Associations between digestibility, feed efficiency, nitrogen efficiency, and nitrogen partitioning in dairy cows on Dutch commercial farms

Enhancing feed resource efficiency and cows' ability to digest fiber is essential for advancing a circular food system that reduces feed-food competition and the environmental impact of dairy farming. This study investigated associations of feed efficiency (FE) and residual feed intake (RFI) with apparent total-tract digestibility (ATTD) of DM, OM, NDF, gross energy (GE) and nitrogen (N) in 480 cows across 4 Dutch commercial farms. In addition, associations of milk N use efficiency (NUE) and residual N intake (RNI) with N partitioning variables were analyzed. Three farms were visited twice, one farm once, resulting in 7 sampling sessions. Individual feed intake was recorded using automatic feed intake bins. During each session, milk yield and composition were determined, and spot samples of urine and feces were collected at 6 4-h intervals over 4 consecutive days, covering all 4-h periods within a 24-h cycle. Creatinine and acid-insoluble ash contents were analyzed and used to estimate urinary and fecal output, respectively. Rumen microbial N supply was estimated using purine derivative excretion in urine. On average, cows had a parity of'1.5 ± 0.74, were 138 ± 52.5 DIM, produced 38.8 ± 6.82 kg/d of fat- and protein-corrected milk, and had a DMI of 24.0 ± 3.40 kg/d, FE of 1.62 ± 0.189, and NUE of 306 ± 39.8 g/kg. The diet contained on average 181 ± 9.8 g/kg DM CP and 308 ± 27.4 g/kg DM NDF. In general, at the inter-individual level, higher FE was associated with greater nutrient digestibility; however, interactions with session were often observed, indicating that these associations varied across sampling sessions. In addition, higher NUE or lower RNI were generally associated with greater microbial N efficiency, reduced N retention, and a shift in N excretion from feces to urine, again depending on sampling session. These patterns were associated with the dietary CP to NEL ratio, with lower ratios associated with more efficient use of digestible N and reduced urinary N excretion. The frequent interactions between variables and sampling session demonstrate that efficiency and N partitioning metrics are context-dependent, emphasizing the need to account for farm- and time-specific conditions when developing strategies to improve nutrient use efficiency and reduce the environmental impact of dairy production.