Renal energy excretion of horses depends on renal hippuric acid and nitrogen excretion.
Authors: Hipp B, Südekum K-H, Zeyner A, Goren G, Kienzle E
Journal: Journal of animal physiology and animal nutrition
Summary
# Editorial Summary Understanding energy losses through urine is fundamental to accurate nutritional modelling in horses, yet predicting renal energy excretion has remained challenging—particularly when forage-derived phenolic compounds (which form hippuric acid) may influence urinary losses. Hipp and colleagues investigated this relationship in four adult ponies fed diverse diets (fresh grass, various hay types, straw, silage and protein supplements), with quantitative collection and analysis of feed intake, faeces and urine over three-day periods to determine correlations between renal energy, nitrogen and hippuric acid excretion. The researchers found a strong predictive equation (r = 0.95) showing that renal energy excretion depended significantly on both urinary nitrogen and hippuric acid excretion, with fresh grass producing substantially higher hippuric acid losses than all other feed types, despite no direct link to aromatic amino acid intake. This work has important implications: feeds can now be categorised into four groups with distinct energy loss profiles per gram of crude protein (ranging from 4.2 kJ/g for soybean meal to substantially higher losses from fresh grass), meaning ration formulation and metabolisable energy calculations must account for feed type, not merely crude protein content. For practitioners, this suggests that grazing horses or those receiving predominantly fresh forage may experience greater urinary energy losses than those on preserved forages or concentrate supplements—a factor that warrants consideration when assessing energy balance and performance in working or breeding stock.
Read the full abstract on PubMed
Practical Takeaways
- •Fresh grass diets result in higher renal energy losses than preserved forage or concentrates, which should be accounted for when calculating metabolizable energy available to horses
- •Different feed types have predictable effects on nitrogen and hippuric acid excretion patterns, enabling more accurate dietary energy calculations for horses on mixed rations
- •Protein supplements produce the lowest renal energy losses per gram of protein intake, while preserved grass products show high variability depending on their crude protein content
Key Findings
- •Renal energy excretion correlates strongly with both renal nitrogen excretion and hippuric acid excretion (r=0.95), described by the equation y=14.4+30.2x₁+20.7x₂
- •Fresh grass produces significantly higher hippuric acid excretion compared to all other tested feed types, with the ratio of hippuric acid to creatinine and hippuric acid per gram dry matter intake both highest for fresh grass
- •Feed categories show distinct energy loss patterns per gram crude protein intake: protein supplements 4.2-4.9 kJ/g, alfalfa/grains 6.4 kJ/g, hay/preserved grass 5.2-12.3 kJ/g, with fresh grass as a separate category
- •Hippuric acid excretion is not related to aromatic amino acid intake, suggesting phenolic acids from forage cell walls rather than protein metabolism drive increased renal energy losses