LAUSR

Abstract The objective of this study was to determine the relative partitioning of N in individual feed within a diet (alfalfa silage [AS], corn silage [CS], corn grain [CG] and soybean meal [SBM]) into milk, urinary and fecal N in lactating dairy cows. For 11 days, twelve multiparous Holstein cows (means ± SD; 264 ± 18 DIM) were fed once a day an unlabeled TMR formulated to contain (DM basis) 335, 325, 190, 125 and 25 g/kg of CS, AS, CG, SBM and a mineral-and-vitamin premix, respectively. On the morning of day 12, cows were blocked by milk yield and randomly assigned within block to one of four dietary treatments constructed by replacing one feed ingredient of the unlabeled TMR with its corresponding 15 N-labeled ingredient (grown with 15 N-labeled fertilizers). Cows were fed dietary treatments for four days (day 12–15) and the unlabeled TMR from day 16–19. Feed intake and lactation performance were measured daily whereas total fecal and urinary collections were conducted on each cow every 6 h from day 12–19. Feeding 15 N-labeled ingredients had no effect on DMI (mean ± SD; 22.0 ± 2.0 kg/d), milk yield (26.4 ± 5.2 kg/d), N intake (631 ± 25 g/d), milk protein concentration (34.7 ± 3.3 g/kg), and N use efficiency (milk N/intake N; 235 ± 46 g/kg). By the end of sampling, 61% of 15 N was recovered in milk (13.6%), urine (24.1%) and feces (23.3%) suggesting substantial distribution of 15 N in tissues with slow turnover rates or growing actively (e.g., fetal tissues). The ratio of 15 N atom% excess (APE) in urine to 15 N APE in milk and the ratio of 15 N APE in feces to 15 N APE in milk measured on the fourth day of feeding the treatment TMR were used as indicators of relative N partitioning. The 15 N APE urine/milk ratio was greater for AS (1.51) than for CS (1.30), which in turn was greater than for the concentrates (1.02 for CG and 0.94 for SBM). In addition, the APE 15 N feces/milk ratio was greater in silages than concentrates (2.12 vs. 1.20, respectively). Interestingly, the main route of 15 N excretion from AS was fecal rather than urinary. Overall results suggested more excretion of urinary N and fecal N relative to milk N secretion for silages than for concentrates.