Determining N losses through breath ammonia production in lactating cows

In N balance studies, retained N, calculated by subtracting N secretion and excretion from N intake, has been generally considered to be overestimated in lactating cows. Various studies have investigated potential sources of unaccounted N losses; however, these sources have not been fully identified. In the current study, we evaluated N losses via oral NH 3 -N production which includes NH 3 -N originating from both eructation and from pulmonary exhalation (defined as breath NH 3 -N) as a potential contributor to unaccounted N fraction in lactating cows. Experiment 1 was conducted to assess breath NH 3 -N production and its diurnal pattern with 8 mid-lactation Holstein cows in a crossover design (21 d per period). Cows received a diet supplemented with saturated fatty acids or oleic acid (2% of dietary DM). Experiment 2 was conducted to assess the effects of dietary CP level (15.3 vs. 17.0% of dietary DM) on breath NH 3 -N production with 16 mid-lactation Holstein cows in a randomized complete block design (21 d). Multiple breath samples (8 time points and 5 time points in a 24-h cycle during Experiment 1 and 2, respectively) were collected from individual cows using a spot breath collection procedure with a facemask system (3-min breath collection for each event). In Experiment 1, breath NH 3 -N production (0.82 g/d) was not affected by dietary treatment and had no interaction between diet and collection time in a 24-h feeding cycle. However, breath NH 3 -N production was affected by collection time, indicating a diurnal pattern of breath NH 3 production. The production decreased immediately after feeding, gradually increased until 6 h after feeding and then remained relatively stable until the next feeding time. In Experiment 2, although N intake was greater (802 vs. 701 g/d) for cows fed the high-CP compared with the low-CP diet, breath NH 3 -N production did not differ (0.61 g/d) between treatments, resulting in greater breath NH 3 -N production as a proportion of N intake (0.086 vs. 0.075%) for cows fed the low-CP diet. Data from Experiment 1 and 2 were combined and analyzed to determine the relationship between N intake and breath NH 3 -N production. The linear regression analysis showed a poor linear association (R 2 = 0.05; CCC=0.11) between N intake and breath NH 3 -N production. In conclusion, N losses in breath were observed, with breath NH 3 exhibiting a diurnal pattern. However, daily production of breath NH 3 -N was not affected by dietary CP or N intake in the current experiments. Although N losses through breath NH 3 were measurable, the loss was ignorable because its proportion of N intake was trivial.