Variance Components for Nitrogen Utilisation in Grazing Dairy Cows

The overall aim of this thesis is to quantify the extent of inter-animal variability in how efficiently grazing dairy cows use ingested nitrogen and to determine how this variability can be exploited in breeding programs to deliver better nitrogen utilisation. Two key nitrogen utilisation metrics—nitrogen use efficiency (NUE) and nitrogen balance (Nbal)—will be investigated. Nitrogen use efficiency is defined as the ratio of nitrogen output (in milk and meat) to nitrogen intake, while Nbal is represented as the difference between intake and output, acting as a proxy for nitrogen excretion. Using data from 2,255 lactations from 1,296 cows collected from four Teagasc research farms in Ireland between the years 2007 and 2018, a total of 4,600 daily nitrogen utilisation metrics were generated. On average, a pure Holstein-Friesian in the fifth month of the first parity had an NUE of 0.21 and excreted 402 grams of nitrogen daily. Results from the thesis reveal significant inter-cow variability in both NUE and Nbal. During lactation, the additive genetic contribution to NUE variability ranged from 7% to 48%, while it increased from 5% to 23% for Nbal. A notable outcome of the thesis is that the phenotypic correlation between NUE and Nbal was only moderately negative (-0.47), while no genetic correlation (0.16, SE = 0.23) was evident; this suggests that both traits should be jointly assessed when considering dairy cow nitrogen utilisation. The need for a breeding goal incorporating both nitrogen utilisation metrics and production traits is further supported by the positive genetic correlation between milk yield and Nbal (0.46, SE= 0.15) and also between NUE and milk yield (0.52, SE= 0.11). Finally, while measuring (nitrogen) feed intake in grazing dairy cows is not trivial, results from this thesis indicate that neither milk urea nitrogen (MUN) content nor predictions from milk mid-infrared (MIR) spectroscopy are good proxy measures of nitrogen utilisation. Overall, the findings from this thesis provide a deeper understanding of nitrogen utilisation in grazing dairy cows and offer valuable guidance for incorporating nitrogen-related traits into future sustainable dairy cow breeding programs.