Milk urea nitrogen is genetically associated with production and reproduction performance of dairy cows: A meta-analysis

This study aimed to conduct a meta-analysis using the random-effects model to integrate published estimates of genetic parameters for milk urea (MU) and milk urea nitrogen (MUN). The study used a total of 49 heritability and 147 genetic correlation estimates from 30 papers published between 2003 and 2023. The heritability estimates for MU and MUN were low and equal to 0.202 and 0.181, respectively. The genetic correlation estimates between MU and production traits were low and ranged from -0.039 (between MU and milk lactose percentage) to 0.102 (between MU and milk protein yield), but the genetic correlation estimates between MUN and production traits were low and varied from -0.070 (between MUN and somatic cell score) to 0.357 (between MUN and milk protein yield). The genetic correlation estimates between MUN and calving to first service, days open, and first service conception rate were low and equal to 0.093, 0.329, and 0.069, respectively. The results of present study showed that available additive genetic variation in MU and MUN could potentially be exploited in dairy cow genetic selection schemes. Although the correlations of MU and MUN with production and reproduction traits were low, there were some cases where they were unfavorable, and they should not be neglected when planning selection programs.