Influence of different concentrations of milk urea nitrogen on the reproductive performance of Holstein cows

The purpose of this study was to assess how various MUN levels affected the reproductive performance traits of Iranian Holsteins. The dataset used in this study included reproduction performance records for 16,000 Holstein cows and was gathered by the Animal Breeding Center and Promotion of Animal Products of Iran on 30 large dairy herds during 2018-2021. The MUN concentrations were grouped into four classes: [Formula: see text] mg/dL (MUN1), [Formula: see text] mg/dL (MUN2), [Formula: see text] mg/dL (MUN3), and [Formula: see text] mg/dL (MUN4). The study took into account various reproductive and fertility traits, such as the number of inseminations per conception (NINS), the percentage of pregnant cows till 120 days in milk (PP120), the calving interval (CI), the days open (DO), the days from calving to the first service (DCFS), the first service conception rate (FSCR), and the abortion rate (ABR). The impact of MUN levels on the incidence of abortion and non-pregnancy in dairy cows was investigated using logistic regression models. Also, to evaluate the effect of MUN concentrations on reproduction traits, a generalized linear model was used. The MUN2 group had the greatest odds of non-pregnancy (P < 0.05; OR = 2.81; MUN2 vs. MUN1). Also, the MUN4 group had the greatest odds of abortion (P < 0.01, OR = 3.36; MUN4 vs. MUN1). The MUN2 and MUN4 groups had the greatest (51.91%) and the lowest (34.32%) FSCR, respectively. MUN classes had a significant influence on DO (P < 0.05), and MUN4 had the largest value (145.44 days). The results indicated the negative association between MUN with reproduction and fertility in Iranian Holsteins.