While frost on winter crops is often growers’ main concern in August and September, this is also the time when some summer weeds start germinating if conditions are favourable. A spring rainfall event, followed by a week or two of warmer weather, can quickly kick off the season for summer weeds.
Mark Congreve, Consultant with Independent Consultants Australia Network, says fleabane, sowthistle and feathertop Rhodes can all start germinating as early as August in northern regions when temperatures are suitable.
“Establishment at this time of year may result in plants that are large and very difficult to control with knockdowns if control is left until after the busy harvest period,” he says. “Once this happens the only options for control are a robust double-knock herbicide strategy, or tillage.”
The full canopy cover in a dense winter crop generally prevents most germinations within the crop, but these weeds can establish in open crops, in missed rows or wide guess rows, around crop edges or in winter fallows.
Mark suggests that pre-emergent herbicides applied in late winter or early spring fallow, before the first spring storms, can play an important role in managing these early germinations of ‘summer’ weeds, helping create a weed-free winter-spring fallows until it is time to sow a summer crop.
“This is easiest when a paddock has been ear-marked for a specific summer crop,” he says. “Rotation planning is really important – where you know what you will be planting, there are normally one or more options with acceptable plant-back periods for most crop choices. Where you are unsure about what crop will be planted into the paddock, then decisions are more difficult.”
To ‘keep the options open’ growers are restricted to using products with shorter plant-back periods, and therefore less residual control. If using a product with potentially damaging residual activity on subsequent crops, growers are reliant on further rainfall to breakdown the herbicide in the soil prior to summer crop planting.
“In some situations, it may be possible to plant the summer crop any time after the residual is applied in spring,” says Mark. “A good example of this is using Dual®Gold for feathertop Rhodes grass control in paddocks going to sorghum.”
For other combinations of residual herbicides and summer crops a plant-back period may be required. Mark said it is very important to use the label information to determine the level of risk involved in applying a particular product andjudge whether it is safe to plant the summer crop or not.
“Where plant-back periods exist, the breakdown of these herbicides needs a combination of time and soil moisture over the warmer months, so it is important to look at how the rain has fallen, as well as the totals,” he says. “Having the soil surface wet for a few weeks from regular rainfall events during these warmer months will support more microbial breakdown of the herbicide than one storm event that delivered the same quantity of rainfall, followed by weeks of dry weather.”
Ideally, a well-timed spring residual herbicide will keep the fallow clean until the summer crop planting window opens. Assuming the appropriate plant-backs have been met, an effective knock-down herbicide may be needed to remove weeds germinating on the planting rain, should the spring residual herbicide be running out.
The decision around the choice of additional pre-emergent applied at planting will depend upon the length of residual expected from the spring application, the known weed pressure in the field, the availability of inter-row cultivation or post-emergent in-crop herbicide options and the predicted rainfall outlook.
Growers and agronomists interested in learning more about the benefits and risks of pre-emergent herbicides can access a free online course at www.diversityera.com, presented by Mr Congreve and Dr Chris Preston.