The purpose of this study was to evaluate the association between milk urea nitrogen (MUN) and fertility of dairy cows using field data. The data came from 24 dairy herds belonging to Ohio Dairy Herd Improvement Cooperative Inc. Reproductive data and MUN measurements from cows that calved between June 1998 and May 1999 and that had been bred at least once were included in the study. Survival analysis, using the Cox proportional hazards model, was performed and days from calving to conception or to the end of the study was used as the outcome. Cows that had not been reported pregnant during the study were considered censored. The mean of monthly MUN values of cows before conception (or the end of the study for censored cows) was used to reflect the MUN status of a cow. Animals were categorized into quartiles based on MUN values in these data. Parity, calving season, peak milk yield, number of services, and herd were included in the models as fixed effects. Cows with MUN levels below 10.0 were 2.4 times more likely and cows with MUN levels between 10.0 and 12.7 mg/dl were 1.4 times more likely to be confirmed pregnant than cows with MUN values above 15.4 mg/dl. Our results indicate that increasing MUN levels appear to be negatively related to dairy cow fertility and are associated with a lower risk of detectable pregnancy at herd checks. They also suggest that the levels of MUN that are adversely associated with fertility might be lower than reported earlier.

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A spreadsheet-based model was developed to estimate the economic effect of varying reproductive performance in dairy herds. Scenarios were created to model an average cow with respect to production, herd lifetime, and reproductive events. Average milk yield per day of life as well as lifetime calf and replacement heifer production were examined. Additional inputs representing milk, feed, semen, calf, and salvage prices were used to calculate net cash flow for each day of herd life for the average cow in a scenario. Economic comparison of different scenarios was accomplished using an equivalent annual cash flow (annuity) methodology.Herd performance measures and prices representative of Ohio dairy herds were used to establish a baseline average cow that had a 160-d calving-to-conception interval [days open (DO)]. Alternative scenarios that differed from baseline in DO, annual culling rate, and feed and milk prices were created to characterize the effects of changes. Under scenario inputs representative of typical Ohio dairy herds, the model indicated that a lower annual culling rate (25%) was preferable to higher annual culling rates (34 or 45%). The model estimated maximum average milk yield per day of life to occur at 110 DO. At 34% annual culling rate, calves and replacement heifers produced per lifetime declined as DO increased; beyond 150 DO, the modeled cow produced less than 1 replacement heifer per lifetime. The model also estimated a loss of $1.37 per cow per year for a 1-d increase in DO beyond 160 d. At 20% higher feed and milk prices, the model estimated a loss of $1.52 per cow per year; at 20% lower feed and milk prices, the model estimated a loss of $1.23 per cow per year. Furthermore, the model suggested that the loss associated with a 1-d increase in DO changed as DO changed. Using baseline inputs, the model calculated losses for a 1-d increase of $0.44 per cow per year at 130 DO and $1.71 per cow per year at 190 DO. The nonuniform nature of the cost of additional DO is important to veterinarians and producers. The implication is that inefficient reproduction becomes marginally more costly to producers as performance declines and warrants increased attention. Conversely, marginal benefits of improved reproduction decrease as performance improves. Herds with strong reproductive performance have less opportunity to capture economic benefits of improvement.

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