Winter crop profits hinge on effective summer weed management

A weed-free summer fallow conserves soil moisture and nitrogen, doubling the yield potential for the following winter crops in Central NSW. In fact, complete control of summer weeds is the main driver of grain farm profitability. 

Colin McMaster, research agronomist with NSW DPIRD says capitalising on summer rainfall replenishes both soil moisture at depth and nitrogen levels, effectively ‘buying a spring’.

“Complete summer weed control translated into the best possible gross margin for winter crops,” he says. “Missing the first flush of weeds after harvest or spraying 24 days after rain instead of the recommended 10 days, reduces gross margins by around 40 to 50 per cent compared to achieving complete weed control. Leaving summer weeds uncontrolled resulted in a negative gross margin, averaged across four sites and three years.”

In these trials, the protocol for complete weed control was to apply herbicide approximately 10 days after every rainfall event over 20 mm between harvest and planting. The nil-control treatment involved no summer sprays other than a knockdown just before sowing.

“When application costs of $12/ha are included in the analysis, the average return on every dollar invested in complete summer weed control is $4.80, using 2025 input costs and grain values,” says Colin. “This result might be further improved if some or all sprays were applied using optical spot-spraying technology in low weed density fields.”

For maximum effect on summer weeds, implement tactics from the ‘spray effectively’ pillar in the WeedSmart Big 6 strategy. These include spraying small actively growing weeds soon after rainfall and following spray application guidelines to ensure the herbicide hits the target.

What supports the extra 50 per cent grain yield?

Controlling summer weeds resulted in crops being planted into a soil profile (measured to a depth of 1.2 m) with an additional 22 to 85 mm of plant-available water (PAW), compared to the nil weed control treatment.

As a result of the higher soil moisture levels, an extra 31 to 70 kg N/ha was mineralised and available for plant uptake, compared to the nil weed control treatment. The combination of increased moisture and higher nitrogen levels gave the crops the best possible start.

Ample soil water and nitrogen boost grain yield through increased grain number (more tillers and more grains per head) and grain size.

Making the most of the benefit from stored soil moisture relies on excellent crop agronomy – optimal sowing time, row spacing, variety choice and so on – all of which also underpin a truly competitive crop able to suppress in-crop weeds.

Does the season make a difference to the benefit from summer weed control?

Weeds transpire soil moisture from a depth of 1.2 to 1.5 m, which would be otherwise protected from evaporative losses. This is the moisture that winter crops need to fill grain in spring.

The impact of summer weed control on crop yield was always positive, but the magnitude varies according to when the summer rain falls. If there is high rainfall late in the fallow period, from February to April, the evaporative losses are less than at the peak of summer. This means more water will be available to the crop at planting, regardless of weed control efforts.

If the summer rain events are sufficient to germinate weeds, but the moisture does not infiltrate past the evaporation zone (20 to 30 cm depth), most will be lost to evaporation if there is no follow-up rain to push it deeper. Retaining stubble can improve infiltration rates and reduce evaporation, but it must be coupled with summer weed control to preserve stored soil moisture at depth.

Saving money plus making money equals a great return on investment

Controlling summer weeds effectively and efficiently saves money by reducing nitrogen inputs, and makes money by increasing crop yield, compared to poor summer weed control.

The saving in nitrogen fertiliser comes from minimising weed biomass decay, which ties up nitrogen, combined with enhanced nitrogen mineralisation in moist soils with high microbial activity. As a rule of thumb, every mm of water lost through summer weed growth costs 0.67 kg N/ha.

Allowing summer weeds to grow unchecked effectively tied up 31 to 70 kg N/ha compared to the complete weed control treatment. Replacing this with urea fertiliser would cost $120/ha to $268/ha (assuming urea at $880/t and 50% nutrient efficiency).

To make money, complete weed control effectively doubles crop yield compared to the nil weed control treatment. The longer weeds persist, the greater the reduction in grain yield. These trials highlighted the importance of controlling the first weeds to germinate after harvest. This is because those early weeds are more difficult to kill later in the season and are more likely to set seed.

Controlling young weeds is cost-effective and reduces the risk of herbicide resistance in weed populations. A delay in spray timing after rainfall has less impact than allowing those first weeds to survive.

Leaving summer weeds unchecked resulted in a gross margin loss of $35/ha, due to reduced yield, even though there was no weed-control expenditure. In contrast, every dollar spent on full summer weed control earned $4.80 and resulted in a gross margin of $341/ha.

Using optical spot spraying ‘green-on-brown’ technology to control summer weeds could further improve the return on investment for growers.