False seedbed, how effective is it?

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

Cereal crops Cereal crops Mechanical weeding

Tractor performing stubble cultivation

False seedbed consists of stimulating weed emergence with shallow soil tillage to destroy them before the crop, thus reducing the seed bank.

Its effectiveness varies depending on the species: relevant for grasses (ryegrass, blackgrass), but of little use for other weeds.

Not very effective in autumn, it is more justified before a spring crop, provided its cost is evaluated (20–35 €/ha).

Effective for stimulating ryegrass/blackgrass emergence

Ryegrass and blackgrass emergence is greater after shallow tillage / false seedbed (here, rotary harrow at 5 cm) than after deep tillage (here, independent disc cultivator at 20 cm).

Weed management during intercrop period: false seedbed put to the test - Perspectives-agricoles.com 2019

Effectiveness is not the same for all weeds

No soil tillage results in better emergence of geranium or oilseed rape regrowth during intercrop compared to a false seedbed

False seedbed practice only promotes grass emergence. If dormancy is not broken, the false seedbed technique is totally ineffective. Seed dormancy periods are specific to each weed species. It is very low for autumn grasses (ryegrass, blackgrass, brome) especially if temperatures were high during seed maturation (see article on ryegrass dormancy)

What about weed emergence during the crop?

False seedbed is relevant if a reduction in weed infestation is observed in the following crop.

Considering all trials, no positive correlation (scattered points) is observed between the number of weed emergences during intercrop and the number of weeds in the following crop.

False seedbed certainly stimulates grass emergence and thus reduces the seed bank, but this does not guarantee a reduction in weed infestation afterwards.

This phenomenon is partly explained by the difference in magnitude between weeds emerging during intercrop (24 to 1600 plants/m²) and the number of seeds returned to the soil during a season (23,000 seeds/m²). It is easy to imagine that the number of seeds eliminated by false seedbed represents only a tiny fraction of the weed seed bank in the soil.

One solution would be to repeat several successive false seedbeds to exhaust the seed bank, especially in particularly weedy fields. However, seed bank reduction is nuanced by Merfield (2015) who estimates the potential impact of a false seedbed at 0.7% of the seed bank, assuming that 10% of seeds are non-dormant at any time and that only 7% of these are in the top 3 cm of soil and thus in conditions to germinate. If false seedbeds are irrigated (40 mm/week) seed bank reduction is much more effective, 40% reduction with four successive false seedbeds versus 3% without irrigation.

False seedbed before spring crop

Spring false seedbed is fully justified, it allows a real reduction in weed pressure in the crop as shown in the graph below:

ECOHERBI trial

No clear difference in effectiveness is observed between the different treatments.

False seedbeds reduce weeds in the crop by a factor of 4 compared to no false seedbed.

What about the cost of a false seedbed?

A pass with an disc cultivator costs about 25 €/ha (varies between 20 and 35 €/ha depending on the model). This practice has a fairly significant cost when considering its effectiveness on certain weed groups.

Costs of crop operations

Conclusions

Autumn false seedbed is rarely justified because its effectiveness is low compared to the costs involved, especially if the field is particularly infested with grasses (ryegrass, blackgrass).

Before spring/summer crops, false seedbeds are relevant because their effect on reducing weed infestation is demonstrated.

In all cases, this practice allows reducing the soil seed bank but this does not necessarily have a direct impact on weed infestation in the following crop.

Sources

La version initiale de cet article a été rédigée par Jasmin Razongles.