Total and Water-soluble Phosphorus Excretion in Horses Consuming All-Forage Diets

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Introduction

Disposal of animal manure is under increasing scrutiny because of growing public concern over the long-term effects on phosphorus (P) accumulation in soils and its contribution to water quality degradation and eutrophication. When manure is applied to pastures or deposited by the grazing animal, soils will retain most of the P applied in excess of plant uptake. Despite the generally high adsorption capacity of most soils, P leaching can occur and P losses to runoff or surface erosion can be high.

Potential P loss is not only related to the amount of P excreted in manure, but also how easily the manure P is dissolved in storm water and subject to runoff loss. Specifically, the chemical forms of P and their relative proportions in manure influence runoff risk. Sharpley and Moyer [1] found a strong correlation between water soluble P in livestock and poultry manures and the amount of P leached from soil following simulated rainfall events. Thus the soluble P fraction in manure is thought to pose the greatest environmental risk.

Water soluble P has been shown to be highly correlated with daily P intake in livestock and poultry.[2] Although total P excretion has been positively correlated with P intake in horses,[3] the soluble P content of horse manure, as well as the effect of diet on fractional P excretion in horses is unknown. This study was undertaken to evaluate total and soluble P excretion in horses consuming an all-forage diet.

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Materials and Methods

Six mature horses (mean ± SE, 530 ± 14 kg) were fed 3 different hays in a 3 X 3 Latin square design. In each of 3 periods, 2 horses were fed the same hay. Hay treatments were switched at the end of each period. This permitted representation of all hays in each period, and n = 6 observations for each hay. The 3 hays evaluated were Coastal bermudagrass (CBG; 10.2% CP, 73%NDF, .29% Ca, .16%P), Tifton-85 bermudagrass (T85; 8.6% CP, 77% NDF, .29 %Ca, .2%P), and perennial peanut (PEA; 12.1% CP, 46%

Results and Discussion

Daily intake of DM (P = .12) and P (P = .14) were not different between hays, but Ca intake was 5-times greater (P = .0001) when horses were offered PEA compared to CBG or T85. Only trace amounts of P (<.04 g/d) were excreted in urine and urinary P concentration was similar between hays (mean 5.2 ± .5 mg/L). Urine volume was highest (P = .0004) with PEA hay, resulting in greater (P = .02) urinary P excretion (mg/kg BW/d) with PEA than CBG and T85 hays. In agreement with others,[6] feces was the

Conclusion

Mature horses fed an all-forage diet in amounts that met the maintenance P requirement excreted a high proportion of total P as soluble P in feces. Legume forage, possibly acting through the higher Ca content, may permit a lower level of soluble P excretion in the manure compared to grass hays. The environmental impact of different forage-based diets remains to be determined and should also take into account the impact of other nutrients excreted in manure (e.g, nitrogen).

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