Updating analysis of nitrogen balance experiments in dairy cows

Nitrogen balance (NB) experiments allow calculation of N retention in the body by subtracting excreted feces (NF), urine (NU) and milk (NM) from intake (NI). In a previous study, we found that NB data with lactating dairy cows were generally high and, current meta-analysis, update our earlier study last 2 decades investigate probable causes error. A total 83 publications, 86 307 dietary treatments, selected top-ranked scientific journals reported all components. The components analyzed linear regression model used NI as an independent variable experiment random effect. NF, NU NM each represented 27 to 34% NI, remaining accumulated was equal 38.5 g/d (overall SD = 43.2 g/d). Retained (as or % NI) increased linearly this led unlikely retentions, especially at NI. Both NF (g/d) increasing assume some unaccounted. Only ~22% measured wet samples when analysis dry samples, no mention corrections due potential volatile losses during drying reported. No experimentalists preserved immediately prevent volatilization collection. Moreover, ~27% estimated volumes concentration creatinine spot these experiments, ~12% lower than those where collected (168 vs. 191 40 volume acids for preservation, 33 furnished incomplete information, remainder did not describe preservation method. conclusion, results using overestimate retention, are most reason.