Plasma and milk urea nitrogen in relation to pregnancy rate in lactating dairy cattle.

TLDR

The study aimed to determine how plasma and milk urea nitrogen concentrations relate to pregnancy rates in lactating dairy cows and to evaluate different analytical methods for measuring milk urea nitrogen. Researchers collected blood or milk samples from 160 and 155 Holstein cows on the day of artificial insemination, compared three laboratory and dipstick methods for MUN, and monitored plasma progesterone and MUN in two subsets of 51 and 23 cows over five days post‑AI. Both laboratory chemical procedures produced similar MUN values, while the dipstick overestimated them; MUN concentrations above 19 mg/dL in plasma or milk were linked to roughly a 20 percentage‑point drop in pregnancy rates, with pregnant cows showing lower and declining MUN and higher progesterone levels compared to non‑pregnant cows.

Abstract

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 < .02) pregnancy rates (18 and 21 percentage point reduction in the two experiments). In two subset groups of cows (n = 51 and n = 23, respectively), plasma progesterone or MUN concentrations were monitored during the 5-d period after AI. Plasma progesterone concentrations increased 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 (CV = 8%) but were lower in pregnant cows and had a decreasing trend over time compared with nonpregnant cows (P < .05). Based on this study, plasma and milk will yield similar results for monitoring urea nitrogen in dairy cows; PUN and MUN concentrations > 19 mg/dL were associated with approximately a 20 percentage point decrease in pregnancy rate after AI in lactating dairy cattle.