Determining relationships between milk urea nitrogen, feeding management, reproduction and environmental indicators in commercial dairy herds

Cow-level milk urea nitrogen (MUN) is widely available for monthly milk testing on many dairy farms and therefore it provides an opportunity to be utilized as a predictor for other farm parameters if there was a clear understanding of the relationships between MUN and these parameters. This thesis examined the associations between MUN and reproductive performance, fecal nitrogen (N) and groundwater nitrate-nitrogen contamination in order to determine whether MUN was indicative of reduced reproductive performance or elevated fecal N or groundwater nitrate-N. The perceived and actual utility of MUN as an effective tool for monitoring protein and energy imbalances in dairy cows was also investigated.

The first analysis utilized mixed logistic regression models to evaluate the association between first service breeding success (FSBS) and the MUN test value closest to first service, using all herds that were receiving MUN testing on Prince Edward Island (PEI), controlling for clustering effects of cows within the 186 study herds. High MUN concentrations were associated with decreased odds of pregnancy at first service in commercial dairy herds. A change in MUN from 10 to 20 mg/dl was associated with a 14.3% reduction in the odds of first service conception, while controlling for the negative effect of milk production (and other possible confounders) on FSBS. Parity 4 plus cows had significantly worse breeding success than parity 1, 2 and 3 cows. The odds of conception increased as days in milk (DIM) at service date increased.

A second analysis on the association between MUN and reproductive performance was conducted, with similar methodology but using 83 of the 186 herds. These 83 herds were part of an intensive analysis of the relationship between MUN and nutritional data, in order to allow us to control for nutritional factors as well. This second analysis also showed that increasing MUN levels were negatively associated with fertility of dairy cows, as measured by FSBS. In this analysis, MUN levels were categorized into 2 groups (high and low groups) based on an obvious cut point in the data. The high group, which had MUN concentrations above 15.5 mg/dl, had a detrimental effect on FSBS (OR = 0.63), even after controlling for herd effects and other significant risk factors, including nutritional factors. (Abstract shortened by UMI.).