The application of pre-emergent Group B herbicides can lead to root pruning, reduced crop vigour and increased incidence of root disease in cereal crops. Herbicide resistance and changes to the weed spectrum, have also reduced the need for these products. Consequently, products such as Logran and Glean have a declining role in most cropping programmes.
In many parts of southern NSW where soils have received multiple applications of lime, the resulting increase in soil pH slows the breakdown of some Group B herbicides, which in turn leads to increased crop effects due to this delayed breakdown. This is most common where soil pH is greater than 5.5 (CaCl2).
In cereal crops, the increased crop effects particularly from later sowings, are seen as root pruning, reduced vigour, transient micronutrient deficiencies, plus increased root disease such as Rhizoctonia. These crop effects may or may not effect yield, however the reduced crop vigour does decrease crop competition with weeds.
Historically Group B herbicide such as Logran and Glean were widely and effectively used to control a range of grass and broadleaf weeds. Many of these weeds such as ryegrass and Wild Radish have developed (or are developing) resistance to these products. In addition, increased cropping intensity and a focus on winter cleaning pastures has shifted the weed spectrum. Weeds such as Capeweed and Paterson’s Curse are less prevalent. Weeds which are not well controlled by Group B’s such as Sow Thistle, Prickly Lettuce and Fleabane, are more common and are best controlled post emergent.
Given these issues, in most cases there is little benefit to be gained from adding pre-emergent Group B products unless there is a specific reason for doing so.