SummaryThe prediction of renal energy excretion is crucial in a metabolizable energy system for horses. Phenolic acids from forage cell walls may affect renal energy losses by increasing hippuric acid excretion. Therefore, the relationships were investigated between renal energy, nitrogen (N) and hippuric acid excretion of four adult ponies (230-384 kg body weight (BW)) consuming diets based on fresh grass, grass silage, grass cobs (heat-dried, finely chopped, pressed grass), alfalfa hay, straw, extruded straw and soybean meal. Feed intake was measured; urine and faeces were quantitatively collected for three days. y = 14.4 + 30.2x 1 +20.7x 2 (r = .95; n = 30; p < .05). Renal hippuric acid excretion was highest after intake of fresh grass and lowest after intake of soybean meal. The ratio of hippuric acid to creatinine in urine and the excretion of hippuric acid per gram of dry matter intake was significantly higher for fresh grass than for all other rations. There was no relationship between aromatic amino acid intake and renal hippuric acid excretion. The results of the present study and literature data suggest that feed can be categorized into four groups with regard to the energy losses per gram CP intake: (i) protein supplements (e.g., soybean meal): 4.2-4.9 kJ/g CP intake (ii) alfalfa hay, grains, dried sugar beet pulp: 6.4 kJ/g CP intake, (iii) hay, preserved grass products, straw: 5.2-12.3 kJ/g CP intake (mean 8) and (iv) fresh grass. For group (iii) a negative relationship was observed between renal energy losses per gram of CP and the content of CP or neutral-detergent-insoluble CP in dry matter.
K E Y W O R D Shippuric acid, metabolizable energy, nitrogen excretion, phenolic acids
| INTRODUCTIONRenal 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