PLASMA AND MILK UREA NITROGEN IN RELATION TO PREGNANCY RATE IN LACTATING DAIRY-CATTLE

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 measur ing MUN. In two experiments, blood or milk samples were collected on t he day of AI from Holstein cows (n = 160 and n = 155, respectively). T hree methods of MUN analysis were compared. Two laboratory chemical pr ocedures yielded similar results, whereas a quick dipstick method over estimated chemical analyses. Before and after milking strip samples ha d MUN concentrations equivalent to those in composite milk. Concentrat ions of PUN or MUN greater than 19 mg/dL were associated with decrease d (P < .02) pregnancy rates (18 and 21 percentage point reduction in t he two experiments). In two subset groups of cows (n = 51 and n = 23, respectively), plasma progesterone or MUN concentrations were monitore d during the 5-d period after AI. Plasma progesterone concentrations i ncreased similarly during the period for cows divided into low vs high PUN but were greater in pregnant than in nonpregnant cows on d 4 and 5 (P < .04). The MUN concentrations showed low within-cow variation (C V = 8%) but were lower in pregnant cows and had a decreasing trend ove r time compared with nonpregnant cows (P < .05). Based on this study, plasma and milk will yield similar results for monitoring urea nitroge n in dairy cows; PUN and MUN concentrations > 19 mg/dL were associated with approximately a 20 percentage point decrease in pregnancy rate a fter AI in lactating dairy cattle.