Phosphorus excretion by mares post‐lactation

Across the equine literature, estimates of true P digestibility range from −23% to 79%. This large range cannot be explained by differences in P intake or phytate‐P intake alone. However, differences in endogenous P secretion into the GI tract may explain the variation. In horses, excess absorbed P is not excreted in the urine but is re‐secreted into the GI tract, increasing faecal P and leading to estimates of low P digestibility. Thus, accurate estimates of P digestibility can only be obtained if absorbed P is retained in the horse. The objective of this study was to examine P digestibility in post‐lactational mares and control mares that were fed similar amounts of P. It was hypothesized that post‐lactational mares would have greater P retention and higher apparent P digestibility than control mares. Prior to the study, four lactating and four non‐lactating mares were fed a diet that provided 100% of the control mares’ P requirement, but only 55% of the lactating mares’ P requirement. During the study, both groups were fed P at the rate recommended for non‐lactating mares. Post‐lactational mares did not retain more P than control mares but tended to excrete more P than control mares ( p  = .082), presumably due to differences in endogenous P secretion into the GI tract. Metabolic changes occurring during mammary gland involution may have contributed to the increase in P excretion. However, faecal P excretion exceeded P intake in both groups ( p  = .08) and both groups lost weight during the study. Tissue mobilization during weight loss may have influenced P secretion into the GI tract.