Notion de TAD et de labour :levers for grass management
Ploughing can effectively reduce grassy weeds in large crops by relying on a better understanding of the Annual Decay Rate (TAD), an indicator of seed viability loss. Ryegrass, with a TAD of 60%, requires ploughing at least every 4 to 5 years. Effective ploughing must be upright, about 20 cm deep, and done slowly. Proper adjustment of the plough, especially the share, optimizes burying seeds outside the germination zone (top 5 cm).
Concept of TAD
The Annual Decay Rate (TAD) corresponds to the percentage of seeds still capable of germinating from one year to the next. Each year, the number of viable seeds decreases proportionally to the TAD value, expressed as a percentage of seeds in the soil.
For example, if a weed species has a TAD close to 100% (brome), after one year post-seeding, almost all seeds are no longer capable of germinating.
Increasing TAD and controlling grasses
Most weeds germinate in the 0-5 cm soil horizon. Below that, only a few exceptions like wild oat can develop.
Knowing the TAD of different weed species is essential to implement a weed management strategy.
Ploughing is a powerful method to eliminate certain weed species in case of infestation provided that a sufficient time interval between two ploughings is respected, which will be defined based on the TAD.
The lower the TAD, the longer the interval between two ploughings should be. Indeed, one must wait until all seeds buried deep enough (to prevent germination, see figure 1) during the previous ploughing are no longer viable to avoid bringing them back to the surface where they can develop.
In the above diagram, only viable seeds of a species with high TAD are shown just after the type of soil work. In short, the superficial soil layer where most weeds can germinate. In dark shading, the soil part containing viable seeds but not in germination conditions because it is too deep.
The case of ryegrass
Ryegrass is one of the most problematic weeds in large-scale cropping systems in France, notably due to emerging resistances and cycle shifts (see ryegrass biology). It is all the more important to know and update knowledge on its biology to adjust control strategies.
Contrary to previous beliefs, ryegrass has a TAD close to 60% instead of 75%. This results in increasing the interval between two ploughings if one wants to bury all viable seeds on the surface, assuming no major soil inversion/mixing.
It is therefore recommended to plough every 4/5 years to effectively control ryegrass populations, compared to the 3 years previously advised.
Plough adjustment
Depending on the ratio between working depth and furrow width, three types of ploughing are identified:
- The upright ploughing: This is the ploughing where the furrow slices are most vertically inclined. For this, the depth should correspond to 3/4 of the space between shares.
- The flat ploughing: With a depth corresponding to half the furrow width, the slices are more horizontal.
- The intermediate ploughing: corresponds to a midpoint between the two above.
The ideal ploughing
As upright as possible
This avoids placing plant residues at the bottom of the furrow. Flat ploughing favors the formation of a gley zone under wet conditions or a hollow zone in drying soil with organic matter at the furrow bottom, situations that are impassable or poorly penetrable by crop roots.
Generally, especially in wet or drying soils, upright ploughing avoids this pitfall and promotes good crop establishment and development, among other things, so that it quickly competes with weeds.
Medium depth, around 20 cm
Ploughing should not be too deep (maximum 25 cm) to avoid mixing aerobic and anaerobic soil horizons: mixing layers promotes organic matter fermentation at the expense of mineralization. But it must bury surface seeds deep enough to be effective. About 20 cm is recommended for grass weed control.
Slow speed, 4-5 km/h
To allow effective inversion and burying from a weed management perspective, a slow speed is preferred as explained in the diagram below:
Three positions are possible:
- Forward position: ideal for good burying
- Rear position: ideal to limit clogging in presence of many plant debris
- Intermediate position: compromise between burying and clogging
In conclusion
For maximum effectiveness, ploughing should be upright and as shallow as possible. This requires a narrow spacing between shares. Then, proceed slowly with the share in the forward position (if not too much material to bury) to achieve optimal inversion and bury weed seeds deep, outside their germination zone.
Sources
This article was written by Jasmin Razongles, agronomy engineering student in apprenticeship at the Centre National d'Agroécologie.
