Cattle Review: March–April

Genomic selection increases accuracy and decreases generation interval, accelerating genetic changes in populations. Genetic trends of American dairy cattle breeds were examined by Guinan et al (2023) (Journal of Dairy Sciencehttps://doi.org/10.3168/jds.2022-22205) to determine the genetic gain since the implementation of genomic evaluations in 2009. Inbreeding levels and generation intervals were also investigated. Breeds included Ayrshire, Brown Swiss, Guernsey, Holstein and Jersey. Mean genomic predicted breeding values were analysed per year to calculate genetic trends for bulls and cows. The data set contained 154 008 bulls and 33 022 242 cows born since 1975. Generation intervals and inbreeding levels were also investigated since 1975. Milk, fat and protein yields, somatic cell score, productive life, daughter pregnancy rate, and livability predicted breeding values were documented. In 2017, 100% of bulls in this data set were genotyped.

Overall, production traits have increased steadily over time. The Holstein and Jersey breeds have benefited most from genomics, with up to 192% increase in genetic gain since 2009. Owing to the low number of observations, trends for Ayrshire, Brown Swiss, and Guernsey are difficult to infer. Trends in fertility are most substantial; particularly, most breeds are trending downwards and the daughter pregnancy rate for Jersey breed has been decreasing steadily since 1975 for bulls and cows.