Polymorphism of the CSN3 3’UTR in Dairy Cows Causes Changes in bta-miR-708 Binding Ability and κ-Casein Expression

κ-casein (<i>CSN3</i>) polymorphisms are closely related to productive performance and dairy processing performance. In this study, <i>CSN3</i> 3'UTR polymorphisms in Chinese Holstein cows (n = 50) and the effects of these polymorphisms on the gene expression of four caseins, the protein expression of κ-casein, the binding of microRNAs (miRNAs), and cow production performance were investigated. There were two main haplotypes for the <i>CSN3</i> 3'UTR in Chinese Holstein cows: Haplotype 1 (0 mutations, n = 16) and Haplotype 10 (a combination of 7 mutations, n = 17). The gene and protein expression levels of <i>CSN3</i> in Haplotype 1 were significantly greater than those in Haplotype 10 (<i>p</i> < 0.05). The ability of bta-miR-708 to bind to the <i>CSN3</i> 3'UTR with Haplotype 10 was significantly greater than that with Haplotype 1 (<i>p</i> < 0.05), potentially explaining why the gene and protein expression of <i>CSN3</i> with Haplotype 10 were lower than those with Haplotype 1. Dairy herd improvement (DHI) data, including 305-d milk production, fat-corrected milk, fat (%), protein (%), and fat/protein ratio, were not significantly different between the two haplotypes (<i>p</i> > 0.05). Results suggest that the two major <i>CSN3</i> 3'UTR polymorphisms in Chinese Holstein cows cause significant changes in <i>CSN3</i> gene expression, CSN3 protein expression, and bta-miR-708 binding ability but no significant alterations in macroscopic DHI data. Therefore, <i>CSN3</i> 3'UTR polymorphisms should be investigated further.