Effective nitrogen preservation during urine collection from Holstein heifers fed diets with high or low protein content

Six Holstein heifers (body weight = 535-625 kg) fed a total mixed ration containing either high protein (13.4%) or low protein (9.0%) were used to evaluate the effect of 3 urine collection methods (chilled, acidified before collection, or acidified after 6 h of collection) on urinary N preservation. In a 2-period crossover design, 16-d diet adjustment stages preceded five 24-h collections. Urinary catheters were inserted 1 d before the collection periods. Urine collection tubes were configured to split urine to 3 collection containers: 1 acidified with 6 N HCl before collection at a rate calculated to reduce pH to below 2, 1 acidified every 6 h during collection to pH below 2, and 1 located in a large cooler of ice. Collection method did not affect urinary concentration of N or urine urea-N (9.2 ± 0.9 g/L and 6.58 ± 0.9 g/L, respectively) or urinary excretion of N or urea-N (82 ± 3.8 g/d and 59.5 ± 3.8 g/d, respectively). These 3 collection methods are equally effective in preserving N during urine collection, but the “chilled immediately” approach may be useful for studies focused on ammonia emission. Urinary and fecal N excretion were significantly different across collection days; fecal N was more highly variable than urinary N. Intake and apparent N digestibility decreased during the collection week, and excretion of urinary and fecal N increased, particularly on d 5. (Stable rectal temperatures suggested no urinary infections.) Improvements in total collection methodology will support continued progress in the understanding of livestock N utilization and post-excretion changes in manure N.