1. Breed Composition Codes for Crossbred Dairy Cattle in the United States John B. Cole,* Melvin E. Tooker, Paul M. VanRaden, and Joel H. Megonigal, Jr. Animal Improvement Programs Laboratory, Agricultural Research Service, USDA, Beltsville, MD 20705-2350. FOUR-GENERATION BREED STACKS The four-generation breed stack is a text string describing an animal's breed composition for the four most recent generations. This allows for the easy identification of rotational crossbreeding systems. The breed stack for a HO x GU F1 cow is: Breed Composition ------------------------------------------------------------ HOGUHOHOGUGUHOHOHOHOGUGUGUGUHOHOHOHOHOHOHOHOGUGUGUGUGUGUGUGU In this example, the sire and the dam are purebred animals for the previous four generations. INTRODUCTION Crossbreds are animals whose genes are derived from more than one breed, such as a cow with a Holstein sire and a Jersey dam. A system for storing breed composition data for US dairy cattle has been developed and the necessary software implemented. This system is necessary to facilitate implementation of an across-breed genetic evaluation. Crossbred animals in the US dairy cattle population do not currently receive genetic evaluations. Two ways that crossbred animals may be handled in an evaluation are: they may be included in the breed- of-sire evaluation, or all animals may be combined into a single evaluation. For each crossbred cow (n=246,218) the database stores animal identification, breed of animal registry, breed fractions for 18 dairy breeds and an “unknown” category, a four-generation breed stack, a sire breed stack, and a heterosis value. Standard two-letter breed codes are used in the breed stacks. Breed fractions were calculated from pedigree information in the USDA database as far back as data were available. SUMMARY STATISTICS There are currently 246,218 crossbred animals stored in the AIPL database. Results are presented for the six major dairy breeds in the US: Ayrshire (AY), Brown Swiss (BS), Guernsey (GU), Holstein (HO), Jersey (JE), Milking Shorthorn (MS). The number of animals in the table with non-zero breed fractions are presented in Table 1. The number of F1 animals for each of the six major breeds are presented in Table 2. SIRE BREED STACKS The sire breed stack is a summary of the breed of sire and sires of the maternal line as far back as pedigree data are available. The sire breed stack for the HO x GU F1 from the previous example is: Sire Breed Stack ------------------------------------------------------------ HOGUGUGUGU Due to the absence of breed information about paternal ancestors, rotational crossbreeding systems cannot be identified from the sire breed stack. However, when purebred sires are used this information provides a complete description of the mating system being used. GENETIC EVALUATION OF CROSSBREDS There are three basic approaches to using records from crossbred animals in a genetic evaluation program: Do not use the records (the current system). Use the crossbred animals in the breed-of-sire evaluation. Use records from all animals in a single across-breed evaluation. The current system, under which crossbred records are not used, is the least desirable option because it discards information in the form of records. The third option is the most technically challenging but appealing option. The second option is not as satisfying as the third, but it is the simplest to implement. CONCLUSION There is now a system in place for easily identifying crossbred dairy cattle in the United States, and research into methods for genetic evaluation of these animals is ongoing. Table 2.Number of F1s for each major breed cross. Table 1.Number of animals with a non-zero breed fraction for each major breed.