Estimation of Genetic Parameters for Milk Urea Nitrogen in Iranian Holstein Cattle Using Random Regression Models

Reducing nitrogen excretion in dairy cattle is a critical factor for improving the environmental sustainability of the livestock industry. This research aimed to estimate the genetic parameters over time for the milk urea nitrogen (MUN) trait in Iranian Holstein dairy cattle. Data from 347,639 test-day records of 52,219 first-parity Iranian Holstein dairy cows (spanning 2018 to 2023), were sourced from the Iranian National Animal Breeding Center. A single-trait random regression test-day animal model was used for the genetic evaluation of MUN. Three orders of Legendre orthogonal polynomials (ranging from 1 to 3) were tested to fit the fixed curve, additive genetic effects, and permanent environmental effects. Based on the AIC, BIC, and residual variances to compare the models, the third order was considered as the appropriate order for this dataset. The average heritability and repeatability of the MUN trait were estimated to be 0.027 and 0.081, respectively. The average estimates for additive genetic variance, permanent environmental variance, and phenotypic variance were 0.14, 0.28, and 5.17, respectively. The genetic trend analysis revealed that the MUN trait exhibited fluctuations across birth years (2016–2021), with an overall negative trend. Importantly, the average MUN levels remained within the desirable range of 13–16 mg/dL for Iranian Holstein cows across calving years from 2019 to 2023. Despite the low heritability estimates, the genetic parameters obtained in this study are valuable for improving MUN in Iranian dairy cattle. These findings provide critical insights for designing effective breeding programs aimed at reducing nitrogen excretion and promoting environmental sustainability in the dairy industry.