By Esther Hegt and Dr. Pieter B. Pelser ( University of Canterbury- Biological Sciences, Christchurch, New Zealand). Thanks to Lex van de Weerd, grasslandspecialist (Barenbrug Holland b.v.) for his advice.
Common ragwort (Jacobaea vulgaris) is a common weed native to Europe, and north and western Asia. It usually grows in places where the original vegetation has been disturbed, in nutrient poor pastures, and areas that have been recently transformed into nature preserves. This species, just like all other ragworts in western Europe, contains pyrrolizidine alkaloids of the senecionine type. Upon ingestion, these chemical compounds are poisonous to most vertebrates and insects (1, 2, 3). When growing in pastures that are used for grazing or hay production, ragwort species are therefore a potential health hazard to cattle and horses. Although horses usually don’t feed on living ragworts, dried plants are not recognized as poisonous (3, 5, 6, 7, 8) and alkaloid poisoning may therefore occur when ragwort ends up in hay that is meant for consumption (4).
Common ragwort is difficult to control
Once common ragwort has established itself in a horse pasture it is very hard to control the plant. It is, for instance, not easy to remove the plant manually. To be effective, this requires removing the entire plant, including all its roots, because even from small root fragments new plants can sprout. Manual removal is therefore generally only effective when fighting seedlings and rosettes, because larger plants are often too deeply rooted to be removed without leaving root fragments behind. An additional disadvantage of pulling ragwort is that this disturbs the soil, which may result in the surfacing and subsequent germination of tansy ragwort seeds that may be present in the seed bank.
Furthermore, soil disturbance creates a suitable environment for incoming common ragwort seeds to germinate. Another form of ragwort control is preventing the plant from producing seeds. Because ragwort is a plant with a short life span, it is sometimes thought that if the ragwort cannot propagate itself, it will eventually disappear from a field. Unfortunately, also this method is not always successful. In a natural situation common ragwort dies after flowering and subsequent seed production (7, 8, 9). Removal of the inflorescence by mowing or cutting, however, may cause the plant to sprout again the same of following year, prolonging its life span (7, 8, 9).
Using herbicides to control ragwort is also no guarantee for success. The products that are frequently used are absorbed by the leaves and are subsequently transported to the other parts of the plant. This, due to the large size of full-grown common ragwort plants, might take some time (10, 11, 12, 13) and is often only effective when the plants are still seedlings or rosettes. When applying herbicides, it is important to make sure that the plants are not under stress (for example, by drought or extreme temperatures). It is therefore best to apply herbicides in favorable weather conditions when the plants are actively growing. Selective herbicides such as 2.4.D and MCPA do not kill grass and are therefore preferable.
The best defense is a good offence
Because ragwort is hard to control once it has established itself in a horse pasture, it is important to prevent the plant from settling. Maintaining a dense layer of grasses and other ground covering plants is in this respect a good strategy, because this reduces the chances that ragwort seeds will germinate.
Unfortunately, it is not always easy to maintain such a dense layer of vegetation in a horse pasture, because horses damage the ground cover by running, playing, and grazing. Good field management is therefore of utmost importance (14, 15, 16, 17). Because grass only grows well when soil fertility levels are up to standard, it is important to know how fertile the soil of your horse pasture is. This information can be obtained from a company that provides fertilization advice based on soil samples of your pasture. Proper fertilization is not only a good method to reduce the chances that ragwort will establish itself in a field, it also allows grasses to compete better with other weed species and prevents erosion (14, 15, 16, 17).
In addition to maintaining soil fertility levels, it is important to make sure there is a good mixture of grass species. Different grasses have different characteristics. For instance, some grasses grow predominantly upwards whereas others grow, through offshoots, in a more horizontal direction. Grasses also vary in their potential to regenerate after damage. Sowing the right combination of grass species is therefore important to obtain a strong and resilient grass layer (18). Unfortunately, the number of grass species that can be used in a pasture is rather limited, because not all grasses are suitable for horses. Some grasses contain too much fructane, which increases the risk of founder (15, 17, 19, 20, 21). Special grass seed mixtures are, however, available for horse pastures.
Rotating grazing is a good principle
Even in fields where the soil is fertile and there is a good mixture of grasses, horses will damage the grass layer. This is especially a problem when there is only a small area available per horse, because in such fields the risks of over grazing and soil disturbance are higher. The grass simply doesn’t get the time to recover. This can be prevented by dividing the horse field in compartments. In this way, the horses can be moved to another part of the field when the grass becomes too short or when the soil is damaged: rotating grazing. This will give the damaged part of the field some time to recover. It is recommended to make sure that the grass does not get shorter than 5-6 cm. As soon as the grass has grown back to 15-20 cm it can be used for grazing again. It is best to cover the damaged parts of a field with old hay or manure. This will give new grass plants the chance to grow and prevents the germination of weeds such as ragworts.
Another strategy to prevent over grazing and soil disturbance is to limit grazing to only a few hours per day. This method works best in combination with rotating grazing, because even with moderate grazing grass needs a few weeks to fully recover from grazing.
Common ragwort is a common and native plant in the Netherlands. In the past decades, this plant has become more abundant in especially the northern and eastern parts of our country. This relatively recent expansion in an area where this plant is not yet well known by people, the danger that ragwort plant can form for horses when it ends up in hay, and the fact that Common ragwort does well in horse fields and other man-made environments, are causing concern among horse keepers. It is, however, impossible to exterminate Common ragwort in the Netherlands.
Because Common ragwort predominantly grows in places where the natural vegetation is damaged or removed, it will be impossible to make sure that this species becomes less common, let alone that it will disappear from the landscape, as long as the large-scale construction of infrastructure, houses, and industrial areas continues and agricultural land is still being converted into nature preserves. In our opinion, the solution to the ‘ragwort problem’ is to provide accurate information about Common ragwort and introducing proper legislation. It is, for instance, of the utmost importance to monitor the origin and composition of hay to make sure that ragwort-contaminated hay will be sold as food for horses. In the absence of effective methods to remove Common ragwort from a pasture, field management is in our opinion a good way to reduce the nuisance that this species can cause.
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