What’s the value of summer weed control?

In areas with high evaporation rates over summer, it’s reasonable to think that summer weeds might not significantly affect stored soil moisture. But experience demonstrates otherwise.  

Josh Hollitt, of Hollitt Consulting, based at Port Lincoln on the Eyre Peninsula, South Australia, says timely summer weed control has changed farming on the Peninsula. Now, farmers are storing more soil moisture to benefit crops for multiple seasons following a wet year.

“The problematic summer weeds in this region are mostly C4 plants, which means they can continue to grow under high-stress environmental conditions,” he says. “They also tend towards greater glyphosate tolerance as they mature, which means they must be treated while they are small for the herbicide to be effective.”

To maximise the return on investment in summer weed control, it is critical to have the capacity to do the job on time and under the right conditions. This usually means spraying soon after summer rain events when the weeds are small and actively growing, and the daytime temperature is slightly cooler.

“Different weeds respond to different herbicide mixtures, so it is worth making those decisions with the weed profile in mind rather than using the same ‘brew’ each time,” says Josh. “With lentils now playing an important part in the rotations on the Eyre Peninsula, we need to be particularly mindful of how we manage milk thistle.”

Diversity is the central component of the WeedSmart Big 6 strategy – diversity in farming systems, diversity in herbicide use and diversity in cultural controls combine to keep weed numbers low and reduce the risk of herbicide resistance.

How can you measure the value of summer weed control?

Two specific scenarios in recent years have clearly shown the value of summer weed control on the Eyre Peninsula.

In the 2016–17 summer, one of Josh’s clients at Kimba on the upper Eyre Peninsula missed spraying approximately five hectares on a heavier red-brown soil, and Josh thought the evaporation rate would be the dominant effect on soil moisture across the paddock. When it came to harvest, though, there was a 600 kg/ha reduction in wheat yield where the weeds had been left untreated.

More recently on a farm at Kyancutta, the grower applied two summer sprays, then a boom spray breakdown interrupted the third application in March. When Josh measured soil moisture in the area sprayed on time and the area where the third spray was delayed, there was a 25 mm difference in plant available moisture in April. This translated into a 1.2 t/ha difference in wheat yield.

Following a good season where the soil profile is filled to depth, the benefit of preserving that stored moisture through timely summer weed control can pay dividends over multiple seasons.

How can I get the job done on time, every time?

Having the equipment and staff available is an important first step in covering the cropping area within a few days of a rain event. Several years ago, many growers saw the benefit of having a self-propelled spray rig to start spraying immediately after each paddock was harvested.

Another step that growers can take is to increase their spray capacity in line with their increased cropping program and to find ways to move machinery and chemicals around the farm easily.

A less commonly discussed consideration is the need to have a clear plan and to communicate that plan with everyone involved in the weed management program. People are feeling the pressure of getting their spray applications done at the right time and under the right conditions, and in some years, there is a risk of burnout because they aren’t taking a break. Time to relax with your family is essential and needs to be built into the calculations regarding spray capacity.

I already have glyphosate-resistant summer weeds; what can I do?

We know that many glyphosate-resistant weeds are more susceptible to glyphosate if it is applied when the weeds are small. By targeting this growth stage, growers can drive down the weed seed bank over a few seasons.

Sprayers with weed detection capability give growers extra flexibility to spray more frequently while keeping herbicide costs down. Mixing and rotating herbicide modes of action is also very important for summer weed control, particularly where the number of options is restricted. Residual chemistry certainly has a place in summer weed control programs on the Eyre Peninsula, keeping in mind the potential plant-back restrictions within the crop rotation.

The summer weed spectrum includes a range of species, and different weeds will dominate in different years. It’s necessary to assess the weed types and density, then respond to what’s there – being vigilant to remove survivor weeds and new weed species, like feathertop Rhodes grass.