Milk Urea Nitrogen Concentration: Heritability and Genetic Correlations with Reproductive Performance and Disease in Holstein Cattle

The objectives of this study were to estimate the heritability of milk urea nitrogen concentration (MUN), describe the genetic and phenotypic relationships between MUN and reproductive performance, and estimate correlations among MUN breeding values and Danish breeding values for disease in Holsteins. Dairy Records Management Systems in Raleigh, NC provided lactation data. The Danish Agricultural Advisory Center provided breeding value estimates for disease. Heritabilities, genetic correlations and phenotypic correlations were estimated with an animal model using ASREML. Infrared (IR) and wet chemistry (WC) data were analyzed separately. Heritabilities were estimated with all lactations, as well as separately for parities one and two. Genetic and phenotypic correlations were estimated separately for parities one and two. Herd-test-day effects, age at calving, and days in milk were included in all models. Heritability estimates for WC MUN were 0.15 for all lactations, 0.14 for first lactation, and 0.09 for second lactation. Heritability estimates for IR MUN were 0.22 for all lactations, 0.22 for first lactation, and 0.23 for second lactation. Genetic correlations between first and second lactation MUN values were greater than 0.97 for both WC and IR. Genetic correlations for WC MUN and various measures of reproductive performance, including days to first service (DFS), first service conception (FSC), services per conception (SPC), and interval from first service to conception (IFC), were generally found to be not different from zero. The genetic correlation between WC MUN and days open (DO) in first lactation was 0.21, and between WC MUN and DO in second lactation, was 0.41, indicating higher WC MUN values were associated with increased days open. Phenotypic correlations were near zero for all measures. Genetic and phenotypic correlations for IR MUN and reproductive performance measures were not reported due to limited number of observations. Correlations among MUN breeding value estimates and Danish disease breeding values identified no significant relationships. Further investigations to identify possible non-linear relationships between MUN breeding values and Danish disease breeding values revealed no significant trends. While the results of this study indicate that heritable variation for MUN exists, the inability to identify significant genetic relationships to metabolic disease, reproductive performance, or foot and leg disease appear to greatly limit its use in selection for dairy cattle improvement in these areas at the present time