Selection of Embryo Transfer Recipients Utilizing Milk Composition at Peak Lactation and During the Breeding Period: An Analysis Based on Production Records

Optimal recipient selection is crucial for embryo transfer (ET) efficiency in dairy herds, yet objective criteria remain limited. This retrospective study examined the associations of milk composition at peak lactation and during breeding period with subsequent ET pregnancy status. Data from 453 ET records from Holstein cows were analyzed using milk fat, protein, solids-non-fat, and milk urea nitrogen (MUN) concentrations from Dairy Herd Improvement (DHI) records. Multivariable logistic regression analysis, adjusting for parity and environmental factors, identified MUN as the sole parameter independently associated with ET pregnancy. Cows in the lowest MUN quartile at either peak lactation or during the breeding period had significantly lower odds of pregnancy than those in the highest quartile. Notably, cows maintaining consistently low MUN from peak lactation through the breeding period (Low–Low group) showed lower pregnancy odds (odds ratio: 0.474) than the High–High group. Conversely, higher MUN levels had no adverse effect on pregnancy. These findings suggest that the absence of a physiological rise in MUN may reflect metabolic maladaptation—such as prolonged negative energy balance—that compromise uterine receptivity. Consequently, monitoring MUN transition patterns via routine DHI records provides an objective tool to identify suboptimal recipients and improve overall ET efficacy in commercial dairy operations.