Renal energy excretion of horses depends on renal hippuric acid and nitrogen excretion
Renal energy losses in horses range between 7% and 12% of digestible energy (Kienzle & Zeyner, 2010). Presumably, the reason for the high renal losses is that phenolic acids of plant cell walls can be released in the digestive tract, absorbed, metabolized to hippuric acid and eventually be excreted via urine. In addition, nitrogen (N) from excess crude protein (CP) intake is excreted as urea. Thus‐—like in other species—a high CP intake increases renal energy excretion. The Society of Nutrition Physiology introduced a metabolizable energy (ME) system for energy evaluation of horse feed and energy requirements for horses (GfE, 2014). A subtraction of 8 kJ/g CP in the feed is made for all feed to predict renal energy losses, as suggested by Kienzle and Zeyner (2010). The content of phenolic compounds in plants and their release and uptake from the animal digestive tract may vary considerably amongst feed stuffs (Jung & Fahey, 1983; Oestmann, Südekum, Voigt, & Stangassinger, 1995). In addition, the CP content can vary considerably between feedstuff. There is a well‐known effect of plant age on the CP content in forages, especially in grass and grass products (Jeroch, Flachowsky, & Weißbach, 1993). Given these differences, it is likely that the ratio of CP to absorbable phenolic acids will vary, and it is likely that the ratio of renal hippuric acid excretion to urea excretion of horses will be affected. The hypothesis therefore is that feed‐specific renal energy losses per gram CP intake can be observed. In the present study, the renal energy, nitrogen and hippuric acid excretion after the intake of various feeds were investigated in ponies.