Abstract – The genetic structure of eight Spanish autochthonous populations (breeds) of beef
cattle were studied from pedigree records. The populations studied were: Alistana and Sayaguesa
(minority breeds), Avileña – Negra Ibérica and Morucha (“dehesa” breeds, with a scarce
incidence of artificial insemination), and mountain breeds, including Asturiana de los Valles,
Asturiana de la Montaña and Pirenaica, with extensive use of AI. The Bruna dels Pirineus
breed possesses characteristics which make its classification into one of the former groups
difficult. There was a large variation between breeds both in the census and the number of herds.
Generation intervals ranged from 3.7 to 5.5 years, tending to be longer as the population size
was larger. The effective numbers of herds suggest that a small number of herds behaves as a
selection nucleus for the rest of the breed. The complete generation equivalent has also been
greatly variable, although in general scarce, with the exception of the Pirenaica breed, with a
mean of 3.8. Inbreeding effective population sizes were actually small (21 to 127), especially in
the mountain-type breeds. However, the average relatedness computed for these breeds suggests
that a slight exchange of animals between herds will lead to a much more favourable evolution
of inbreeding. The effective number of founders and ancestors were also variable among breeds,
although in general the breeds behaved as if they were founded by a small number of animals
(25 to 163).