Can increasing wheat sowing rate maintain yield and suppress weeds in low rainfall areas

Crop competition is one of the cheapest and most broad-spectrum weed control tactics broadacre grain growers can deploy without compromising yield. There are six ways to give the crop a competitive edge – the easiest and often the cheapest is to increase the crop density through a higher sowing rate.

Maurie Street, a research agronomist and CEO of Grain Orana Alliance (GOA), says many growers in the central western grains regions of NSW and other drier cropping areas may lose wheat yield if the plant density is below the recommended range.

“In drier cropping regions, there is a common perception that establishing higher density crops comes at a risk of the crop haying off if the end of the season is hot and dry, losing yield and increasing screenings,” he says. “Many growers choose lower than recommended sowing rates to avoid this perceived risk. However, an unintended consequence of this practice is increased weed establishment and seed set, including problematic weeds like fleabane, milk thistle and ryegrass.”

“Plenty of research shows that increased crop density suppresses weed numbers and seed production. GOA was concerned that growers in the Central West were missing out on the benefits of crop competition, relying more on herbicides and potentially missing out on wheat yield.”

To test the theory, GOA conducted wheat plant density trials with support from the Grains Research and Development Corporation from 2018 to 2024. This period included the end of the 2017–19 drought when late breaks and low stored moisture were common.

“Crop competition is hard to see and hard to measure. Research conducted in a wide variety of environments, crops and soil types repeatedly demonstrated that increasing crop density maintained or increased grain yield and grain quality whilst suppressing weeds,” says Maurie. “When crop competition is coupled with the other WeedSmart Big 6 tactics, weeds no longer call the shots, even in low rainfall regions.”

What effect did increasing plant density have on wheat yield and screenings?

The 12 trials included 15 common mid-season wheat varieties, tested at a range of crop densities to measure the impact on crop yield and quality. At each trial site, all varieties were sown on the same date.

Four plant densities (from the region’s lowest common density to a non-commercial high density) were tested in lower rainfall environments where hotter and drier grain-filling conditions are more common.

Analysis of the data showed that in 99 per cent of yield and 97 per cent of screenings comparisons, moving from the lowest plant density to the next highest had no negative impact, for example, a change from 60 to 100 plants/m² or from 30 up to 70 plants/m².  Conversely, yield loss was common at the low crop density of 30 plants/m².

There were cases where yields did decline when crop density increased, mainly in extreme circumstances, such as in the 2018 trials that were sown very late (5 July) with little to no subsoil moisture. Even under these circumstances, the negative impacts were marginal, and the poorest result was not necessarily from the highest crop density.

This result should give growers the confidence to move towards the higher target plant populations that NSW DPIRD recommend for wheat in lower rainfall environments, i.e. 70–90 plants/m².

Did higher plant populations create a more competitive crop?

The trials were maintained weed-free, so the research team used a handheld normalised differential vegetation index (NDVI) meter to compare the competitiveness of each variety x plant density treatment across the 12 trials.

In all cases, increasing plant populations increased the vegetative index (VI), suggesting increased competitiveness. Different varieties had different responses to plant density, and some varieties may provide better weed competition earlier in the crop cycle when weeds first establish and have the greatest impact on the crop.

A clear learning from this set of 12 trials is that growers should increase sowing rates to establish 70+ plants/m², even in lower rainfall years.

When calculating the sowing rate (kg/ha) for a target plant density (plants/m²), consider the seed weight and germination rate of the planting seed.

Can I increase crop competition without financial risk?

In low rainfall regions, profit margins are often slim and understandably, growers are often reluctant to spend money on more expensive or complex weed control options. Low crop density has been seen as a risk management strategy to protect yield. However, this research suggests that increasing crop density will give a ‘free kick’ for weed control without losing yield. Moving up from the lowest crop density often resulted in higher yields and less screenings.

Increasing sowing rates is easy, but it does incur some additional costs for seed, seed dressing, seed storage and transport. The sowing rate can be adjusted to suit the weed pressure in each paddock, and is much easier and cheaper to adopt than altering row spacing.

Higher crop density through increased sowing rate is a proven tool to drive down the weed seed bank and to keep it low, reducing weed control costs in the current and future seasons.

WeedSmart promotes six tactics that growers can use to increase crop competitiveness – higher sowing rate, narrow row spacing, east-west orientation, vigorous crop traits, minimising soil constraints, and sowing early in the recommended window.

In most situations, several competitive tactics can be adopted for an additive effect. As a rule of thumb, competitive crops can halve weed seed production.

GRDC Crop Update paper :  Does increasing wheat sowing rates to combat weeds lead to yiled or quality instability in the low rainfall regions of Central West NSW 

Related GRDC Update paper : Increased wheat plant population: the interaction with variety Fusarium crown rot and nitrogen 2025

Online course : Crop competition 101