Estimation of genetic parameters for milk urea nitrogen and its relationship with milk constituents in Iranian Holsteins

Abstract

The objectives of this study were to estimate the heritability of milk urea nitrogen (MUN) concentration in the first three parities and describe the genetic and phenotypic relationship between MUN and other milk constituents in Iranian Holstein dairy cows. Test-day records including MUN data obtained from 57,301 dairy cows on 20 large dairy herds in Iran between January 2005 and June 2009. Genetic parameters for MUN and productive traits were estimated using random regression test-day models. Regression curves were modeled using Legendre polynomials of order 3. Coefficient of variation for MUN was 38.76%. Average estimated heritabilities for MUN were from 0.14 to 0.21 over the parities. Phenotypic trend of MUN increased significantly over the years. Also, average phenotypic correlations of MUN with milk production traits were low and generally close to zero. The average genetic correlation was close to zero for MUN and lactose percentage (−0.01); was moderately positive for MUN and net energy concentration of milk (0.31), fat percentage (0.21), protein percentage (0.30), total solids (0.29), solids-not-fat (0.26), and milk yield (0.24), and was negative for MUN and somatic cell score (−0.04). Herd-test day explained 46% to 52% of the variation in MUN across the parities, which suggests that management adjustments at herd-level can reduce MUN. This study shows that it is possible to influence MUN by herd practice and by genetic selection.