Relative excretion of nitrogen from alfalfa silage, corn silage, corn grain and soybean meal in urine and feces by lactating dairy cows
The main objective of this trial is to determine the partitioning of nitrogen (N) from different feed ingredients in milk, feces and urine. This abstract focuses on relative excretion of N in feces and urine. Twelve multiparous late lactation Holstein cows (means±SD; 264±18 DIM) were fed a pretreatment TMR once a day for 11 days containing (DM basis) 35.5, 28.6, 20.3, 12.9 and 2.6% of corn silage (CS), alfalfa silage (AS), corn grain (CG), soybean meal (SBM) and a mineral and vitamin premix, respectively. On day 12, cows were grouped by milk yield and randomly assigned to one of four dietary treatments corresponding to each feed ingredient at natural abundance of 15N being replaced by its homologue ingredient enriched with 15N (except for CS treatment for which only 75% of the unlabeled CS was replaced by 15N enriched CS). After 4 days feeding the 15N-enriched TMR’s, cows were fed the pretreatment non-enriched TMR during days 16 to 19. Total fecal and urinary collection was conducted on each cow every 6 hours during days 12 to 19. Feed intake and lactation performance were also measured from day 12 to 19. Data were analyzed as a complete randomized design with treatment as a fixed effect and cow as a random effect. Corn silage and CG had the highest 15N enrichment (atom % 15N of 1.857 and 2.040, respectively) whereas AS and SBM had the lowest (atom % 15N of 0.730 and 1.385, respectively) due to 15N dilution by the atmospherically-fixed N by these legumes. Feeding 15N-enriched ingredients had no effects on DMI (23.2±2.4 kg/d, P=0.39), milk yield (26.1±5.2 kg/d, P=0.85), N intake (601±61 g/d, P=0.41), protein yield (0.89±18 kg/d, P=0.80) and N use efficiency (milk N/N intake; 0.23±0.05, P=0.86). Cumulative 15N recovery (% of total 15N fed) in feces between days 12 and 19 was similar (P=0.61) between AS and CS treatments (29.8 vs. 28.2%), which were greater (P<0.05) than CG (21.5%) and SBM treatments (12.5%). Although not significantly different (P=0.12), greater cumulative 15N recovery (%) in urine was measured in AS (27%), it was intermediate in CS and CG (21.6 and 19.4%, respectively) treatments, and smallest in the SBM (17.0%) treatment. Results from this study suggested that AS and CS contributed most to fecal N excretion and AS contributed most to urinary N excretion.
Keywords: N partitioning, N use efficiency, 15N