Horses are typically ingesting more than half a kilogram of soil a day, findings of recent research show, raising questions over the wisdom of grazing them on short swards.
Stefan Jurjanz and his fellow researchers noted that soil ingestion has been well documented for most animals reared outside, but not in horses.
In horses, ingestion of soil may cause gastrointestinal disorders such as sand colic or intestinal damage. There is also the risk of taking in soil contaminants.
The study team, with the University of Lorraine in France and the University of Limerick in Ireland, devised an experiment to assess soil ingestion in six adult Irish sport horses grazing at three levels of herbage – two three or four percent of their body weight.
The researchers, reporting in the journal Animals, found that their soil intake amounted to about 4% of the total ingested dry matter, corresponding to 543 to 648 grams of dry soil per animal per day.
This intake, they said, is quite similar to cattle in normal grazing conditions.
“Such amounts would clearly be an issue for food safety in areas with contaminated soil, but also an animal welfare issue due to gastrointestinal damage,” they said.
The study team calculated soil ingestion by the faecal recovery of a soil natural marker – the amount of acid-insoluble ash.
Horses had 4.5% of soil in their total intake when offered herbage at 2% of their body weight; 4.1% of soil at 3% of body weight; and 3.7% soil when offered herbage at 4% of their body weight.
The 4% offering presented significantly less soil intake (543 grams per day) compared to the more restricted food offerings, with 624 grams for 3% herbage and 648 grams for 2% herbage.
The post-grazing sward height was significantly lower on the 2% offer (3.1cm) compared to the higher offers (4.1cm and 4.4cm respectively for 3 and 4%).
Thus, restricted herbage allowance required grazing closer to the ground and consequently increased soil ingestion.
The sward height appeared to be a reliable indicator to manage horse withdrawal from a pasture to limit soil ingestion and reduce the risk of gastrointestinal problems caused by it, they said.
Discussing their findings, the study team said the ingestion of soil seems quite notable and should be integrated in risk evaluations for food safety and health.
“Indeed, such amounts could transport significant amounts of harmful compounds if the soil contained any cadmium or arsenic for example, but could also disturb the gastrointestinal milieu by its mechanical and microbiological adverse effects.”
It seems likely that poorer grazing circumstances, such as muddy conditions underfoot, would increase the soil ingestion in horses even more, as reported in studies in cattle and sheep, but the level of such increases had yet to be studied.
They noted that ingestion of soil by horses has often been reported, especially with harmful effects on health. Soil ingestion risk should be considered more carefully in the evaluation of animal welfare, they said.
They said that although sandy soils are linked to sand ingestion, soil ingestion can also occur on a wide range of soil textures.
The authors said more detailed work is necessary to precisely quantify the effects on soil ingestion. There are different grazing management systems to consider, differences between types of horses, and the influence of restricted pasture access through stabling.
They concluded that the amount of soil ingested by horses is not negligible. The known risks from soil ingestion show the need to limit this involuntary intake in order to ensure horse welfare.
“The amount of herbage offered and the sward height appear to be useful tools to limit soil ingestion as previously shown in other herbivorous animals.”
The study team comprised Jurjanz, Claire Collas, and Cyril Feidt, all with the University of Lorraine; and Carol Quish and Bridget Younge, with the University of Limerick.
Jurjanz, S.; Collas, C.; Quish, C.; Younge, B.; Feidt, C. Ingestion of Soil by Grazing Sport Horses. Animals 2021, 11, 2109. https://doi.org/10.3390/ani11072109