Characterization of grazing Holstein-Friesian dairy cows based on a combination of nitrogen use efficiency and...

Awareness and interest in the use of nitrogen in dairy production systems is intensifying. Cows with high nitrogen use efficiency (NUE; nitrogen used and stored divided by nitrogen available) are generally considered to have a lower nitrogen footprint. This, however, may not always be true on a per-animal basis. The objective of this study was to characterize lactating dairy cows based on the combination of NUE and nitrogen balance (Nbal; nitrogen available less the nitrogen used and stored). Data were available for 1,472 weekly NUE and Nbal estimates from 711 lactations of 278 Holstein-Friesian grazing dairy cows. Random solutions for each cow from separate mixed model analyses of NUE and Nbal were used to categorize each cow into 1 of 2 strata for NUE and separately for Nbal; cows were then categorized on a combination of strata for NUE and Nbal. Data from 248 cows were used. Marginal means for a range of production traits were computed for each of the 4 strata. The population mean (SD) for NUE and Nbal was 21.3% (4.4) and 436 (97) grams of nitrogen per day, respectively. The correlation coefficient between NUE and Nbal was -0.47 demonstrating that the relationship between efficiency (i.e., NUE) and excretion (i.e., Nbal) is, indeed, not perfect. Cows with high NUE and low Nbal were assumed more desirable cows because they had, on average, a higher (i.e., better) or similar NUE to the other 3 strata and excreted, on average, 53 g of nitrogen less per day (SE = 5.1 g N/d) compared with the cows stratified as high NUE and high Nbal. Nevertheless, cows with high NUE and low Nbal were, on average, in greater negative energy balance than cows with low NUE and high Nbal, with the former also having, on average, a lower BCS than both strata of cows with low NUE, suggesting potential poorer health or reproductive capabilities compared with their counterparts. In conclusion, it is important to consider both NUE (high is desirable) and Nbal (low is desirable) when defining nitrogen-efficient dairy cows.