With showers and storms across the cotton growing valleys recently, it's important to monitor fields and fallows for summer weeds.

Trial results from Colin McMaster from NSW DPI provide some sobering facts around the benefits of summer weed control.

Colin found that summer weeds can remove valuable soil moisture down to 1.2m and deeper. And for every millimetre of moisture lost, mineral nitrogen is also depleted (for example, saving 75mm of moisture in the profile can provide 50kg of nitrogen/ha to the following winter crop).

It doesn't just make good agronomic sense. If you put a dollar figure on it: for every $1 spent on summer weed control you can expect up to $8/ha return in the following crop.

Summer weeds also act as hosts for pests and diseases in cotton crops. With an expected La Nina (waiting, waiting..) summer weed control will be important. We can expect to see increased germinations over the summer months if the predicted wet conditions eventuate. Selecting the right herbicide and adding a mix partner such as a group G can add extra punch, especially when chasing broadleaf weeds.

So what can you do to help stop summer weeds? 

Be on time: 

Timeliness is the key for all summer weed control. A few days can make a big difference to the condition of the target weed. Often growers have a narrow window of 5-7 days before the weeds are stressed and the efficacy of the herbicide is compromised.

With any spray application, a small proportion of fine droplets remain in the air for a period of time. This can cause major problems over the summer period, especially when a widespread rainfront occurs. As a result all growers will be spraying summer weeds in the same two-week window. This can lead to low levels of herbicide in the atmosphere across a large area, sometimes called ‘community drift’. We can reduce the amount of spray droplets in the air by changing to very coarse nozzles: effectively reducing the potential of off target movement.

Consider your set up:

The correct set up of your boom - especially nozzle selection, boom height and travel speed - are key ways to reduce herbicide levels in the atmosphere. Most importantly, avoid potential inversion periods just prior to sunset and sunrise. Cotton Australia has developed a range of resources for growers to access when considering summer weed control options. The Nufarm spray wise clock is also a handy reference tool (see here). 

Many of the problematic summer weeds require an understanding of their lifecycle and ecology to target the weak links for maximum control. CottonInfo has produced fact sheets for three of the hardest to control weeds: feathertop Rhodes grass, windmill grass and sow thistle

Map your fields:

To reduce the risk of off target damage it is important to know what your neighbours are growing. SataCrop is up and running again this season. It provides imagery of farming regions overlain with sensitive winter and summer crops, eg: chickpeas, sorghum, cotton and viticulture. This provides a more comprehensive view of the crop types that are in the fields in all farming valleys. Crops are colour coded which helps to inform users about where sensitive crops are when planning spray programs.

Know your 2,4-D requirements:

A reminder for growers of the requirements for 2,4-D application and the risks posed to off target species. In an effort to combat the high incidence of off target damage from phenoxy herbicides, especially 2,4-D, the Australian Pesticides and Veterinary Medicines Authority (APVMA) conducted a review in 2018 and introduced new requirements for growers in 2019.

As of 1 October 2019, the new 2,4-D label instructions came into effect and old labels have been suspended. Users of 2,4-D must comply with the new label instructions, even if they are using products with the old labels.

These changes affect about 220 products, and the new instructions for use include:

a requirement not to spray in inversion conditions and additional information on recognising inversion conditions
downwind mandatory no spray zones for both aquatic and terrestrial off target vegetation (including sensitive crops, gardens, landscaping vegetation, protected native vegetation or protected animal habitat)
a requirement to use nozzles producing droplets no smaller than the VERY COARSE spray quality category
mandatory record keeping requirements, and
advisory statements about spray application over summer.

These requirements do not change or restrict other aspects of the currently approved use patterns and should not affect availability of the product.

For more information, visit the APVMA website

Remember: if we can reduce spray drift we will increase spray efficacy as more of the product is hitting the target.

Use double knock to control survivors: 

Having a good understanding of the herbicide resistance status of summer weeds is important. It's no-good throwing herbicides at weeds if you already have resistant individuals in the population to that mode of action.

Consider the WeedSmart summer BIG 6 as part of best practice. Always look for opportunities for a double knock. The idea of controlling weeds with more than one tactic is referred to as a ‘double knock’. Traditionally this has been an application of glyphosate followed 7-10 days later with paraquat. The double knock strategy could also include harvest weed seed management, spray grazing or cultivation as the second knock. The important thing is that all survivors are controlled. This is the cornerstone of the cotton industry’s Herbicide Resistance Management Strategy (HRMS): 2+2&0 survivors.

Glyphosate is a key component of weed control in cotton farming systems. Recent weed surveys have identified consistently high levels of glyphosate resistance in summer weeds such as feathertop Rhodes grass, windmill grass, barnyard grass, fleabane and sowthistle. The double knock is aimed at protecting glyphosate to ensure we maintain its effectiveness for years to come.

Reported resistance to the traditional glyphosate/paraquat double knock in tall fleabane in cotton farming systems has the potential to derail effective weed control in hard to kill weeds. With the widespread use of paraquat as the double knock partner in summer fallow sprays, it highlights just how important it is to ensure that all plants that receive a double knock of ANY herbicide do not set seed. If we lose efficacy with paraquat the options for controlling problem weeds are quite limiting. It reinforces the importance of the message delivered through the HRMS 2+2 & 0 survivors.

For more information, contact our CottonInfo Weed Management Tech Lead Eric Koetz.