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Nutritive Value of Grass: Considering Pasture in the Horse’s Diet



Horses are evolved to consume bulky plant matter such as grasses. As such, they are well-adapted to meeting most of their nutrient requirements from forages, which is why forage-first diets are increasingly popular. Forage-first diets meet the majority of the horse’s requirements from forages, before turning to other feedstuffs to cover any remaining gaps. While hay commonly acts as the main feedstuff for horses in this area, we cannot forget the nutritive potential that pasture offers. For some horses, pasture can be an incredibly beneficial part of their diet, however, for others, it can also contribute to other health issues if not managed carefully.


BENEFITS OF PASTURE ACCESS


Pasture access offers many benefits in many areas of the domestic horse’s life. Not only is it a species-appropriate way to manage horses, but it can also offer nutritive benefits!


Species Appropriate: Physiology & Behaviour


Horses are designed to eat little and often, which is evident in numerous areas of their physiology and behaviour.


Physiology


Equine digestive physiology is designed for frequent consumption, so the equine stomach continuously secretes hydrochloric acid throughout the day, regardless of whether there is feed material present in the stomach. This is why feed deprivation can cause ulceration - more vulnerable parts of the stomach can get exposed to acid when there is no food present to protect the stomach walls (Andrews, 2020). Equine saliva helps to buffer this effect, so horses who have food available ad lib (ie, high roughage diets) will have more saliva production (stimulated by chewing), which can also help to maintain digestive comfort.


Behaviour


Pasture also provides a safe space to practice species-appropriate behaviour, in both feeding behaviour (little and often; foraging, etc), and social behaviour. Time spent on pasture with other horses provides forage, exercise and social contact, and may reduce the development and frequency of stereotypic behaviour (Whisher et al, 2011; Stanley, Cant and Osborne, 2015).


Management practices which cater to species-appropriate strategies can help promote both equine physical and mental health.


Nutrition


As horses are evolved to meet their needs on forages, it is not surprising that nutritive benefits of pasture exist! Fresh pasture can contribute energy, protein and vitamin content to the horse’s diet.


Energy & Protein


Depending on how long your horse is on pasture, and the quality of the pasture, it is possible for your horse to obtain a large amount of their calories from pasture - which can contribute to obesity in domestic horses! However, for horses with higher energy requirements, maintaining good pasture can help reduce the amount of concentrate fed.


A horse will eat 1.5 to 2.0% of their BW in forage per day, on a dry matter basis (water weight not included). As forage can mean pasture or hay, the availability (and quality) of both in the diet will impact intake.


For pasture, consumption rates range from 0.45 to 0.90 kg (DM) per hour. This can mean energy consumption of 1.2 to 2.4 Mcal per hour, depending on the pasture available.


For a 500 kg horse, maintenance energy requirements range from 15.2 to 18.2 Mcal/d.

For a 200 kg pony, maintenance energy requirements range from 6.1 to 7.3 Mcal/d.


Sufficient pasture can support adult horses, and depending on the season and availability of pasture, pasture can even oversupply energy and protein (NRC, 2007).

It's easy to see how easy-keeping ponies and horses can greatly exceed their energy requirements off just 3-6 hours of pasture access, contributing to the prevalence of overweight easy-keepers on pasture!


Vitamins


Pasture access can also contribute vitamins towards the horse’s requirement. Vitamin E is an important biological antioxidant that protects cell membranes from oxidative damage, and usually, the highest concentrations are found in fresh forages (NRC, 2007). In addition, while Vitamin D2 can be obtained from plant sources, Vitamin D3 can be synthesized in the skin from sun exposure (Puangthong et al, 2020). B vitamins, important for energy metabolism, cell proliferation and tissue metabolism, have also been reported to be higher in horses on pasture when compared to horses who are stalled (NRC, 2007).


On top of contributions toward energy, protein and vitamin requirements, pasture can contribute a substantial amount of water to the horse, as grass can be around 80% water (NRC, 2007). As the hindgut is largely water, hydration is important for digestive function and health.

  • Note: ample amounts of clean, fresh water should be available to all horses, regardless of housing situation.


RISKS OF PASTURE ACCESS


Digestive Upset & Pasture-Associated Laminitis


Consumption of rich grasses can sometimes result in digestive upset, and may also contribute to the development of laminitis, due to intake of water-soluble/non-structural carbohydrates (sugars), such as starch and fructans.


Different types of plants and grasses store sugars differently. Fructans are the main storage carbohydrates in cool-season plants, while warm-season plants use starch as their main storage carbohydrate (NRC, 2007).


Fructan storage is non-limiting, while starch production is a self-limiting process, which means that warm-season plants tend to have higher non-structural carbohydrate content. While plant type can impact the amount of fructans/storage water-soluble carbohydrates (WSC) factors such as time of year and time of day can also cause further variation, due to patterns of plant energy storage and use. Seasonally, WSC content is highest in the spring, intermediate in the fall, and lowest mid-summer. Daily variation follows sun presence, with WSC content rising during the morning, peaking in the afternoon, and declining overnight. WSC levels in the afternoon can contain 2-4x the WSC of other time points, which definitely poses a concern for overconsumption. Excessive consumption of water-soluble carbohydrates may give rise to laminitis, similar to the effect of excess undigested starch in hindgut function (NRC, 2007). Pasture turnout can trigger a bout of laminitis even in lean horses with no history of laminitis, but horses who are prone to laminitis (IR/PPID/EMS/obese/etc) are at a higher risk (KER, 2017).


To manage these risks, it’s important to consider the type of horses and their individual health, as well as pasture suitability and level of access. All horses should be gradually exposed to pasture, starting with 15-minute intervals, and increasing the duration over a period of weeks. This can allow digestive adaptation and present overconsumption.


For horses who are at risk of laminitis, risk mitigation strategies include avoiding grazing and using dry lots (feeding hay instead), using grazing muzzles, and considering the time of day the horse is out on pasture. Due to WSC concentrations peaking during the afternoon, laminitis-prone horses should not be on grass during the afternoon (KER, 2017).


PASTURE MANAGEMENT


Pasture availability & quality is a big concern, as pastures are commonly overgrazed/not rested adequately. Pasture can only support the nutrient requirements of adult horses if the pasture is managed properly, and if the pasture is stocked with the appropriate number of horses (NRC, 2007). In order to properly manage a horse on pasture, quality of plant matter, stocking rate, grazing management and pasture maintenance must be considered.


Quality of Plant Matter


Quality of plant matter refers to whether there is a sufficient amount of plant matter present to sustain the horse without impacting the regrowth of the plant. The ideal plant height to begin grazing depends on the plant species. (Martinson and Peterson, 2021). For short-grass species such as Kentucky bluegrass or perennial ryegrass, you can begin grazing when there is 4-6 inches of grass. For tall-grass species such as Smooth Brome or Orchard, you can begin grazing when there is 8-10 inches of plant.


To avoid overgrazing, and damaging the plant’s ability to regrow (Penn State Extension, 2022), horses should be removed from the pasture when short-grasses are grazed to 1-2 inches, and tall-grasses are grazed to 3-4 inches. Resting the pasture until growth has recovered can help maintain pasture quality, and will take 2-6 weeks depending on season, climate and health of the soil.


Stocking Rate


Stocking rate refers to the number of horses per acre of pasture space. The stocking rate will impact the pasture’s ability to support your horse’s requirements. Depending on plant growth and number of horses, a pasture with a higher stocking rate (more horses per acre) may not be able to support their intake. Supplemental hay will be required for horses during insufficient pasture availability. Generally, 2 acres per horse is the average recommendation, however less productive soil may warrant 5 acres per horse (Martinson and Peterson, 2021).


Grazing Management


Pasture management such as use of a sacrifice field, rotational grazing to prevent overgrazing and allow recovery, and fertilization/re-seeding can also help maximize forage output (Ministry of Agriculture, 2023). High presence of weeds in your pasture can indicate that a pasture needs extra care. In such cases, the Ontario Ministry of Agriculture has some great tips for rejuvenating your pastures should this be the case, and for those in other geographical areas, your local agricultural extension office will have recommendations suited for your area.


SUMMARY


To be maintained on pasture alone, a 500 kg horse will require 10 kg of plant matter (on a dry matter basis) per day. This accounts for intake, as well as losses due to trampling and defecation (NRC, 2007). Unfortunately, due to space allowances at many equine facilities, high stocking rates without the option for resting or rotating pastures is common. This results in pastures which cannot contribute substantially to the horse's diet.


So, while quality pasture can support the requirements of adult horses and ponies, this largely depends on proper pasture management and pasture availability. In addition, ad lib pasture access is not an appropriate feedstuff for horses who have metabolic disorders or who are laminitis-prone. Even otherwise healthy adult horses need to be gradually transitioned to pasture to reduce the risk of pasture-associated GI upset and laminitis.

Careful management, consideration of the individual horse and attention to the pasture available is key when pasture is part of your horse’s diet.

 

REFERENCES

Andrews, FM. 2020. Equine Gastric Ulcer Syndrome. American Association of Equine Practitioners. Accessed online from: https://aaep.org/horsehealth/equine-gastric-ulcer-syndrome


Clauss M, Hummel J. 2008. Getting it out of the (digestive) system: hindgut fermenters and calcium. Proceedings of the Comparative Nutrition Society 7:30-36


Clauss M, Lang-Deuerling S, Kienzle E, Medici EP, Hummel J. 2009a. Mineral absorption in tapirs (Tapirus spp.) as compared to the domestic horse. Journal of Animal Physiology and Animal Nutrition 93:768–776


Clauss, M. 2013. Digestive physiology and feeding behaviour of equids - a comparative approach. In: Horse Health Nutrition - European Equine Health Nutrition Congress, Gent, Belgium, 1 March 2013 - 2 March 2013, 25-33.


KER, 2017. Five Tips for Avoiding Pasture-Associated Laminitis in Horses. https://ker.com/equinews/five-tips-avoiding-pasture-associated-laminitis-horses/


KER, 2020: Calculating Pasture and Forage Consumption of Horses.


Lane JG. 1994. A review of dental disorders of the horse, their treatment and possible fresh approaches to management. Equine Veterinary Education 6:13–21


Manthe, BN., Youngs, CR. 2014. An Overview of Vitamin Requirements of the Domestic Horse. Natural Sciences Education 42(2013): 179-184.


Martinson, K., Peterson, P. 2021. Managing established horse pastures. University of Minnesota Extension. Accessed online from: https://extension.umn.edu/horse-pastures-and-facilities/managing-established-horse-pastures


Ministry of Agriculture, 2023. Managing horse pastures. Accessed online from: https://www.ontario.ca/page/managing-horse-pastures


NRC, 2007. Nutrient Requirements of Horses: Sixth Revised Edition. Washington, DC: The National Academies Press. 2007.


O’Neill HVM, Keen J, Dumbell L (2010) A comparison of the occurrence of common dental abnormalities in stabled and free-grazing horses. Animal 4:1697-1701

Penn State Extension, 2022. Basic Pasture Management for the Equine Owner. Accessed online from: https://extension.psu.edu/basic-pasture-management-for-the-equine-owner


Puangthong, C., Sukhong, P., Saegnual, P., Srikurea, R., Chanda, M. 2020. A single bout of high-intensity exercise modulates the expression of vitamin D receptor and vitamin D-metabolizing enzymes in horse skeletal muscle. Equine Veterinary Journal 53(4): 796-805.


Schryver HF, Foose TJ, Williams J, Hintz HF (1983) Calcium excretion in feces of ungulates. Comparative Biochemistry and Physiology A 74:375-379


Stanley, SO., Cant, JP., Osborne, VR. 2015. A Pilot Study to Determine Whether a Tongue-Activated Liquid Dispenser Would Mitigate Abnormal Behaviour in Pasture-Restricted Horses. Journal of Equine Veterinary Science 35 (11-12): 973-976.


Whisher, L., Raum, M., Pina, L., Perez, L., Erb, H., Houpt, C., Houpt, K. 2011. Effects of environmental factors on cribbing activity by horses. Applied Animal Behaviour Science 135(1-2): 63-69.


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