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E p i c e n t r e
Issue 4

EXMOOR STALLIONS

EFFECTIVE POPULATION SIZE.

The study carried out by Sneha Maroo in 1998 for her MSc. Thesis revealed that Genetic Bottlenecks have occurred in the past due to the overuse of certain sires, causing a sudden increase in inbreeding two generations later.

The key to the problems of genetic bottlenecks is understanding the concept of Effective Population size,

Systems of Mating

1 male + 1 female has an effective population size of 2
2 male + 2 female has an effective population size of 4
3 male + 3 female has an effective population size of 6

and so on, but if only 1 male is used on a large population of females it is clear a formula must be used to determine what is obviously an Effective Population size between the 1 male used and the number of females. This is TWICE the Harmonic Mean of the male and female populations, expressed as:-

1/Ne = 1/4Nm + 1/4Nf

where Ne = Effective Population
Nm = Male Population
Nf = Female Population
if we substitute 3 for male and female populations, then:

1/Ne = 1/4(3) + 1/4(3)

1/Ne = 1/12 + 1/12 = 1/6

Thus Ne = 6 as we would expect, but if we use 1 male on 6 females we get

1/Ne =1/4(1) + 1/4(6) = 1/4 + 1/24 = 7/24

Thus Ne = 24/7 = 3.428 less than many breeders would expect.

Similarly if we use 3 males on 60 females we get an Effective Population size of 11.428.

If we were to use 1 male on an infinitely large number of females then we get an Effective Population size of 4.

It is the mathematical limits of the Effective Population using single stallions which causes genetic bottlenecks and reduces the Gene Pool.

Updating the studbook to 1999 reveals that to date 4219 ponies have now been registered, of these 1130 females and 261 males have bred. Caractacus had 31 progeny including Crackshot, he had 65 progeny including Heatherman, who in turn had 93 progeny and 338 grand-progeny. Analyses such as these illustrate one reason for the narrow founder base of the present pedigrees.

Only too often a breeder regards his stallion of having done well by having many progeny, not realising that using the stallion even on an infinite number of mares reduces the Effective Population of his herd to 4. To date the two stallions which have sired over 100 foals are Golden Gorse 23/69 with 151 and Dazzling Boy 85/32 with 102.

It is the second generation of progeny, that is the grand-progeny, of a stallion where we can see the effect on the effective population size. The following shows the stallions with over 300 grand-progeny :-

Stallions with over 300 grand-progeny

Brand Name Progeny Grand-progeny
85/32 Dazzling Boy 102 352
23/33 Forest 76 344
78/2 Heatherman 93 338
14/35 Agrippa 74 324
23/69 Golden Gorse 151 300

According to the studbook there are 118 stallions and 578 mares alive now . Of the 167 foals registered in 1999 they were the progeny of 60 stallions. The discrepancy in stallion numbers is probably due to the fact that the last census was carried out in 1996 and deaths of entires have not been updated since then.

Even now in 1999 the studbook indicates that of the ponies which have bred and are alive now there is a maximum of 578 mares, so groups of over 300 full cousins in 578 represents an enormous reduction in the Exmoor Genome.

The EPS has since 1921 operated a system of Selective Registration for foals, with potential stallions inspected at 2 to 3 years of age. The standards for these inspections must be maintained but at the same time we must be prepared, that to progress without the risk of genetic bottlenecks, to increase the number of stallions available to breeders. A current problem with inspections is that NOT ALL foals are being inspected, the rule 6A of the EPS states :-

  1. " At an authorised inspection ALL pure bred foals in the herd to be inspected must, unless prior permission has been granted by the Secretary, be offered for inspection.
  2. That any pony rejected for registration shall be given an identity number and that the breeding of the pony and the reason for rejection shall be notified to the Society."
It appears that these rules are not being enforced.

Another problem is that in any system of selective registration, for the practice to be worthwhile, we require a failure rate of 15% to 20%, in 1999 of 201 foals inspected 28 failed, (i.e. 14%), but according to the chairpersons comments - 'most pass on reinspection'. Considering the cost of inspections to the Society and to individual inspectors we must question not only the cost effectiveness of inspections but also the value of selective registrations.

My own view is that the present system of inspection should be continued with the above rules enforced. The value of such inspections would be in the data of animals failing inspection, this data being utilised as a progeny test of the parents.

In order to solve the overuse of certain stallions, and their use into old age, the number of foals they produce should be limited. For each stallion all foals born should be presented for inspection and it must be ensured that the above rules are always enforced..

The EPS could pay premiums on the foals which pass inspection for the first 25 foals put forwards, thereafter the EPS pays no further premiums for foals from that stallion and recommends he be castrated. This does not prevent the individual herd owner from continuing to use the stallion, it would only stop premiums being paid.

Such a regime would ensure a broad genetic base for the breed to flourish during and indeed to survive the next millennium.

Stallion Assessment

The Spring 2000 Newsletter contained two items of note regarding show results and shows.

The first was an article on "Stallion Progeny 1999" in which the author used show results to list stallions according to the prizes won by their progeny at shows. The shows were mainly those Agricultural shows local to Exmoor and the South West of England with a few others in the South of England and EPS area shows in Wales, Midlands, and the North of England.

The second item was on the back page, this was "Show Dates 2000". This again revealed that the further one moved away from the Exmoor area the less attention was paid to Agricultural or Equestrian shows. This to the extent that only two shows were listed in the North of England, two in Scotland, one in Wales, and nothing in Ireland.

The Autumn 2000 Newsletter under "Show Results" defends not sending members of the EPS the Show Results Booklet unless they send an A5 S.A.E. because "very few members are interested in showing".

The show ring assesses animals by conformation, at best, if all judges were equal and had consistent standards, we would be valuing an animal only by its Phenotype, i.e. by what it appears to be. Conclusions reached by this kind of estimation are of little or no use to the animal breeder, the breeder requires an evaluation which gives an indication of the Genotype. i.e. the genetic characters the animal is likely to pass on to its offspring.

The problem with awarding points by the number of prizes won by the progeny is that someone with plenty time on their hands and plenty of money could trail the same pony around every show in the country which would gain its sire a maximum number of points. This could be the only progeny of that sire that had a conformation that was accepted by the judges, every other sibling of that prize winner could not only have failed its inspection but have four white feet and a white star etc.

At best the show ring offers a channel for publicising the breed but it only reflects the ideals held by a minority of breeders, the show ring does not lead , it merely follows current views on what an animal should look like.

With data from inspections and the ability to analyse such information are we not better to improve inspection procedure, gather progeny data, and legislate for a more open and honest assessment of a pony's genetic potential.

It is of more value to a purchaser to know that the sire of a foal has had no or very few of his foals rejected at inspection rather than that he got prizes in the show ring .

written by Alex.N.Copland

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