Genetic connections among herds in a selection nucleus of mutton type Merino sheep in Spain: A case of indirect connections through a connector herd in an extensive production system

摘要

A study was conducted to quantify the levels of connection among herds, reached through a connector herd, in the Selection Nucleus of the Mutton Merino breed of sheep in Spain. The database, provided by the Asociacion Nacional de Criadores de Ganado Merino (National Association of Spanish Merino Sheep Breeders), contained 86,773 lambing and weight records and pedigree records of 110,510 animals (86,879 lambs progeny of 2011 rams and 40,257 ewes) registered between 2000 and 2011 in 47 herds which constitute the Selection Nucleus of the breed. The ratios of genetic similarity (GS) between herds and the comparison of the means of the coefficients of determination (CD), a measure of the degree of connection of two herds through common ancestors, obtained using an animal model and the Criterion of Admission to the Group of Connected Herds (CACO) methodology, were used to quantify the levels of connection among herds and to cluster them. A direct genetic connection between herds of 55.3%, an indirect connection reached through the connector herd of 95.7%, twenty nine herds (61.7%) sharing parents with any other herd, 14 herds (28.6%) sharing paternal grandfathers and 27 herds (57.4%) sharing maternal grandfathers, were the main results of the GS analysis. The CACO analysis showed that the estimated values of the CD strongly depended on the heritability of the trait and the number of offspring per sire. Relatively high average values (0.548, 0.678 and 0.745) of the CD of contrasts, clustering 86.0%, 98.3% and 100% of the herds in the SN, were obtained under the low (0.10), moderate (0.25) and high (0.40) heritability scenarios was another result obtained with this method. For the main selection criterion presently being used in this breed (75 day-weight with h(2) = 0.356) the average CD of contrasts was 0.729, with a minimum value of 0.436 and a maximum value of 0.940, connecting 100% of the herds. Assessment of connections based on reproduction systems for natural mating using the animal model for 75-days weight in Merino lambs, provides high reliability in comparing the breeding values, despite the low use of artificial insemination in this breed. Results obtained in this work show that, for the purpose of estimating the BV for the main selection criterion of the breed (weight at 75 days), most Merino flocks in the SN are genetically well connected and unbiased EBV can be obtained through the use of the connecting herd, despite of lacking A.I. sires