ADSA-SAD Original Research Oral Competition

摘要

High levels of urea in blood, milk, and urine have been linked to poornitrogen efficiency, increased feed costs, poor reproductive performanceand increased environmental impact of dairy farming. Bulk tank averagemilk urea nitrogen (MUN) is often used to manage herd nitrogen efficiency.Current recommendations suggest MUN should be between 8 and14 mg/dL to maintain milk production and reduce nitrogen losses, but aprevious study found that commercial analysis of MUN ranged from 6.5to 14.9 mg/dL for the same sample set. The objective of this study was toevaluate the precision and accuracy of milk testing lab MUN measurements.Milk samples were collected from multiparous Holstein cows (n =16) 3 times daily (06:00, 14:00, and 22:00) over 7 consecutive days duringearly lactation (average DIM of 40 d). Samples were sent for analysis at 2labs (A and B). Both labs tested samples using mid-infrared spectroscopy(MIR). Lab B also employed an enzymatic spectrophotometric method formeasuring MUN on de-fatted and de-proteinated milk, here consideredto be the gold standard protocol. The mean (sd) of MIR MUN for Lab Aand B are 8.05 (1.33) and 6.41 (1.78) mg/dL respectively. The differencesbetween the MIR MUN values and the enzymatic assay MUN for each labwere calculated and regressed on mean-centered enzymatic MUN valuesin a mixed model with random effect for day. Results of the linear regressionare presented below. Labs A and B had significant (P < 0.01) negativeslope coefficients, indicating MIR methods over predict MUN at low valuesand underpredict at higher values. The intercept estimates suggest LabA significantly (P < 0.01) over predicts MUN by 2 mg/dL while Lab Btends (P < 0.10) to overpredict MUN by less than 0.5 mg/dL. Estimates ofthe residual variance and random effect of date indicate similar precisionbetween labs. In spite of existing bias, results show the MIR accuracyhas improved, however, sampled data is below industry average MUNand further work on samples with a wider range of MUN is required.