Catsear : a poisonous plant

Catsear (Hypochoeris radicata) is an extremely common perennial herbaceous plant, with yellow flowers from the daisy family (Asteraceae), and from the same family as the dandelion.

Leaves

Arranged in a low-lying rosette the leaves are :

• Simple
• With multiple lobes : each half leaf has several rounded lobes
• Hispid : Sparsely covered in hairs on both sides.

Their size can vary from a few centimetres to a couple of dozen centimetres in length.

Stems

Catsear has multiple branching flower stems topped by a capitulum (inflorescence resembling a single flower, but in fact composed of numerous tightly packed florets). The stems do not have leaves, but sometimes have scales. They can grow to between 30 to 70 cms in height.

Flowers

The flowers are yellow, and arranged in 20 to 40mm wide capitula at the end of each branch.

The involucre (part surrounding the base of the flower) is long. It is composed of long, hispid bracts, pointed at the extremity and irregularly distributed. Bracts are a sort of « small intermediary leaf » between the real leaves and the petals, they are present around the flower, which they partly encompass before flowering. The florets are longer than the bracts.

Fruit

Catsear generally appears all through the warmer weather, from the end of Spring/ beginning of Summer (May/June) to the end of Summer/ beginning of Autumn (September/October), especially after a dry period.

It is a perennial plant, and therefore survives through the winter as a rosette of low lying leaves, before flowering again in the following Spring.

                                         
The deep taproot system of catsear, enables it to resist particularly dry summers, as opposed to annuals which will suffer and dry up.

Horses will usually ignore the plant, but in dry conditions it may become attractive when grass becomes scarce. Some horses then start to actively seek it out and to eat the flowers and stems, and even in some cases the leaves. It appears the plant has an addictive effect, as once a horse has tasted the plant, they come back all the more readily, and become « addicted ».

Australian stringhalt

It is solely ingesting catsear which is toxic to horses. Ingesting the plant is one of the factors involved in the appearance of Australian stringhalt, an atypical movement caracterised by involuntary hyperflexion of one or both hind legs. France saw an increase in cases of this afflictionin 2003, year when a heatwave and subsequent dry period saw increased development of catsear. The common factor in the outbreak of Australian stringhalt, was the presence of the plant in the pastures. The toxic principle of the plant is still unknown, but the aetiology of the Australian stringhalt has shown a link between ingesting the plant, and the appearance of the affliction.

This starts off with the horse having difficulty giving its hind feet, turning, moving backwards, and loading in a trailer for example. In extreme cases the horse moves like a rabbit with its hind legs. (see fact sheet on Australian stringhalt)

Prevention measures remain the best way of limiting the development/ proliferation of catsear. This means good pasture management, and good upkeep of land for grazing and or for production of hay.

Good pasture management

This entails preserving the quality of grazing land so that catsear does not have the chance to develop. When the grass is thick and of good quality, weeds like catsear do not develop as readily as when the cover of grass is sparse, therefore the following measures are recommended :

• Avoid overgrazing, by reducing the grazing load (number of horses/ ha) and increase the pasture’s resting time.
• Restrict grazing in dry conditions
• Cut the roughs (places the horses do not graze) so as to promote better regrowth of grass.

Good upkeep of fields

• Over sow with a grass mix in places where the cover is sparse,a field can even be ploughed and replanted when it is too damaged (> 20 % of the surface with a poor cover of grass, or a low ratio of grass/ weeds)

• Chemical control : when the area is invaded with catsear, a chemical weedkiller for dicotyledons can be used.
  Localised spraying plant by plant works best, as this restricts the use of weedkillers to the areas or to the plants to be treated.

• Eliminate any remaining waste after treatment, and do not place on the compost or on the manure heap. This could lead to disseminating any seeds or fruits of the weeds when fertilising. The best course of action is to burn them.