Phosphorus excretion by mares post-lactation

Ashley L. Fowler, Morgan B. Pyles, Susan H. Hayes, Andrea D. Crum, Laurie M. Lawrence

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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.

Original languageEnglish (US)
Pages (from-to)1912-1918
Number of pages7
JournalJournal of Animal Physiology and Animal Nutrition
Issue number6
StatePublished - Nov 2020

Bibliographical note

Publisher Copyright:
© 2019 Blackwell Verlag GmbH


  • digestibility
  • endogenous phosphorus loss
  • equine
  • lactation
  • phosphorus excretion

PubMed: MeSH publication types

  • Journal Article


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