The objectives in this study were to measure urea nitrogen levels in blood serum (SUN) and milk (MUN) of lactating cows and to fine the relationship between their concentrations and pregnancy rates. Blood and milk samples were collected on the day of insemination from 80 lactating cows of a dairy farm in a period of four weeks. Cows were detected as to be either pregnant or non pregnant 45 days after insemination. Cows were also grouped by milk production (<8000lit and >8000lit), lactation number (first, second and third or further) as well as interval between calving to sampling (<60, 60-120 and >120 days) and within pregnants and non- pregnants. Mean SUN and MUN concentrations in non-pregnant cows were significantly higher than in pregnant cows (p<0.05). A highly significant correlation (r=0.79) was observed between the levels of MUN and SUN. Neither the effect of milk production, lactation number and sampling day relative to calving (DIM), nor their interactions with pregnancy status were significant. Concentrations of SUN and MUN greater than the mean (15 and 13 mg/dl respectively) were associated with a decreasing (p<0.05) pregnancy rate. Serum creatinine levels did not indicate any probable renal disease. Based on this study, high levels of MUN and SUN can be detrimental to pregnancy in lactating cattle; MUN can be employed for monitoring urea nitrogen in dairy cows.
)