ROLLING COVER CROPS - A RICH TECHNIQUE OF OPPORTUNITIES WITH MUCH STILL TO MASTER
Written by Matthieu ARCHAMBEAUD, TCS Magazine No 64 September/October 2011
The research institute Agroscope of Changins in Switzerland (Canton of Vaud) has been conducting a trial since 1969 with four different types of tillage and since 1976 also comparing various forms of fertilization and plant cover. The results that have been learned so far are proving to be a very valuable insight for all farming systems whether conventional, conservation agriculture based systems or organic.
An Idea pioneered in South America, the physical destruction of a cover crop by rolling has developed all around the world and in France. The technique is so promising that we can now see it on all continents, in both in organic and now conservation agriculture systems as well as more conventional agriculture. Unlike the destruction by mulching, a crimper roller doesn’t touch the ground and therefore kills the plant material without disturbing soil, in some situations with or without additional chemistry, and especially without the risk of reviving a fresh germination of weeds.
Conservation of soil cover after rolling keeps the ground protection against heavy rain, erosion, and evaporation losses and extremes in temperature. Finally, keeping the terminated biomass on top of the soil decreases the risk of nitrogen hunger for the next crop, which is often not the case when mulching to destroy the living vegetation. In a high biomass cover the purpose of the crimper roller is to injure the plants in extreme weather conditions of heat or cold so they will not regrow.
The South Americans use the technique in dry and hot conditions, in Europe, it’s the cold, even the frost, which improves efficiency. The technique interests naturally the organic farmers and farmers practicing conservation agriculture. However we are seeing interest now with more trials in conventional agriculture. Indeed, successful termination using a crimper roller on high biomass material can replace mowing or mulching: the rolling becomes a solution which is both economic and relatively simple means of control for most farming systems.
However several conditions must to be brought together to ensure effective destruction of the cover. Firstly, the rolling is generally only effective on a high and developed vegetation. For mechanical termination of either weeds or cover crops a well developed plant will be effectively killed. There are several versions of crimpers which have been imported from South America soon after direct sowing was introduced and both the technique and method of crimping has been researched by technicians of the INRA farm from Mirecourt, Vosges, Eastern France.
The INRA have been looking at multiple effects from cold, heat, pathogens or pests. Secondly, we know that after flowering, the plants are a lot more susceptible to this physical mode of destruction because the plant reserves are then redirected towards reproduction rather than vegetative growth. This sensitivity is necessary for both crimping and rolling in the frost and equally sometimes to mowing or traditional mulching.
Of course, for the plant to reach this critical stage, the cover must be given every opportunity to achieve maximum biomass in good time in order for it to fulfill its role as a cover. Depending on the date of destruction, it is therefore essential to establish the cover crop in good time to allow the crimper to work effectively. For overwintered cover crops with long growing periods, relatively late sowing dates are conceivable. On the other hand, for short term covers preceding a cereal, a very early sowing date is essential.
The frost is an extremely valuable tool in assisting a crimper to work effectively but not essential. It is usually recommended to roll or crimp during a frost for plant species that can survive a normal winter, this will see their tissues sufficiently damaged by the roll, resulting in certain death. Freezing temperatures are much more necessary when the tool being used is not specialized (for example a flat or cambridge roller, which can be too light and not as reliable.) or that the cover is still immature due to a late date of sowing, resistant species and / or a lack of soil fertility.
We find also that the crimping must take place on frozen ground to avoid physical damage to the soil structure and machine. In general, a well-covered soil will not freeze over hard as the high biomass cover itself will serve as a buffer. However the weight of tractors that are used during this period should be both as light as possible and the power requirement will be minimal for this type of operation. When considering a specific crimper roller there are a number of factors which should be considered for success. A simple flat roller will not be as effective as a purpose made cross kill or bar type crimper.
A flat roller has the potential to work efficiently on a white frost when compared to blade crimper. However, in order to achieve an effective kill in a wider range of weather conditions and the stage and type of vegetation, the more specialized the roller will have to be. A large diameter roller allows the operation to act at a higher forward speed with less revolutions but more of blades around the circumference meaning the tool will jump less maintaining stability and an even pressure on the biomass which is very important.
A wide diameter of the rolls also leaves the ability to add a ballast of water or sand according to the weather and the toughness of the canopy (otherwise tractor weights or dual ballasted wheels do just as well). The inclination of the knives compared to the diameter plays an important role. Depending on the angle of attack, the tool will be more or less aggressive: in most cases, a reducing angle will be sufficient to damage the stems but by increasing the angle this will increase the effectiveness of the knives of the crimper on woody stems.
However if the tool is too aggressive in relation to the volume of vegetation this may produce varying results as will an excessive forward speed. The distribution of the blades around the diameter also comes into play when using a “straight” mounted pattern (perpendicular to the direction of travel) promotes vibration and therefore reduces stability; the pinch pressure is more unevenly spread over the entire rolling width. A helical assembly of blades, or more simply a mounting of offset fin blades will bypass this problem.
Choosing suitable species and susceptible varieties.
The last point to be addressed concerns the sensitivity of species and even individual varieties. The question is even more important when the cover sometimes has to be destroyed outside periods of cold, before an autumn sowing or in late spring for example. The tall vertical species like rye will therefore be more easily destroyed than those that remain at ground level; a tillage radish will be easily killed, unlike a turnip or rapeseed, where the underground architecture is protected from cold and rolling. The physiology of the plant and stage of development also play a role, delicate hollow stems being more easily destroyed that those who are full or already lignified.
A green mustard is destroyed easily, which is more the case than a mustard which has finished flowering and whose stem has hardened. The same idea applies to a crimson clover or a bean which are destroyed easily by rolling, as opposed to a red clover or an alfalfa plant that are perennial plants. In the case of cereal plants it is even more interesting; rolling (using a conventional or cambridge roller) of a cereal grass can enhance tillering and therefore the vigor of the plants, while it can be destroyed by a simple pass of a roller (blade crimper roller) at the flowering stage.
Aiming to bruise the cereals stem and not chop it up, the plant is then exhausted and dies trying to feed damaged tillers. If the stem is cut it will try to produce new secondary tillers, as would happen if the plant is cut for silage or grazed. Choosing varieties that have been selected for this role is essential. The importance of variety choice within plant species has been briefly mentioned previously, it is necessary to choose covers who will reach the appropriate stage at the date of destruction required while being neither too advanced (with a risk of running to seed) or neither too immature (making it difficult to destroy by rolling).
In the end, if we want to increase the chances of success, that is, total destruction of the cover without the use of cultivation or a contact herbicide and in addition to not being able to rely on a frost such as in frost-free areas of the west of France. Then choice of species and a dedicated type of crimper roller is paramount. If conditions are not met with the first pass of a crimper, a second pass can be used or even the use complementary reduced doses of herbicide applied at the same time or as a stand-alone operation.
Mechanical destruction of cover crops is an unexploited technique with many opportunities to reduce the need for chemical herbicides. However it does have limits, rolling can also become a powerful tool for plant species management without the need for total destruction of the canopy. A roll passage in the autumn or spring can help suppress vegetation considered too forward and to postpone destruction, mechanical or chemical, until a later time in a season and allow an adaptive approach to mechanical destruction.
For example differences in sensitivity to crimping which is often seen as an inconvenience, can be considered from another angle making the roller a selective weed killer which will eliminate non target plants sensitive to being rolled while retaining the resistant plants. We could consider the destruction of tall
plants (such as a mature cereal) in favour of smaller plants lurking in the bottom of a canopy (rapeseed, clover or alfalfa for example), the weakening of flowering perennials in a fallow or in a meadow without mowing and so without the stem re growing (thistles and dock).
Why not also replace the coulters of the weeder with a crimper roller to ensure localized weeding on or in between the row without touching the ground and maintaining the cover crop as a mulch? Finally the only limit as to the potential uses of plant species are in the mind; many cash crop plants can provide an excellent cover such as; cereal stubble, forage maize/grain maize, sunflower and rapeseed.
Conclusion
Rolling of cover crops seems simple at first, in fact it is not and it’s a complicated set of parameters that determines its success. However, rolling will undoubtedly go from a simple tool of terminating cover crops to a versatile tool capable to reducing certain species and selecting others; selective weeding by mechanical means alone or combining it with a herbicide. There is a huge amount of complexity in this technique still to be explored. However, before rushing to purchase a purpose built machine its worth exploring what is already in your yard.
Combined rolling and chemical weeding
JJérôme Labreuche and his team worked on the synergy between rolling and the application of herbicides (glyphosate with or without 2-4 D) to destroy cover crops. The first trials (2009-2010) focused on the effects on the weeds controlled by rolling on and the subsequent weed populations. However the plots of mustardfaba bean cover developed poorly, they had been killed by the frost and overtaken by volunteer oats and corn. So a second trial (2010-2011) focused on the effectiveness of a combination of rolling and glyphosate on cover species usually difficult to destroy by simple rolling (mustard, undeveloped oats, etc.).
The rolling was carried out during a period of frost with a range of rolls used across the three experimental sites, using cambridge, packer or cross-kill rolls, with different doses of herbicides and a spray application between two hours and several days after rolling. Several interesting points stand out of these tests; the effectiveness of rolling before an herbicide application depends on the sensitivity of the species to glyphosate. The technique is most reliable on sensitive weeds (cereals, oats and lentils) and the results are more consistent. However, with species usually more resistant (established broafleaf weeds, mustard, rapeseed and radish) the results vary.
On volunteer rapeseed rolling followed by 360 g / ha of glyphosate is as effective as 720 g / ha without rolling, ie 1 l /ha of glyphosate saved. In the end, the effectiveness of destruction has been improved by 40% for the broadleaf weeds, from 70 to 99% for cruciferous plants and 95% for crimson clover. Note that for a dose of 360 g / ha of glyphosate (efficiency of 10% on rapeseed), the addition of 480 g / ha of 2-4D is more efficient (70%) than rolling (40%). The time between rolling and spraying is another key element since the
sprays carried out within two hours of rolling is the most effective.
If the spraying is postponed twenty-four hours the level of destruction is reduced by 10 to 15%; if the period is fifteen days, the impact is almost nil. The hypothesis formed for the moment by the Arvalis team is that the injuries inflicted by the rolling improve penetration of the product, by bruising the leaves and the exposure of their lower face. Leaving too long a gap between rolling allows the plant to heal, or if rolling has not been effective, the activity of the he glyphosate is reduced. Last but not least, Arvalis has calculated the cost of rolling at €12 euros ha, 3litres ha of fuel and 20 minutes ha for a 6 meter Cambridge roller.
This cost is not insignificant, nor is it exorbitant and reduces or even eliminates the need for further herbicide applications. To conclude, these tests give very interesting results which will undoubtedly be confirmed and reinforced by the use of more specific rollers on more developed cover crops. The amount of glyphosate used to destroy the cover crop can be reduced while reducing the effect of shading by large leaves when treating a diverse cover mix than would otherwise have to be used. Also rolling improves the overall destruction of all plants meaning a reduced dose can be used on species which would otherwise require a larger rate of herbicide. The growing period for covers can be extended even until sowing time particularly when sowing in the autumn, allowing the technique of roll-sow apply total herbicide to be used.
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