Put out to pasture: Effects of a grass-hay diet on gut bacteria assessed in study

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Horses thrive on high-forage diets through the microbial fermentation of fibre and other compounds in the hindgut.
Horses thrive on high-forage diets through the microbial fermentation of fibre and other compounds in the hindgut.

The diversity and composition of the gut-related bacterial population of horses changes over a year when kept on pasture, researchers have found. It is driven by changes in the nutrient make-up of the grass, which in turn is affected by climate.

Horses thrive on high-forage diets through the microbial fermentation of fibre and other compounds in the hindgut. This requires a high food intake level to maintain proper gut function, with forage comprising most of the diet.

Research has shown that the balance of microbial populations in the hindgut is important to maintain digestive health, immune function, and performance.

Over the past decade, research has focused on understanding the structure and composition of these microbial communities in the hindgut. Several studies have indicated that the structure of the equine hindgut microbiota is complex, comprising a highly diverse community dominated by bacteria, amongst other microbial species such as archaea, protozoa and fungi.

Karlette Fernandes and her fellow researchers at New Zealand’s Massey University and the nearby Grasslands Research Centre devised an experiment to explore seasonal variations in the faecal microbiota of mature adult horses kept on pasture.

Ten mares (eight Standardbreds and two Thoroughbreds) were used in the study at the Veterinary Large Animal Teaching Unit at the university.

They were kept for a year on a paddock with a standard New Zealand pasture mix of predominantly ryegrass and white clover species. The animals were provided with supplementary hay in the colder months, from June to October, when pasture cover was lower.

Each month, the study team measured how much pasture was present and collected pasture and hay samples to assess their nutrient content. Monthly faecal samples were also collected from all horses to investigate the diversity of the bacterial species present using next-generation sequencing technology.

The researchers said the effects of season and the more subtle effects of the month on the diversity of the bacterial community appeared to be driven by the variation in the nutrient composition of the pasture.
The researchers said the effects of season and the more subtle effects of the month on the diversity of the bacterial community appeared to be driven by the variation in the nutrient composition of the pasture.

Overall, the pasture and pasture-plus-hay diets showed a high diversity of bacterial genera in the faeces. However, the researchers found that the diversity of faecal bacteria was greater during the months when the horses were kept exclusively on pasture compared to when they received a hay top-up.

“The supplementation of hay appeared to dampen the effects of variations in pasture composition,” they said.

“This dampening effect could be associated with the relatively consistent nutrient composition of hay usually harvested at a fixed time during the year when compared to the variation observed in pasture grazed over several months.”

They said caution was warranted in extending this finding to other sources and types of hay, because there may be substantial variations in nutrient composition depending on the type of grass/legume, the stage of growth when harvested, climate factors, the time of harvest, and the method of processing and storage.

The diet, and the season and month, all had a major influence on the diversity of the species of bacteria in the faeces.

While there were some differences between the horses, generally the bacterial populations could be grouped together in samples obtained during May, June, and July (late-autumn to winter), and January, February, and March (a period of drought).

“More specifically, we were able to show an association between specific bacterial species, nutrients (dry matter, protein, and structural carbohydrates), and climatic conditions (rainfall and temperature),” they reported.

They said the effects of season and the more subtle effects of the month on the diversity of the bacterial community appeared to be driven by the variation in the nutrient composition of the pasture.

When the pasture was growing, Firmicutes dominated the bacterial community, whereas when the pasture was drought-stressed, the abundance of Bacteroidetes increased, at the expense of Firmicutes.

In essence, when the dry matter percentage of the pasture increased, the relative abundance of Bacteroidetes increased while Firmicutes decreased, whilst the relative abundances of other bacteria remained fairly constant.

This finding suggests that major shifts in bacterial diversity and abundance can occur with Firmicutes and Bacteriodetes, they said.

“Our study showed that the diversity and composition of the bacterial population of horses kept on pasture changes over a 12-month period, and this reflects changes in the nutrient composition of the pasture, which in turn is influenced by climate,” they concluded.

“The population structure of the bacterial community, albeit dominated by two bacterial phyla, was dynamic with relative abundances of bacterial genera that fluctuated over time.

“These fluctuations appeared to be driven by seasonal changes in pasture composition associated in turn with climate factors such as rainfall and temperature.”

It would be interesting, they said, to investigate the resilience of faecal microbiota following dietary alterations — that is, how do their abundances bounce back after a shift during the dry pasture periods?

“The outcome of this study suggests future studies can be designed to examine the rate of change in the diversity of faecal microbiota and its resilience following dietary alterations and investigate the functional roles played by the dominant bacterial genera at different times of the year.”

They said their findings may have implications for managing horses on pasture and the use of forages for horses susceptible to digestive problems.

The study team comprised Karlette Fernandes, Erica Gee, Chris Rogers, Patrick Biggs, Charlotte Bolwell and David Thomas, all with Massey University; and Sandra Kittelmann and Emma Bermingham, with the Grasslands Research Centre, part of AgResearch Ltd.

Fernandes, K.A.; Gee, E.K.; Rogers, C.W.; Kittelmann, S.; Biggs, P.J.; Bermingham, E.N.; Bolwell, C.F.; Thomas, D.G. Seasonal Variation in the Faecal Microbiota of Mature Adult Horses Maintained on Pasture in New Zealand. Animals 2021, 11, 2300. https://doi.org/10.3390/ani11082300

The study, published under a Creative Commons License, https://creativecommons.org/licenses/by/4.0/ can be read here. https://doi.org/10.3390/ani11082300

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