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The objectives of this study were to relate concentrations of plasma (PUN) and milk (MUN) urea nitrogen to pregnancy rate in dairy cows and compare various methods of analysis and preparation of milk for measuring MUN. In two experiments, blood or milk samples were collected on the day of AI from Holstein cows (n = 160 and n = 155, respectively). Three methods of MUN analysis were compared. Two laboratory chemical procedures yielded similar results, whereas a quick dipstick method overestimated chemical analyses. Before and after milking strip samples had MUN concentrations equivalent to those in composite milk. Concentrations of PUN or MUN greater than 19 mg/dL were associated with decreased (P 19 mg/dL were associated with approximately a 20 percentage point decrease in pregnancy rate after AI in lactating dairy cattle.
Butler et al. (Mon,) studied this question.