Lactation-wide genetic insights into nitrogen utilization in grazing dairy cows.

Societal and economic pressure to reduce nitrogen use in the dairy cow sector is increasing. One potential contributing solution is to breed more nitrogen-efficient dairy cows that excrete less nitrogen. Although genetic variability for nitrogen utilization has been documented in dairy cows, how the phenotypic variance is partitioned into its causal components across lactation has not been previously explored in dairy cows. The objective, therefore, of the present study was to use 4,600 nitrogen utilization phenotypes across 2,255 lactations from 1,296 grazing dairy cows to explore whether the genetic variation in nitrogen utilization changes throughout lactation. Nitrogen utilization in the present study was evaluated using 2 metrics: nitrogen use efficiency (NUE), the ratio of nitrogen used and stored relative to the nitrogen available, and nitrogen balance (Nbal), calculated as nitrogen available minus the nitrogen used and stored. A random regression model was used to explore the variability of each studied trait over the lactation as well as the within-trait covariances across the lactation. The genetic variability in both NUE and Nbal did indeed change throughout lactation. The genetic SD of NUE decreased rapidly from 0.018 (SE = 0.003) at 6 DIM to 0.007 (SE = 0.001) at 82 DIM, then remained relatively constant until 191 DIM before increasing thereafter to 0.016 (SE = 0.002) at 268 DIM. Mirroring the change in the genetic variability across the lactation, the heritability of NUE decreased from 0.35 (SE = 0.09) at 6 DIM to 0.07 (SE = 0.03) at 80 DIM, before it increased up to 0.48 (SE = 0.10) by the end of the lactation. The genetic SD of Nbal increased from 11.87 (SE = 0.05) g/d of N at 6 DIM to 29.92 (SE = 3.61) g/d of N at 268 DIM, which was reflected in a gradual increase in heritability from 0.05 (SE = 0.04) at 6 DIM to 0.23 (SE = 0.05) at 268 DIM. Moreover, the within-trait genetic correlations between different DIM for NUE ranged from strongly positive to -0.25 (SE = 0.16), which was between 6 and 148 DIM. The within-trait genetic correlations for Nbal between different DIM were all strongly positive, with the weakest value being 0.61 (SE = 0.19), which was between 6 and 268 DIM. This profile in genetic correlations, combined with the sign of the eigenfunctions of the genetic covariance matrix for both nitrogen efficiency traits across lactation, suggest a greater potential to modify the shape of the lactation profile for NUE through genetic selection compared with Nbal. Nevertheless, there is still a good opportunity to genetically modify the height of the Nbal profile throughout lactation. Finally, given the limited evidence of a nonzero genetic correlation between NUE and Nbal, should both traits be deemed important, then both should be considered in a breeding objective to improve the nitrogen utilization metrics of dairy cows.