Horseshoe Vetch:  the Facts

1) The scientific name of this plant is Hippocrepis comosa
2) Its appearance depends on its habitat, sometimes forming upright clumps and at other times sending prostrate leafy runners over extensive areas of the downs
3) Its common name comes from its horseshoe-like pods (at least this is the popular book rendition: it looks more like a hoe-seed-drill that would be hauled by a horse to me!?)
4) Its small yellow flowers are in flower for a period of two weeks in May
5) It could be confused with the Bird's Foot Trefoil, Lotus sp. that also has small yellow flowers.
6) It is a long lived perennial legume. At least 40 years has been suggested.
7) It has a variable relatively low and sometimes negligible seed production, but seedlings is the main method of expanding its range. It seeds on a minimum of bare chalk/mosses/shallow soil and loses out in competition in lightly disturbed soil.
8) It has a low germination rate in the wild, although this can be improved in nurseries.
9) It is the exclusive food plant of the Chalkhill Blue and Adonis Blue Butterfly caterpillars and a probable food plant for the caterpillars of Dingy Skippers.
10) In England, Horseshoe Vetch is a calciole, found only on chalk and limestone.
11) The prostrate downland tetraploid race of Hippocrepis comosa is not harmed by moderately heavy sheep grazing and is resistant to moderate trampling, but doesn't persist after ploughing or disturbance of the ground, or in areas grazed by cattle (in the medium term, it disappears quickly or after several years depending on grazing intensity).
12) Horseshoe Vetch has a symbiotic relationship with certain soil bacteria, these bacteria form nodules on the roots and fix atmospheric nitrogen. Some of this nitrogen is utilized by the growing plant but some can also be used by other plants growing nearby
13) Horseshoe Vetches are loaded with a toxin called 3-nitropropionic acid (3NPA). Apart from blue butterfly caterpillars of the Chalkhill Blue and Adonis Blue, very few things actually eat it. On the south Gower cliffs it is avoided by sheep, ponies and rabbits. The extent to which the ruminant digestive system of cattle can deal with this toxin is debatable (one research paper indicates that cattle shows signs of poisoning after ingesting a few grams of 3NPA). With large numbers of cattle, trampling could be a problem, but eutrophication would probably be a more significant one. Hippocrepis comosa is a nitrogen-fixing legume and prefers soils that are deficient in nitrogen. Addition of fertilisers, deliberately or through run-off would have a big effect (1) by stimulating the growth of course grasses and (2) by decreasing the competitive advantage of nitrogen fixation in the Hippocrepis population.
14) Populations of Horseshoe Vetch supporting Chalkhill Blue Butterflies are on long standing (at least a century*) ungrazed meadows, quarries, edge of paths and wasteland.
15) Only Horseshoe Vetch in southern England and the midlands on chalk supports the Chalkhill Blue Butterfly and its larvae. (The Gower in south Wales and Yorkshire Horseshoe Vetch do not support the Chalkhill Blues because of adverse climatic conditions, i.e. too cold.)
16) Horseshoe Vetch is problematical to sow directly into the ground from seed and re-establishment of a colony from local provenance seed is best attempted by planting individual plants (grown from chipped seed) with no guarantee of success within 20 years and even less likelihood that the new plants will be used by significant populations of Chalkhill Blue Butterflies within 50 years.

(* On Mill Hill, cattle grazing occurred briefly on the upper part in 1947. Sixty years on in 2007, the land contains Horseshoe Vetch, but it has not recovered properly. It has not been colonised by the Chalkhill Blues either.)
 

by Gillian M Fearn (Dept. of Botany, University of Sheffield)

Text is a description of the plant and briefly its bionomics and habitat.

Reproduction is mostly from seed, 40 year life span is not confirmed. At least 40 years seems about right.

Prostrate habit is "apparently" adapted in response to grazing. (Observational evidence indicates this is highly unlikely)

Intolerant of deep shade on north-facing slopes. Flourished in phosphorous deficient slopes and to a certain extent in nitrogen deficient slopes.

It seems it is the euthrophication and trampling effect of cows that makes them incompatible.

Seed production is sometimes prevented by grazing animals.

Look for wild seedlings in May and June.

Chipped seeds gives 100% (? in practice by private growers where 20% seems more common ?) success rate for germination, unchipped seed only 5%.

Private researchers have found vegetative reproduction by a simple test. This is not mentioned in my source. This may explain the different types, prostrate etc.

More Information

In England, Horseshoe Vetch is a calciole, found only on chalk, limestone and gypsum.
 

Horseshoe Vetch is not really a grassland plant. It comes after the mosses and in ecological succession on bare chalk and the grasses only come when the soil and  nutrition levels increase and then the succession favours the diminution of Horseshoe Vetch until the land becomes too fertile and it disappears entirely.

Its range is from bare chalk with mosses (in chalk pits and chalk cliffs inland) through original chalk herbland* (maximum amounts) and then a diminution on extensive sheep pastures and Rights of Way and wasteland and inaccessible (to livestock) hill tops, to occasional clumps in wildlife meadows, rare on conservation cattle pasture and a complete disappearance on modern agricultural pastures and ploughed land.

* It is interesting to note that the clumps of Horseshoe Vetch do not grow from the bare soil created by moles and rabbits on the Shoreham Bank but from the chalky outcrops when they are mossed over. The bare bits are either uncolonised, occasionally occupied by Oil Seed Rape and Great Mullein, and with nutrition this would be Vervain and Ragwort.
 
 

2 May 2007 In this study on the  the lower slopes of Mill Hill the chalk hole was probably created by the uprooting of a Hawthorn or other bush. The depth of soil of about 70 mm can be discerned. The Horseshoe Vetch seems be around the top ridges of the soil and this pattern is repeated over the slopes, where Rabbits have created similar micro-habitats. The bare soil is sometimes settled by ruderals, Great Mullein and  Hounds-tongue with seem to survive and Thistles which usually perish.

14 May 2008 The path through the Horseshoe Vetch was created by humans. Just a small amount of disturbance creates conditions where the Horseshoe Vetch does not persist, but this applies to most other herbs as well.

Horseshoe Vetch Hippocrepis comosa	Common Bird's Foot Trefoil Lotus corniculatus
Flowers: Yellow. Up to 10 flowers.	Flowers: Yellow, sometimes streaked with red. 2-7 flowers.
Flowers:  May - June	Flowers: May - September
Leaves:	Leaves:
Pinnate. Blunt-ended.	Trifoliate (two of the five leaves are bent back so it appears trifoliate) Elliptic (not quite circular) = Oval, ending, rounded or with a short point.
Pods:	Pods:
Pods radiate from the top of the stem, strongly wavy, breaking up into horseshoe-shaped sections after flowering (see below).	Cylindrical spreading out from the stem like a bird's foot.
Comments:	Comments:
Long-lived perennial	Long-lived perennial

2 June 2006
The Horseshoe Vetch is now just past its best on the the lower slopes of Mill Hill. I took some measurements and my estimate of the number of Horseshoe Vetch flower heads (each with seven or eight flowers) is 25 million. In the patches which were covered by flowers there were about 500 flower heads every square metre. However, it was only about 30% of the main Horseshoe Vetch area that was actually covered in the yellow flowers and some parts of the slopes did not have any Horseshoe Vetch at all.
 

28 May 2006
 

The Horseshoe Vetch was prevalent on the lower slopes of Mill Hill, at about 70% of its luxuriance. Some flowers had not yet opened and it has appeared at the northern end which it usually does first and has not yet covering much of the steeper banks, which are always at least a week later. Over a dozen Honey Bees were attracted to the Horseshoe Vetch. The Horseshoe Vetch was flowering late compared to 2003, 2004 and 2005.

Dear Andy,

Thanks for your note. I am not sure I can answer your questions adequately
but I can offer the following as pointers. The most important of these is
that the leaves of Hippocrepis comosa are loaded with a very nasty toxin
called 3-nitropropionic acid (3NPA). Apart from the butterflies you mention,
very few things actually eat it. On the south Gower cliffs it is avoided by
sheep, ponies and rabbits. In experiments where locusts are offered
Hippocrepis as fodder, they prefer to starve or eat each other first ! It is
very unlikely that cattle will eat Hippocrepis in preference to grass.
However, the extent to which the ruminant digestive system can deal with
this toxin is debatable. If they were hungry enough, I guess they would eat
some. As far as Lotus is concerned, it is a bit more complex. Some plants of
Lotus corniculatus contain cyanogenic glycosides which generate hydrogen
cyanide when they are eaten, but not all individuals in a population do.
However, Lotus pedunculatus, which probably doesn't occur on your site,
contains 3NPA, but not in such high concentrations as Hippocrepis. With
large numbers of cattle, trampling could be a problem, but eutrophication
would probably be  a more significant one. Hippocrepis comosa is a
nitrogen-fixing legume and prefers soils that are deficient in nitrogen.
Addition of fertilisers, deliberately or through run-off would have a big
effect (a) by stimulating the growth of course grasses and (b) by decreasing
the competitive advantage of nitrogen fixation in the Hippocrepis
population. The extent to which fertilisation via natural manure would cause
eutrophication on site might be an important factor. On the Gower cliffs,
rabbit grazing probably has a beneficial effect, by keeping the swards
fairly trim.

Hope this is helpful to you.

Charles Hipkin

-----Original Message-----
From: British Marine Life Study Society [mailto:Glaucus@hotmail.com]
Sent: 17 May 2004 19:09
To: c.r.hipkin@swansea.ac.uk
Subject: Hippocrepis comosa
 

Dear Dr Hipkin,

It might be a bit presumptuous to write to you, but I obtained your name as
a result of an Internet search.

The enquiry concerns consumption of the prostrate forms of Hippocrepis
comosa by cattle, sheep and ponies.

The first one is the most important. My observations go only as far as when
cattle are introduced to grazing land, herbs with grass, that the
Hippocrepis comosa seems to disappear in time, quite a short period of time
with limited observations.

This is backed up by one research paper. However, the exact mechanics of
this disappearance are not clear, or it is not even clear that there is a
connection, or sometimes there may and sometimes there may not be.

Questions:

1)  Do cattle eat the green leaves of Hippocrepis comosa?

2)  Do cattle eat it preference to grass?

3)  Do cattle consider Hippocrepis and Lotus to be choice foods in winter?

4)  Do cattle trample the plants and destroy them this way?

This actually refers to plan to bring cattle on land which is Hippocrepis
with grass and other herbs on a steep slope. The Hippocrepis grows on
shallow soil and supports thousands of Chalkhill Blue Butterflies and a
hundred or so Adonis Blues and up to 30 butterfly species in all. (30 have
been recorded and 29 seen by myself).

One argument seems to be that cattle will eat the long grass that swamps out
the Hippocrepis. I do not know if this is right, as I would have thought
they would simply eat both, and they might even eat Hippocrepis ahead of
some grasses. On this slope there are no long grasses so this argument does
not work, but the conservation board is saying that anyway. Not that they
have been anywhere near the site in the first place, the decision makers.

Any pointers would help, even to research papers which I can peruse.
 

Cheers

Andy Horton
glaucus@hotmail.com <mailto:glaucus@hotmail.com>
Adur Valley Nature Notes   (including Shoreham-by-Sea, West Sussex)
http://www.glaucus.org.uk/Adur2003.htm
<http://www.glaucus.org.uk/Adur2003.htm>
http://www.glaucus.org.uk/Adur2004.html
<http://www.glaucus.org.uk/Adur2004.html>
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