Non-linear models of 15N partitioning kinetics in late lactation dairy cows from individually-labeled feed ingredients

Authors: REED, KRISTAN - Cornell University; BARROS, TIAGO - University Of Wisconsin; POWELL, J. MARK - Former ARS Employee; Zanton, Geoffrey; WATTIAUX, MICHEL - University Of Wisconsin; ERICKSON, MARY GRACE - University Of Wisconsin

Submitted to: Journal of Dairy Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/9/2024
Publication Date: N/A
Citation: N/A

Interpretive Summary: Our trial tested kinetics and partitioning of N from individual feeds in late-lactation Holsteins. For each of four dietary treatments, one ingredient (alfalfa silage, corn silage, corn grain, soybean meal) was replaced with an analogous endogenously-labeled 15N feed ingredient. Non-linear models indicated differences between forage and concentrate feeds in N kinetics and partitioning that we compared to NRC (2001) and NASEM (2021) nutritional models. Absorbed N from the concentrate feeds was incorporated into milk at a higher proportion than the absorbed N from the forage feeds, Additionally, approximately 40% of urine and milk N derives directly from body tissue N.

Technical Abstract: Few studies have examined the N kinetics of individual feeds with stable isotope tracing. We hypothesized that N partitioning to milk, excreta pools, and body would differ for alfalfa silage, corn silage, corn grain, and soybean meal, consistent with existing nutritional models. Feed ingredients were endogenously labeled with 15N and included in four diets to create treatments with the same dietary composition and different labeled feed. Diets were fed to 12 late-lactation dairy cows for four days (96 hours) and feces, urine, and milk collection proceeded during the four days of 15N enrichment and for three days (80 hours) after cessation of label feeding. Non-linear models of 15N enrichment and decay were fit to milk, urine, and fecal N in R with the nlme() package and feed-specific parameter estimates were compared. The estimated proportions of feed N that were excreted in feces supported our understanding that N from soybean meal and corn grain is more digestible than N from alfalfa and corn silage. Estimates for the post-absorption N partitioning from the two concentrate feeds (soybean meal and corn grain) indicated N was selectively partitioned to milk N instead of urine N and that the opposite was true for N from the two forage feeds (alfalfa and corn). The modeled proportion of total fecal N that originated from feed N was 83% which is in line with previous research using a similar infusion timeframe. However, the proportion of urinary and milk N originating from feed N was much lower (60% for urine, 58% for milk), suggesting that approximately 40% of urinary and milk N directly originate from body N sources related to protein turnover.