Amount of nitrogen excreted by horses affected by diet – study

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"When feeds with good protein quality and amino acids profile are fed, smaller nitrogen intakes can be applied to minimize the risk of the nitrogen excretion."
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Feed choices affect the amount of nitrogen excreted by horses, researchers have found.

Scientists in Finland studied how protein sources and intake influenced diet digestibility and nitrogen excretion in adult horses.

Markku Saastamoinen, Susanna Särkijärvi and Heli Suomala used six healthy Finnhorse mares in their digestibility trial, in which six typical horse diets were compared.

The diets were 100% haylage; 100% hay; 70% hay and 30% oats; 70% hay with a 30% mix of soybean meal and oats; 70% hay with a 30% mix of rapeseed meal and oats; and 70% hay with a 30% mix of linseed meal and oats.

The experiment was organised into four 3-week experimental periods so the horses could be assessed on the different diets, with dung and urine collected for analysis.

The digestibilities of dry matter and organic matter in the haylage-only diet were lower compared to the other diets, the study team reported in the journal Animals.

“The supplemental protein feeds improved the diet digestibility of crude protein compared to a hay plus oats diet,” they wrote.

The dry matter, organic matter and crude protein digestibilities of the soya-supplemented diet were better than those of the rapeseed and linseed-supplemented diets.

Faecal excretion was greater for haylage (19.3kg of fresh faeces and 3.6kg of dry matter per day) and hay (18.7kg of fresh faeces and 3.6kg of dry matter per day) diets compared with the other option.

Urine excretion was also greater for forage-only diets when compared with the diets including protein supplements.

Horses excreted an average of 14 litres of urine a day on the haylage-only diet and 14.3 litres a day on the hay-only diet.

They found the animals excreted more nitrogen in their urine than in their dung.

Nitrogen excretion differed between the diets, they reported. Horses on a haylage-only diet excreted 51.6 grams of nitrogen in their faeces each day and on the hay-only diet it was 51.4 grams of nitrogen a day. When protein content in forages increased (haylage vs. dried hay), nitrogen excretion via urine increased.

When nitrogen excretion in faeces and urine were counted together, horses excreted less nitrogen with a hay-only diet compared to the hay-and-oats diet supplemented with feeds rich in protein.

“Feeding recommendations should consider not only the horse category and work level, but also the protein source,” they concluded.

“When feeds with good protein quality and amino acids profile are fed, smaller nitrogen intakes can be applied to minimize the risk of the nitrogen excretion, at least concerning adult horses involved in light exercise.

“At the farm level, improved understanding of feed quality, feeding planning and practices, as well as binding the excreted nitrogen in the beddings, is a way to decrease the risk of nitrogen leaching and evaporation.

“Further research on feeding strategies, their applications and selection of protein supplements for horses is essential to reduce the horse industry’s harmful impacts on the environment.”

The study team noted that the horses maintained their body weight and body condition score during the experiment, indicating that the feeding regimes and intake covered the nutritional needs of the horses during the experiment.

Saastamoinen and Särkijärvi are with Finland’s Natural Resources Institute, and Suomala is with the University of Helsinki.

The research was partly funded by the Finnish environment ministry.

Saastamoinen, M.; Särkijärvi, S.; Suomala, H. Protein Source and Intake Effects on Diet Digestibility and N Excretion in Horses—A Risk of Environmental N Load of Horses. Animals 2021, 11, 3568. https://doi.org/10.3390/ani11123568

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

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