Herbicide plant-back is a topic of consideration for most Wimmera and Mallee growers during this sowing preparation period. The dry spring and summer we have experienced has had an impact on the residual herbicide breakdown, hence why this has generated the herbicide plant-back consideration for 2013.
Many herbicides remain in the soil from one year to the next. With reliance upon group B (imidazolinones) herbicides increasing, along with the common use of Lontrel® and Balance®, it is expected that these herbicides will have left a residual effect carrying over from 2012. These herbicides exhibit the ability to control future weed emergence, however, if conditions are not suitable they can also negatively impact the subsequent crop. Group B herbicides are broken down in the soil by microbes in wet, aerobic conditions. In conditions such as soils with low organic matter, non-wetting sands, waterlogging and prolonged dry periods, soil residues will persist and can affect susceptible fallow crops.
Growers need to take note of when they sprayed certain chemicals, consider the subsequent crops in each paddock, and adhere to the herbicide labels. It is also important to consider soil types. Rainfall plays a part in herbicide plant-back and the above figure (Figure 1.) shows the rainfall that has fallen in towns across the Wimmera and Mallee.
Applying OnDuty® at a 20g/ha rate requires 150mm of rain from the date of spraying to sowing for chickpeas, faba beans, field peas, lucerne, lupins, vetch and cereals (except varieties with Clearfield technology, all with a plant-back period of 8 months). Assuming the application of OnDuty was at the start of July (green column) then Hopetoun, Sea Lake and Tooleybuc may be restricted by residuals if there is not adequate rainfall between now and the time of sowing.
Applying Lontrel® at a rate of 300ml/ha requires a plant back interval of nine months for clover, chickpea, faba bean, field pea, lentils, lupins and vetch. It also requires a minimum of 25mm of rainfall in the post-harvest-autumn period; this has to be followed by at least one week where the top 10cm of soil is moist. A similar rainfall is required for wheat, barley, oats and canola however, the plant-back period is only one week. All of the above locations have received the 25mm rainfall, but growers need to be mindful that residues may still be present due to the moist topsoil requirement.
With interest in helping growers understand this issue, BCG has begun a demonstration at the 2013 main research site, located on the Birchip-Sea Lake Rd at Watchupga East. The aim of the demonstration is to simulate the effects of herbicide residuals on subsequent crops after a dry summer. The BCG plant-back trial will demonstrate the residual effect on fourteen crops/varieties and eighteen herbicide treatments.
Herbicides being tested in this demonstration include Ally®, Atrazine®, Balance, Ester 680®, Lontrel and On-Duty, each of which were applied at a quarter, half and full rate on February 28. There has been no rainfall since the treatments were applied.
The BCG team hopes to develop results which exploit plant-back issues while demonstrating that herbicide labels need to be fully understood and the likelihood of issues occurring can be minimised through careful planning.
