By Dr Rosemary Waring

Chief Science Officer / Tharos Co-Founder


Why does eating grass make my horse irritable?



My name is Dr Rosemary Waring. I have spent my career as a toxicologist, doing research and teaching whilst being on international committees such as EFSA (European Food Safety Authority) and PRiF (Pesticide Residues in Food) in the UK. 

Toxicologists study the reasons why cells die and I have looked at the interactions of environmental compounds with human tissues. I have a particular interest in digestion and the chemicals produced by the gut microbiome – when this complex mix of bacteria, viruses and fungi goes off-balance, a variety of toxic compounds are produced and these can lead to illness or poor functioning. 

Our original work to develop Equinectar was focused on the metabolites of digestion in performance horses. We discovered something interesting – horses have relatively low levels of an enzyme called amylase which breaks down starch. If horses are fed a lot of starch, most of it goes down to the hindgut, where it is fermented by ‘bad’ bacteria – we have managed to identify some of the bacteria and toxins which are involved. 

We then started to look at horses that ate a low starch diet and were largely pasture fed. Similar situations to over feeding of starch can happen when the grass grows quickly or is particularly high in fructans. The gut can become overloaded and the bacterial composition of the hindgut may worsen, in many cases significantly.

We published a paper following a trial we conducted with Redwings Horse Sanctuary. Horses exposed to spring grass experienced substantial changes to their gut microbiomes. Those horses that were fed EquiNectar did not experience the same dramatic microbiome changes. You can read the study here: Modulation of the equine microbiome by pasture and feed supplements: A metabolomics approach

Over the past few weeks we have been answering questions from horse owners, with a common theme – “Why does eating new grass make my horse irritable?”.

To try to uncover the answer, I have been reviewing the scientific literature and have produced a brief review of my findings.


What the science shows

First, what is in new grass to make it different? New grass contains sugars, glucose, fructose and sucrose, which make it taste sweet, with fructans, which are long-chain polymers of fructose (Kagan 2022).  Grasses grow fastest when the weather is warm and there is plenty of moisture, conditions which especially occur in Spring and also Autumn. Both these seasons can be chilly at night, and fructans protect against cold-temperature stress while acting as an energy source. Several studies have shown that fructan levels are higher in the afternoon than in the early morning when they have supported the plant metabolism overnight, although they may still be at high levels in the morning after frost (Weinert-Nelson et al 2022, Kagan et al 2020).

Fructans have evolved to protect grasses but these compounds are not broken down by the equine digestive enzymes. Instead, after ingestion they are concentrated in the hind gut where they are hydrolysed to fructose (the basic unit) by bacterial enzymes. In this fermentation process, ‘bad’ bacteria take over, the gut contents become more acidic, toxic compounds are released and the gut walls become more permeable, allowing more toxins and fructose to be absorbed into the blood stream.

Fructose is normally converted to glucose in the body but sudden high concentrations may overload this pathway, leading to raised blood fructose. No work seems to have been done in horses, but several studies in rats, mice and human beings have shown that fructose affects metabolism (Spagnuolo et al 2020) and increases the appetite (Payant and Chee 2021) so that horses may actually eat more of the new grass, which they prefer anyway, perhaps because it is sweeter and less fibrous (Cameron et al 2022). Fructose also affects the brain, increasing anxiety and behavioural problems, although the exact mechanisms are not clear (Tang et al 2022, Payant and Chee 2021). Fortunately, the effects are limited as short-term intake of fructose does not lead to lasting damage, at least in rats (D’Ambrosio et al 2023). In agreement with these studies, new green grass with its accompanying fructans was found to be one of the factors associated with misbehaviour and poor performance in horses, together with obesity and competition stress (Buckley et al 2013). The reasons for this pasture-linked aberrant pattern must include the alteration of the gut microbiome, which affects mood and behaviour (Mach et al 2020) while the discomfort of the gut digestion processes will also be a factor and high blood levels of fructose may temporarily alter brain function.

Possible solutions to the problem will include keeping new grass grazing to a minimum. Allowing access to new pasture only in the early morning and keeping grass heights low (Kagan 2022) will minimise fructan intake. Supplements such as Equinectar contain fructanase and will improve the slow release of fructose at an early stage of digestion, avoiding metabolism of fructans in the hind gut and possible behaviour problems.  


1. Buckley P et al Equine Vet Journal 45(1): 9-14 2013

2. Cameron A et al J Equine Vet Sci 110: 103745 2022 03

3. D’Ambrosio C et al Nutrients 15(2): 2023 Jan 16

4. Kagan IA J Equine Vet Sci 110: 103866 2022 03

5. Kagan IA et al J Equine Vet Sci  84: 102858 2020 01

6. Mach N et al Scientific Reports 10(1): 8311 2020 05 20

7. Payant MA and Chee MJ Neuroscience and Biobehavioral Reviews 128: 346-357 2021 09

8. Spagnuolo MS et al Molecular Neurobiology  60(2): 1004-1020 2023 Feb

9. Tang CF et al Nutrients 14(9) 2022 Apr 29

10. Weinert-Nelson JR et al J Equine Vet Sci 110:103836 2022 03

2 thoughts on “Why does eating grass make my horse irritable?”

  1. Given this evidence, would I be better bringing my horse in during the afternoon and leaving him out at night/morning? I don’t want to deprive him to much and risk him gorging if only out for a limited time? He has a supplement specifically to help his body deal with the sugars in the grass, along with a bespoke vitamins/mineral blend and Fulvic acid.

  2. This was so interesting and describes what is happening to my mare. I knew it was the grass because she changes when withdrawn from the grass. Thank you

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