It has been well documented that a negative relationship exists between blood, plasma, or milk urea nitrogen and pregnancy rates in dairy cattle. Traditionally, blood urea nitrogen is used as an indicator of protein status in cattle, with blood urea concentrations above 19 mg/dL and milk urea nitrogen concentrations above 15 mg/dL being considered excessive and linked to suppressed fertility. Although this relationship is not fully understood, likely contributing factors include altered sperm transport, impaired oocyte competence, and impaired uterine environment resulting from excessive dietary protein.
 
While largely unfounded, it has been widely accepted that a similar relationship between blood urea nitrogen and fertility also exists in beef cows. However, recent research at Iowa State University brings this theory into question.
 
Blood samples collected from 1,331 beef females during estrous synchronization across 15 separate experiments were analyzed for circulating blood and plasma urea nitrogen. All females included in the analysis had been adapted to their nutritional management scheme for at least one week prior to synchronization and were maintained on that nutritional program through the first 21 days of the breeding season.
 
Average first-service pregnancy rate across the entire dataset was 55.6%. Regardless of age and whether cows were maintained in a drylot or on pasture, a urea nitrogen concentration that was associated with decreased pregnancy rates could not be established. In fact, there was a tendency for improved pregnancy rate as urea nitrogen concentration increased, and cattle with urea nitrogen concentrations above 16 mg/dL tended to have greater pregnancy rates (57.0%) than those below 16 mg/dL (54.6%).
 
Based on these data, when cattle are allowed to adapt to a nutritional management scheme prior to breeding and maintained on that diet through the first 21 d of the breeding season, blood urea nitrogen concentrations are not negatively associated with first-service pregnancy rates in beef heifers and cows However, because acute changes in dietary protein were not evaluated in this study, caution should be used when changing nutritional management at or near the time of breeding.
 
As always, consult with the team of experts you have assembled including your nutritionist and beef extension specialist for more information on nutritional management of your herd.