First results on the incorporation and excretion of 15N from orally administered urea in lactating pony mares.

This research investigates how lactating pony mares absorb and excrete nitrogen from orally ingested urea. The findings show that the majority of the nitrogen ends up in the horses’ urine and feces, with only a small percentage appearing in their milk.

Research Methodology

• The researchers conducted their study on two lactating pony mares. The chosen mares were given 20 grams of 15N urea orally for six consecutive days. Urea has a natural abundance of 15N isotopes (around 0.37%), allowing for tracing within biological systems.
• They then monitored the routes of excretion, specifically through urine, feces, and milk. They also checked how much of that urea is converted into milk-lysine, which is an essential amino acid.
• The “labelling” mentioned in the article refers to the method of tracing the 15N’ isotopes within the biological samples of the pony mares.

Results and Findings

• Approximately 80% of the ingested 15N’ isotopes were expelled via urine and feces.
• Contrastingly, only about 2% was detected in the mares’ milk. The amount of 15N’ found in milk-lysine was even less, accounting for only 0.04% of the total consumed 15N’.
• 90% of the 15N’ isotopes were recoverable from these pathways, indicating that only a small amount was metabolized or otherwise utilized by the mares. This suggests a high efficiency in tracking the isotopes within the biological system of the mares.
• The researchers noted unexpectedly high labelling (greater than 0.3 atom-% 15N’) in tissues with active metabolism.

Conclusion

• The small quantity of oral urea nitrogen converted into milk-lysine suggests that enteral synthesis (the process of breaking down ingested nutrients in the gut) may not substantially contribute to meeting the amino acid requirements for lactating mares.
• Based on this, the researchers concluded that determining the exact amino acid requirements would be necessary for this specific group of ponies.