Elijah Hail

For horse owners, there is often nothing more satisfying than seeing the mud in the paddocks turn green with fresh pasture after a cold and wet winter. In New Zealand, pasture growth can happen rapidly after only a few days of sun, following what seems like endless weeks of cold and wet weather. While this fresh green grass is highly appealing to them, there are certain components in spring pastures that can contribute to a variety of problems for grazing horses.

Horses evolved as grazing animals, and forage in the form of pasture, hay and other fibre sources should always be the largest part of their diet. Their digestive tract is designed in a way that is reliant on a consistent intake of forage, as the fibre it contains helps to buffer gastric acid in the foregut and ensure the optimum balance of hindgut microbiota. Studies have shown that for optimum digestive health, horses are required to consume at least 1.5% of their body weight in forage daily. Despite the importance of forage, it is necessary to select the correct type for the individual horse and for some, spring pasture may not be the best choice.

The first issue with spring pastures is the lower fibre content they contain. While pasture at any time of the year contains the same ingredients of water, vitamins, minerals, protein, sugars, starch, and structural fibre among other things, the proportion of these changes significantly according to the time of the year. For example, the fibre content of a mature summer grass will be much higher than a lush spring grass, because the spring grass contains a much higher water content. This means that to reach their optimal fibre intake, horses need to consume higher amounts of the rich spring pastures.

Implications of this can often be seen through horses that only have access to lush pastures chewing fences or trees, in search of fibre. To rectify this, a slice or two of hay daily can help to boost fibre content in the diet and decrease the wood chewing and potential fence destruction! Despite the highly palatable rich pasture, horses will often select hay over green grass if they are searching for fibre.

The ingredients in spring pastures that cause them to be so highly palatable for horses are another reason why they might not be the best choice in some cases. The rapid growth of spring pasture means it contains both simple sugars and a specific form of storage sugars called fructans, which can both be responsible for various problems. The high water content of spring pasture means sugars may not be present at high levels, however, the palatability of the pasture can significantly increase intake and therefore the level of sugar consumed.  

Simple sugars are rapidly absorbed in the small intestine and provide a quick release of energy. These are often the cause of behavioural changes in the horse and the typical ‘fresh’ behaviour seen in spring. These behavioural changes occur due to peaks in blood glucose caused by the rapid release of sugars, similar to giving a child a bag of lollies! In this instance, the best solution is to remove pasture from the diet for some or all of the day or to dilute the amount of rich spring grass consumed by including hay in the diet as mentioned previously.

Fructans are a specially adapted type of sugar and are quite a different structure from simple sugars. Because of their longer, more complex structure and the way they are digested, they can cause various digestive and behavioural problems in some horses, as well as a painful and debilitating condition of the hooves called laminitis.

Where most sugars and starches are easily digested by the enzymes in the horse’s stomach and small intestine, the strong bonds of fructans cannot be broken by normal enzymatic mechanisms. This causes them to pass undigested into the horse’s cecum and colon. When fructans reach this area of the hindgut, they are fermented by bacteria that produce lactic acid. Excessive amounts of lactic acid are not absorbed efficiently from the hindgut or used by the body for energy. The accumulation of lactic acid alters the microbial population in the hindgut and produces endotoxins, resulting in a condition known as hindgut acidosis.

Symptoms of hindgut acidosis are varied however they include loose manure, behavioural changes, and possible stereotypical behaviour such as wood chewing, and in more severe cases it is one of the main causes of colic and laminitis in pasture-kept horses. While some horses have a high tolerance to fructans and are unaffected, for particularly sensitive horses even a small amount of fructan-rich grass can quickly trigger laminitis that can be highly debilitating if left untreated. Heavier types such as Quarter Horses and many pony breeds seem to be more susceptible than other horses to fructan-caused problems, however, it is important to understand that all horses with unrestricted access to lush pasture can be at risk, even after years of grazing freely without consequences. 

As a general rule, horses that have high energy requirements can handle fructans as long as the overall energy balance is taken into consideration. The classes of horses that can generally handle high fructan levels if they are allowed time to adapt to them gradually include growing horses (just don’t overfeed grain when fructans are high), lactating mares, hard-working horses, thin horses (that are not compromised by disease or parasites), and breeds that are known to be hard keepers like Thoroughbreds. Those that should generally avoid fructans are easy keepers, ponies, and any overweight horses. However, each horse is an individual, regardless of breed, and all horses grazing lush pastures should be monitored for potential symptoms.

Management of hindgut acidosis depends on the individual horse and the severity of the condition. Minor symptoms such as loose manure and behavioural changes can be managed with some restriction and hay access, however, in the instance of colic or laminitis, it is important to restrict horses from all lush pasture immediately and meet forage requirements with alternative low sugar sources. To avoid problems when introducing horses to spring pastures, introduce access in small segments several times a day and gradually increase the number and length of these access periods.

Managing pastures so that horses do not overgraze will help reduce fructan intake. One simple step is to mow higher than usual during seasons when fructan accumulation may occur. Because plant stems contain high levels of fructan, horses should not be grazed on stubble. The condition of the entire pasture must be monitored daily, as horses tend to select high-fructan plants to graze. It is also important to consider that stressed pasture plants can cause fructan levels to increase. For example, drought or frost can increase fructan levels by 30%, so special attention should be paid to the weather.

One of the most common questions asked is when are the sugars at their lowest in pasture and therefore when is the best time to graze susceptible horses? The answer to this depends on the fructan fluctuations and the temperatures at night.

Fructans are produced by photosynthesis. Photosynthesis occurs in the leaves during daylight hours; the sunnier the day, the more photosynthesis, and therefore the more fructans. During the dark (overnight) phase of photosynthesis, plants use the sugars to grow more leaves and stems. Extra sugars that are not used for growth are stored within the plant tissues. Many cool-season types of grass store fructans in the lower two inches of the stem just above the soil line. However, temperatures at night are critical. If the temperature is not above 4 degrees Celsius at night, the plant will not grow and sugars remain in the lower leaves in high concentrations.

This means when days are warm with temperatures above 4 degrees Celsius at night it is best to graze early in the morning when plants have had the night hours to use up sugars from the day before. Then remove the horse from the pasture before the day gets hot and sugar production starts again. In the case of warm days and cool nights (below 4 degrees Celsius), it is best to limit grazing time for all horses, especially those that are particularly sensitive!

For horses with minor symptoms, continue to offer hay while they are turned out on fresh pasture to ensure they are consuming enough fibre, and monitor manure consistency and hoof temperature. Take the horse off pasture and call the veterinarian if the horse shows signs of hoof discomfort like reluctance to move, shifting from one foot to another, or assuming a leaning-back posture.

When horses are restricted from pasture it is essential that they are provided with additional hay and alternative forage sources to ensure optimal digestive health, and super fibres such as beet pulp and soy hulls are ideal for this situation. McMillan Grain Free is a blend of beet pulp and soy hulls which also contains balanced levels of vitamins and minerals and is a great way of ensuring fibre and nutrient requirements are met. The low starch high fibre formulation is ideal for horses and ponies at risk of laminitis. The use of a product such as KER EquiShure, a time-released hindgut buffer, is also helpful in preventing metabolic problems associated with the overconsumption of sugar-rich grasses.

For further assistance with managing spring pastures and laminitis, consult an equine nutritionist.

Article supplied by Luisa Wood, Equine Nutritionist