In winter, when the sun is travelling at a lower angle the additional shading of the inter-row that occurs when crops are sown in rows running east-west can help suppress weeds growing in-crop. This effect is most noticeable in southern latitudes.
In Western Australia, Department of Agriculture and Food researcher Dr Catherine Borger demonstrated in six trials that east-west sowing can halve annual ryegrass weed seed set, even though weed biomass may not be reduced. This is a rare opportunity for free weed control that could be worth implementing in suitable paddocks.
Weeds researcher Dr Catherine Borger measured the difference in light penetration through the crop and weed growth when the crop was sown east-west compared to north-south. Photo: DAFWA
cent weed biomass reduction when crop rows ran east–west compared to north–south. At Trangie the barley varieties Hindmarsh and Granger had higher yield and were more competitive against weeds when sown east–west. In these trials weed biomass was measured rather than weed seed production, however both are good indicators of the weed response to the treatments.
NSW DPI research and development agronomist, Greg Brooke says that in many cases there are other, more practical, ways to suppress weeds but that row orientation is well worth considering and implementing if it can be easily done.
“The more non-herbicide tactics a grower can use to support their herbicide program the better,” he says. “Crop yield is not compromised under east–west sowing.”
Researchers in NSW also noticed a stark difference in the growth of the weed fumitory, with the weed being prolific in the north–south rows and absent in the east–west rows.
Growing crops in rows at right-angles to the sun decreases the amount of solar radiation that the weed plants can intercept and use for photosynthesis. This translates into lower biomass production and potentially decreased seed production.
Importance of early weed control
A weed seedling can impact crop yield potential well before the weeds are large enough to take moisture, light and nutrients away from the crop.
Canadian researchers Dr Clarence Swanton and Dr Jessica Gal observed this effect in both corn and soybean. The weeds emerging alongside or soon after the crop seedlings reduced the crop yield potential, with the effect occurring at the seedling stage, long before the weeds could be competing directly for resources such as moisture, nutrients and light. The differences in light wavelengths bouncing off bare soil compared to growing weeds triggers a reaction in the crop plants to put a disproportionate amount of resources into growing taller, with more leaf area—resulting in a comparatively smaller root system.
This response can pre-condition the crop to a lower yield, particularly in years where the crop may encounter moisture stress and have limited ability to seek deeper stored moisture.
DAFWA researcher Dr Catherine Borger says that crop plants adopt this ‘shade avoidance growth characteristic’ to allow them to more effectively shade weeds as the plants mature and begin directly competing for light.
“Since crop plants in an east-west orientation physically shade weeds at an earlier growth stage, weeds have less opportunity to trigger the shade avoidance growth characteristics,” she says.
The interrow area is shaded earlier in the season when the crop is sown east-west, suppressing weed germination and growth early.
Choosing competitive crops and varieties
“The row orientation is one part of increasing crop competition,” says Mr Brooke. “The other factors to consider are row width, seeding rate and crop and variety selection.”
“The difference in competitiveness between crop types is well known with general rules of thumb such as triticale, barley and rye being more competitive than wheat; and cereals are generally more competitive than pulses and canola,” he says. “What is less well understood are the differences between varieties. In one trial Skipper barley achieved a 30–40 per cent greater weed biomass reduction compared to Hindmarsh barley, however the competitive ability of different varieties varies widely between sites and seasons.”
Generally, varieties that exhibit rapid early growth and early canopy closure will provide better weed suppression than less vigorous varieties of the same crop type. Seeding rate and row spacing also play a part in increasing the crop’s competitiveness.
Choosing a highly competitive cultivar in one year of a three to five year program can have a significant and lasting effect on the weed seed bank. If this is coupled with harvest weed seed control, and possibly east–west sowing, the result could be impressive and achieved at a very low cost.
“Introducing more non-herbicide tactics into the farming program gives crops the advantage over weeds and reduces the pressure on the herbicide,” says Mr Brooke. “Farmers have started using tactics like double-sowing known weed areas such as along irrigation channels. With GPS, double pass seeding automatically gives narrow rows, doubles the seeding rate and makes those key management areas much more competitive.”