The breed structure and genetic analysis of the pedigree cattle breeds in Australia. II. The Poll Hereford

(i) The breed structure and genetic history of the pedigree Poll Hereford breed in Australia are analysed by pedigree sampling methods. (ii) The basic pattern of the breed structure is similar to that found in other studies, but it is both diffuse and dynamic owing to the present rapid expansion of the breed. There are a number of herds moving into the major breeders' group and there are many new herds yet to find their level in the structure. (iii) An attempt is made to determine why particular herds become major breeders' herds. It is suggested that the use of imported animals is important, while there is evidence that those herds apparently moving into this group show a higher degree of selective registration of males. The relationship between herd size and importance of the herd is considered. (iv) The breed has developed since 1920 entirely from imported Poll Hereford and from imported and Australian-bred horned (pedigree Hereford) animals. The percentages of genes in the breed in 1949 derived from these animals are 64.3 and 35.7 per cent. respectively. (v) The most important herd in 1949 has a genetic contribution to the breed of 26.9 per cent. However, the genetic contributions of the four next most important herds are only 6.7, 6.1, 5.1, and 4.5 per cent. In the four-generation pedigrees from which these figures are derived, the genetic contributions of imported and horned animals are respectively 56.3 and 36.6 per cent. (vi) The animal with the highest relationship to the breed is Woodrow 1st (imp.), with direct relationships of 2.0 and 9.3 per cent. to the 1937 and 1949 samples respectively. Of the 14 sires and three dams whose direct relationships are 3.0 per cent. or over in either of the two sample years, nine sires and two dams are imported but none is horned. (vii) The degree of inbreeding (base year 1920) is calculated. The total inbreeding in 1949 (1.8 per cent.) comprises 0.8 per cent. current inbreeding, 0.6 per cent. long-term inbreeding, and 0.4 per cent. strain inbreeding. There is no evidence of subdivision of the breed into separate strains. (viii) The effective generation length is approximately 5.5 years. Nearly 50 per cent. of the animals in the 1949-50 sample are sired by bulls 4 years old or younger, while nearly 40 per cent. are from dams 4 years old or younger. (ix) It is not likely that there is much genetic variation between herds. Bulls from major breeders' herds are used widely throughout the breed while about 80 per cent. of sires and 60 per cent. of dams are bred in herds other than the one in which they are used.