Studies of the Australian Illawarra Shorthorn breed of dairy cattle. I. Breed structure

The structure of the pedigree Australian Illawarra Shorthorn breed in Australia was analysed by using registration data from the Society's Herd Books. The breed may be divided into three broad levels, namely, Breeders' Herds, Multipliers' Herds and Grade (unregistered) Herds. Of the registered herds, 32% are Breeders' Herds and the remainder are Multipliers' Herds. These are each subdivided into four strata depending on their importance as a source of breeding material, and on their registration practices respectively. Of major importance is the highest stratum of the breed. It comprised only 13.6% of all herds, but accounted for 40.5% of male and 24.8% of female registrations. There is a high ratio of females to males registered of 5.9, which indicates considerable selective registration of males. The degree of this selective registration generally increases at the lower strata, ranging from 3.6 at the top of the structure to 8.7 toward the bottom of the structure. Of all new herds initiated from 1931 to 1971, 44% survived less than 5 years. These short-lived herds averaged only 3.3 female registrations per year, but herd size increased as longevity increased. These points highlight the instability of new, small studs, and indicate that few herds are big enough, or exist for long enough, to contribute significantly to breed improvement. It is not likely that there is much genetic variation between herds. Bulls from major breeders' herds are used widely throughout the breed, while 68% of sires and 24% of dams are bred in herds other than the one in which they were used.