Another reason to properly manage horse dung: Phosphorus runoff

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Horse dung may pose a potential risk of phosphorus run-off into the environment if not properly managed, according to researchers.

Horses excrete most of their surplus phosphorus through their dung, with only 1% or so being passed through urine.

Phosphorus from animal manure runs the risk of getting into waterways, harming the environment. Indeed, several studies have indicated that leaching of phosphorus from horse paddocks and pastures are hotspots for high leaching losses.

Yet horses can suffer problems if deficient in the macro-mineral, given its important role in bone formation, and in energy and fat metabolism.

Three researchers in Finland, Markku Saastamoinen, Susanna Särkijärvi and Elisa Valtonen, set out to learn more about phosphorus use in horses and its excretion in manure when typical feeds and forage-based diets are fed.

They hypothesised that feeding regimes might influence phosphorus digestibility and excretion in feces, and therefore the environmental impact of horse husbandry.

They also studied the nutrient digestibilities of the diets, as well as the proportion of the soluble fraction of the total phosphorus.

Their experiment involved six horses fed six forage-based diets. The study involved a total collection of all horse dung produced.

The diets were 100% hay; 100% haylage; 80% hay with the rest in whole oats; 65% hay with the rest in whole oats; 80% hay with the rest in a commercial pelleted feed; and 65% hay with the rest in a commercial pelleted feed.

All diets, except those including the complete pelleted feed, were balanced with a mineral mixture in
which the phosphorus component was monocalcium phosphate.

All studied diets resulted in a positive phosphorus balance.

They found that digestibility of phosphorus varied from 2.7% to 11.1%. Supplementing forage-diets with concentrates slightly improved phosphorus digestibility. It also improved the digestibilities of crude protein and organic matter.

The horses excreted an average of 20.9 grams of phosphorus per day in their feces, ± 1.4 grams.

Excretion was smallest, 20 grams, in horses on a hay-only diet.

The average daily phosphorus excretion amounted to 7.6 kg in a year.

The soluble part of the total phosphorus in feces accounted for about 88%. This portion, they say, is vulnerable to runoff losses and may leach into waters.

“Thus, horse dung may pose a potential risk of phosphorus leaching into the environment if not properly managed, and is not less harmful to the environment than that from other farm animals,” the trio reported in the open-access journal Animals.

They say that supplementation with inorganic phosphorus should be controlled in the diets of mature horses in light work to decrease the excretion of the mineral in feces.

More research, especially into cost-effective feeding strategies and their applications for horses, is essential, they say, focusing on diet composition and ingredients, to reduce the horse industry’s potential impact on water quality.

Discussing their findings, the researchers say phosphorus excretion in horses is linearly related to its intake, and the intake increases with the increasing concentrate in the diet.

“It is impossible to conclude how polluting the horse industry is compared with other forms of animal production,” they said.

However, in another study, the high proportion of inorganic phosphorus in horse feces indicated that overfeeding the mineral to horses might be more harmful to the environment than overfeeding it in dairy cows.

Saastamoinen and Särkijärvi are with the Natural Resources Institute Finland; and Valtonen is with the University of Helsinki.

Saastamoinen, M.; Särkijärvi, S.; Valtonen, E. The Effect of Diet Composition on the Digestibility and Fecal Excretion of Phosphorus in Horses: A Potential Risk of P Leaching? Animals 2020, 10, 140.

The study, published under a Creative Commons License, can be read here.

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