Renal energy excretion of horses depends on renal hippuric acid and nitrogen excretion

Abstract

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

The effect of crude protein content of the diet on renal energy losses in horses

“…When the equation (Equation 1) of Hipp et al. (2017) was used for the prediction of renal energy excretion per gram crude protein intake in rations containing mainly gramineae (HAY1, HAY2, GRASS, STR), there was a very close correlation between predicted and measured values ( r 2 = .90). When the data for the ingredients which were calculated by difference were included, there was still a remarkably good prediction (Figure 1).…”

“…Urine and faeces were analysed as described by Hipp et al (2017): to determine the gross energy content of urine and faeces, bomb calorimetry was used. Nitrogen in urine was determined by the Kjeldahl method (VDLUFA, 2012).…”

“…Since the study of Hipp et al. (2017) showed high values for renal energy excretion in fresh grass, the present study was focused notably on this aspect.…”

“…Renal energy losses in horses are relatively high, when compared to other mammals (Kienzle & Zeyner, 2010). Hipp, Südekum, Zeyner, Goren, and Kienzle (2017) showed that renal energy losses of horses depend mainly on renal hippuric acid and renal nitrogen excretion. Renal nitrogen excretion is predominantly caused by crude protein (CP) intake.…”

“…Hipp et al. (2017) demonstrated for gramineae that all three factors are linked together and that renal energy excretion per gram protein intake (y; kJ/g crude protein) can be predicted by crude protein content (x; g/kg DM) according to the following Equation 1:…”