Effects of forage phosphorous content on faecal phosphorous excretion and possible markers of low phosphorous intake in foals fed forage-only diets.

Knowledge of endogenous nutrient losses is important when estimating the nutrient requirements of animals. It has been suggested that faecal endogenous phosphorus (P) losses differ between growing and adult horses, but studies on foals are scarce. In addition, studies on foals on forage-only diets with different P contents are lacking. Thus this study: (1) assessed faecal endogenous P losses in foals fed a grass haylage-only diet close to or below estimated P requirements; (2) evaluated use of serum cross-linked carboxyterminal telopeptides of type-I collagen (CTx) as a marker of bone resorption secondary to low-P intake; and (3) examined whether analysis of faecal P concentration on a dry matter (DM) basis could be used as an indicator of P intake. Six foals were fed three grass haylages (fertilised to contain different amounts of P: 1.9, 2.1, 3.0 g/kg DM) for 17-day periods in a Latin square design. Total collection of feaces was performed by the end of each period. Faecal endogenous P losses were estimated using linear regression analysis. There was no difference in the concentration of CTx in plasma between diets in samples collected on the last day of each period. A correlation was found (y = 0.64x - 1.51; r  = 0.75, p < 0.0001) between P intake and faecal P content, but regression analysis indicated that underestimation as well as overestimation of intake is likely if faecal P content is used to assess intake. It was concluded that faecal endogenous P losses in foals are low, probably no higher than in adult horses. It was also concluded that plasma CTx cannot be used to assess short-term low-P intake in foals and that faecal P content cannot be used to assess differences in P intake, at least not when P intake is close to or below estimated P requirements.