Massey University, Palmerston North, New Zealand.
Applying phenoxy herbicides annually to thistles for many years can lead to herbicide resistance developing within thistle populations. Populations of nodding thistle have been located in Hawkes Bay which require six times more MCPA than normal to obtain adequate control. Such populations are known to exist on at least 12 properties in Hawkes Bay and Waikato. A comparison of past spraying practices on these properties and farms where resistance has not developed identified that resistance has occurred due to more frequent use of phenoxy herbicides on the former properties. Cross-resistance exists within these populations to 2,4-D, MCPA and MCPB. Thus such thistles can only be controlled by adding clopyralid to one of these herbicides, which is damaging to pasture legumes. Work with radio-labelled MCPA has shown that resistance is due to an increased rate of herbicide degradation within the plant rather than reduced uptake, so adding surfactants to the herbicide will not overcome the resistance.
Based on research results from a range of sources, the following recommendations could be made. Intense herbicide pressure can be justified on thistle species which are just establishing on a property. However, once populations have well established seed banks, a better strategy may be to rely mainly on good pasture management techniques to reduce the impact of weeds which are present. Sowing pasture species tolerant of dry conditions is probably the most useful pasture management strategy. Ensuring that these pastures are not overgrazed in summer and early autumn should help reduce establishment of the seedlings each autumn. Grazing paddocks with cattle rather than sheep can avoid overgrazing, and goats can reduce seed production by eating the seed-heads. Farmers may need to be more tolerant of low thistle numbers, and spray only in those years when densities are high. Thus resistance will be less likely to develop, so thistles will still respond to herbicides in those years when control is really necessary.
Plant Protection Quarterly (1996) 11 (Supplement 2) 273-275.