Evaluation of between-cow variation in milk urea and rumen ammonia nitrogen concentrations and the association with nitrogen utilization and diet digestibility in lactating cows

Concentrations of milk urea N (MUN) are influenced by dietary crude protein (CP) concentration and intake, and could therefore be used as a biomarker of the efficiency of N utilization for milk production (milk N/N intake; MNE) in lactating cows. In the present investigation, data from milk production trials (production dataset; n = 1808 cow/period observations from 21 change-over studies) and metabolic studies involving measurements of nutrient flow at the omasum in lactating cows (flow dataset; n = 446 cow/period observations from 29 studies) were used to evaluate the influence of between-cow variation on the relationship of MUN with MNE, urinary N (UN) output, and diet digestibility. All measurements were made on cows fed diets based on grass silage supplemented with a range of protein supplements. Data were analysed by mixed model regression analysis with diet within experiment and period within experiment as random effects, allowing the effect of diet and period to be excluded. Between-cow coefficient of variation in MUN concentration and MNE was 0.13 and 0.07 in the production dataset and 0.11 and 0.08 in the flow dataset, respectively. Based on residual variance, the best model for predicting MNE developed from the production dataset was: MNE (g/kg) = 238 + 7.0 × milk yield (MY; kg/d) – 0.064 × MY2 - 2.7 × MUN (mg/dL) – 0.10 body weight (BW; kg). For the flow dataset, including both MUN and rumen ammonia N (RAN) concentration with MY in the model accounted for more variation in MNE than when either term was used with MY alone. The best model for predicting UN excretion developed from the production dataset (n = 427) was: UN (g/d) = -29 + 4.3 × dry matter intake (kg/d) + 4.3 × MUN + 0.14 × BW). Between-cow variation had a smaller influence on the association of MUN with MNE and UN output than published estimates of these relationships based on treatment means, in which differences in MUN generally arise from variation in dietary CP concentration. For the flow dataset, between-cow variation in MUN and RAN concentrations were positively associated with total tract organic matter digestibility. In conclusion, evaluation of phenotypic variation in MUN indicated that between-cow variation in MUN had considerably smaller effects on MNE compared with the responses of MUN to dietary CP concentration reported in the literature, suggesting that a closer control over diet composition relative to requirements has greater potential to improve MNE and lower UN on-farm than genetic selection.