Excretion of faecal, urinary urea and urinary non-urea nitrogen by four ruminant species as influenced by dietary nitrogen intake: A meta-analysis

The quantification of faecal nitrogen (FN) and of urinary urea-N (UUN) and urinary non-urea-N (UNUN) excretion at varying N contents in ruminant rations is an important tool in assessing endogenous N turnover via the rumino-hepatic cycle. Using a statistical analysis based on an extensive database, the aim of this meta-analysis was to evaluate correlations derived previously by deduction. The data were categorised into dairy cattle, growing cattle (bulls and heifers), sheep and goats. Data from 50 publications were considered. The independent variable was the daily N intake (NI, g/day). The dependent variables were the daily quantities (g/day) of FN, urinary N (UN) , UUN, UNUN and N retention. The NI influenced FN to differing extents in goats, dairy cattle, growing cattle and sheep (listed in descending order of influence). Except in sheep, the effect was statistically significant. The influence on UN varied in the order goats, growing cattle, dairy cattle and sheep; the effect was statistically significant only for dairy cattle and growing cattle (P<0.001). The UUN was influenced in the order sheep, goats, dairy cattle and growing cattle (P<0.05). The UNUN could be assessed only in dairy cattle, growing cattle and sheep and was not influenced by NI. The UUN is therefore more strongly dependent on NI than is UNUN and the latter can therefore continue to be seen as obligatory. The FN is indeed influenced by NI but, as a result of higher digestibility of the total ration with increasing crude protein content, an improvement in microbial crude protein synthesis can also be assumed, which is reflected in higher FN levels.