Adjusting Row Spacing and Seeding Density to Manage Weeds

Technical articles - Centre National d'Agroécologie - Conserwa - Weeds

Field crops Field crops Weed management Sowing techniques No-till sowing

Reduce inter-row spacing to increase crop sowing density in order to intensify competition with weeds. This method aims to close the canopy by the crop as quickly as possible to smother weeds through light competition.

Description

This method is complementary to other agronomic levers for integrated weed management. It has proven effective but is not sufficient on its own and there is a lack of technical and scientific references on this practice in France. Despite its advantages, it also presents risks which will be discussed below.

In this graph, we can see the theoretical effect of crop biomass on weed biomass and yield.

If we increase sowing density (reduced inter-row spacing), the yield increases before stabilizing. A correlation between sowing density and weed biomass is observed.

The challenge is to find which density (inter-row and intra-row spacing) offers the best compromise between competition against weeds and yield while taking into account the increased establishment costs when practicing high densities.

In practice

This involves narrowing the spacing between two rows but slightly increasing the spacing within the row as shown in the diagram, here in the case of corn.

This can be more complicated to implement for cereals depending on the seed drill.

Modifying a seed drill to bring sowing lines closer can be more or less easy depending on the models and investments may be necessary to make these modifications.

Advantages

• Improved competition against weeds: reduction in the number of weed emergences and seeds produced by weeds.
• Rapid closure of the vegetation cover.
• More efficient use/interception of light.
• Reduced evaporation / moisture conservation.
• Lower root competition: more efficient use of water and minerals.
• Better yields on average when crop density is higher.

Disadvantages

• Increased risk of fungal diseases: This is due to moisture retention by a dense vegetation cover. A study showed that the severity of wheat septoria is significantly worsened by high density per hectare (studied conditions: for a density of 35 kg/ha, 8% severity observed whereas for a density of 140 kg/ha it rises to 12%) but not by reduced inter-row spacing (studied conditions: 9 cm and 38 cm). It is therefore important to maintain the same densities per hectare when practicing reduced inter-row spacing. Generally, agronomic practices have less impact on the establishment of septoria epidemic foci compared to location, climate, and variety choice.

• Requires specific varieties: Some varieties may be disadvantaged depending on their phenotype and respond poorly to this management.
• Requires irrigation: This practice may not be suitable for systems under Mediterranean conditions without irrigation.
• Higher establishment costs: Increasing the number of rows requires more power to pull the seed drill and thus higher mechanization costs. If seed density per hectare is increased, an additional cost for seed must be expected.

Impact on weed seed bank (ryegrass)

An Australian study monitored ryegrass populations over a 10-year period (2003-2013, with a rotation composed of wheat, barley, rapeseed, pea and chickpea) testing 4 inter-row spacings each year. The weed control program was the same for each treatment.

For the same rotation and similar weed control program, the trend is very clear: a decrease in seed number when spacing is reduced.

Note: The effect of reducing inter-row spacing is attenuated when the spacing is already small. For example, when reducing spacing from 36 cm to 18 cm, i.e., halving it, a reduction of more than 75% of seeds per m² at harvest is observed. Whereas if halved again to 9 cm spacing, the reduction is only 25%.

Nevertheless, this remains an interesting lever (at least 25% less ryegrass) because it is relatively easy to implement and saves on phytosanitary inputs.

Impact on yield

Most articles on the subject show a trend of increased yields when inter-row spacing is smaller even at equal density on the plot, as demonstrated by DEKALB on a corn variety.

Yield differences are not necessarily significant but the trend is improvement and given the competition such practices offer to weeds, this method seems relevant.

The case of no-till sowing

Reducing inter-row spacing in no-till sowing can bring new technical constraints. Indeed, the "rake" effect on crop residues can be greatly amplified if sowing lines are closer, especially with a tine seed drill.

To limit this effect, it is recommended to arrange the tines diagonally on several beams (ideally 4 or even 5) each spaced 60 cm apart and with a spacing of 20 cm or even 25 cm between each tine to ensure sufficient clearance to limit clogging.

Also pay attention to the positioning of gauge wheels. They must be sufficiently spaced from the tines so as not to block residues.

Perspectives for improvements and associated method

To improve the effectiveness of this practice on weed regulation, it can be combined with the east-west sowing method.

Sources

This article was written by Jasmin Razongles, agronomy engineering student in apprenticeship at the Centre National d'Agroécologie.