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Can regular mowing protect from laminitis?

14/08/2017 by Dr Mariette van den Berg, BAppSc (Hons), MSc, PhD (Equine Nutrition), RAnNutr

With Spring upon us in the Southern Hemisphere, many horse owners caring for sugar-sensitive horses will be frantically trying to adopt different management strategies to reduce the intake of sugary pastures to avoid weight gain and/or laminitis. The approaches used probably involve restricting and/or managing their horses’ access to grazing either strip grazing, fitting grazing muzzles and often, by locking horses out of pasture completely or during parts of the day. When the horses' access to fresh pastures is restricted, their owners will be supplementing with soaked/steamed hay and low glycaemic index (GI) horse feeds.

However, we must not ignore the importance of pasture management strategies to reduce some of the risks we expose our horses to. In this article, Dr Mariette van den Berg reviews a recent study which looked at mowing pastures to reduce the intake of non-structural carbohydrates (NSC), and which has caused some confusion amongst horse owners and grass farmers alike.

Managing pastures for lower ‘sugar’ levels

Most owners are aware of the benefits of growing pasture species that accumulate lower levels of non-structural carbohydrates (NSC), such as Australian native grasses and a selection of introduced species. But the reality is that, very often, most horses are kept on properties that were initially set up for grazing and fattening livestock, as well as having pastures that are under heavy grazing pressure (due to the property’s small size) and will, as a result, only grow a few ‘improved’ pasture species.

Under these circumstances, many owners become discouraged and give up on trying to do something about their pastures. They tend to revert to sacrificing paddocks, turning them into so-called ‘Jenny Craig’ paddocks with very short, over-grazed, monoculture grasses. Owners might even mow these paddocks frequently in an effort to reduce their horses’ grass intake (and to try to control weeds that flourish in degraded soils).

The problem is, however, that in these environments, horses may actually be more at-risk, due to the stress the plants experience as a result of the continued removal of their leaves (over-grazing). Furthermore, if the horses are experiencing a lack of roughage (fibre), it could even cause metabolic and may aggravate the inflammatory responses that lead to diseases such as laminitis.

Horses and People readers will be well-versed in the story that short grass is ‘bad’ for sugar-sensitive horses (not to mention the plants and the soil!) and, for that matter, so will many other horse owners since most nutritionist experts have been reporting this in many magazine and peer-reviewed articles. Nevertheless, if this concept is new to you, please keep reading!

As a result of our increasing understanding, both pasture experts and equine nutritionists have been advising to graze pastures that are in a healthy regrowth stage and that slashing can force plants to regrow. This is understood to be good for the pasture and for decreasing the overall sugar content in the plants because after slashing, the plant reallocates and uses its sugar reserves to grow instead of storing it.

This aspect was recently illustrated by a new equine study led by Paul Siciliano from North Carolina University in the United States, who reported that mowing (cool-climate) pasture (reducing plant sward height) reduced non-structural carbohydrate (NSC) concentrations and, subsequently, it lowered the insulin response of horses grazing the pasture. The study is titled: Effect of sward height on pasture nonstructural carbohydrate concentrations and blood glucose/insulin profiles in grazing horses.

The study, however, was promoted on social media with the headline “Regular pasture mowing has potential to reduce laminitis risk for horses”, and it confused some people who understood this to contradict the story I mentioned earlier that ‘short grass is 'bad' because it can lead to stress in plants and greater NSC concentrations’. 

In this review of the full paper, I want to clarify some of the aspects and/or ambiguities of the study and discuss important facts about plant growth, carbohydrate accumulation in grasses, the impact of grazing management on pasture plants and how this may affect their nutritional value. First, let’s review the study.

Study aim

Siciliano and his colleagues studied different sward heights of a cool-climate grass, using mowing to reduce the length of grass with the aim to promote vegetative regrowth. The researchers were specifically testing if the shorter, regrowth grass contained lower forage NSC compared to longer grass. They also hypothesised this would be reflected in the postprandial (after a meal) blood glucose/insulin concentrations of horses grazing these pastures. Thus, a lower insulin response would be expected in the horses grazing the shorter regrowth grass.

Study design

In this study, six mature horses, aged between 6-12, were used and randomly assigned to two treatments (short-grass or long-grass). To make sure all horses received both treatments, they conducted a cross-over design. This means once the horses finished one treatment, it was followed up by the second treatment.

The researchers set up two grazing cells (each 0.37 ha), which they planted with a commercial grass species of tall fescue (Lolium arundinaceum).  Both the short and tall grazing cells were mowed to a similar height (15cm) a month before the start of the experiment. Then, only one grazing cell was mowed again to 15cm, 11 days and one day before the start of the experiment. The grazing cell with the lower sward height of 15cm is referred to as 'short' and the other, which was 30-40cm is referred to in the study as 'tall'.

There were two testing periods of seven days each (due to the crossover), this meant that, in total, the researchers collected six observations per treatment. They did not state what time of the year (month) they conducted this study, but assuming cool-climate grasses have peak growing season in Spring and Autumn, it is likely it was around this time.

The horses had access to the grazing cells for 10 hours per day from 8am to 6pm. The remaining hours of the day they were stabled. Pasture was their sole diet. The researchers reported the horses were healthy and no signs of laminitis were recorded during the study.

Study measurements

A number of samples were collected from the pasture cells for nutritional analysis, including dry matter, crude protein, fibre (acid detergent fibre (ADF) and neutral detergent fibre (NDF)), water soluble carbohydrates (WSC), ethanol soluble carbohydrates (ESC), starch and a selection of minerals.

They recorded the herbage mass of each grazing cell. Herbage mass is used to describe the quantity of pasture available to a grazing animal.

To examine the glucose and insulin profiles, the researchers collected blood samples from each horse before turnout and then at two, four, six and eight hours after turnout.

Results

The grazing cell plant carbohydrate fraction (NSC 12.6%, WSC 12%, and ESC 9.5%) of the shorter sward height was significantly lower compared with the tall (NSC 16.3%, WSC 15.7%, and ESC 11.6%). Pasture plant starch concentrations were unaffected and were similar for both treatments.

The bodyweight between the treatment groups did not differ, but they did observe a drop in weight on the shorter pasture (day one versus day seven), which they said may point to reduced gut-fill, rather than body mass. The herbage mass was expectably lower for the shorter grass compared to the taller grass.

While this study didn’t record the actual dry matter (DM) intake, the researchers computed some estimations and it was likely the horses had less total pasture intake on the shorter grass, compared to the taller grass. Still, it appears all horses received enough DM to meet maintenance requirements in both treatments (most likely because there was enough sward height at the beginning of the week and some regrowth may have occurred during the seven days).

There were no significant differences in the blood glucose and insulin baseline levels between the two treatments (baseline being the sample taken immediately before grazing). It appeared levels were all within the normal range. However, they did find the peak serum insulin was significantly lower in horses grazing short grasses versus tall.

The researchers suggest the greater insulin peak on taller grass may be caused by the greater NSC intake because of the difference in NSC concentrations and herbage mass available for grazing. This suggests the insulin concentrations in grazing horses may be influenced by pasture NSC available for grazing.                  

Based on these findings, the researchers concluded (frequent) mowing of cool-climate pasture could be effective in reducing NSC available for grazing, and may support lower insulin responses in horses and potentially reduce the risk of metabolic disorders.
They specifically address that mowing may be most useful when cool-climate grasses are growing and accumulate carbohydrates, which is during Spring and Autumn.

By stimulating regrowth during these growth periods you potentially can reduce overall NSC concentrations. Both NSC and fructan levels are highest in early Spring and Autumn, and lower in Summer and Winter.

Glucose and insulin concentrations in horses have shown to coincide with the NSC increases in Spring and Autumn, and they are lower in Summer and Winter. In Spring and Autumn, NSC concentrations peak later in the afternoon/early evening (between 4 and 8pm) and reach their lowest levels in the early morning hours (between 4 and 8 am).

Additionally, the research team also indicates other strategies, such as rotational grazing, should be incorporated and over-grazing should be avoided by removing all stock when plants are grazed down to 7.5-10cm in height.

Let’s put this in perspective.

There are number of aspects in this study we need to address so we can clarify some of the confusion related to short grasses and laminitis.

How short is short?

Firstly, in this study,, the ‘short’ pasture was 15cm high. This is still a reasonable herbage mass for a cool-season type of pasture. At this height, it will still contain enough leafy material. This study agrees with the current advice which is not to graze these types of pastures below 5cm (ideally, to a minimum height of between 7.5-10 cm).

The reality, however, is most horse owners struggle getting enough or taller pasture to grow, due to set-stocking, smaller acreage, grazing pressure and/or soil conditions. Over-grazing is a common problem and will stress grasses (and the soil they grow in).

Grazing very short, over-grazed plants can also increase NSC intake because it forces a horse to consume the plant stem and crown closer to the ground, where the plant stores carbohydrates (fructan). The same accounts for frequently mowing grasses short (below 5cm) to create ‘Jenny Craig’ paddocks.

Mowing or slashing?

While this study talks about mowing pastures to reduce sward height to 15cm, in Australia we would refer to this as ‘slashing’. Most horse people would understand mowing to mean very short, like a lawn, or similar to when grass is cut for hay production.
Indeed, slashing cool-climate grasses as well as warm-season grasses during their elongation phase (while they grow and before they begin setting seed, which is known as the reproduction phase), can promote regrowth in plants, which means plants will reallocate their resources to growing more leafy material.

Grass plants’ leaves are lower in NSC when compared to the crown and stem so, overall (and supported by this study), this will reduce available NSC.

A panacea for sugar-sensitive horses?

While slashing may have a useful application in your pasture management, sugar-sensitive horses could still be exposed to threshold levels of NSC that could put them at risk. Factors, such as how much pasture you make available to your horse, the grazing behaviour of your horse and his/her genetics could still contribute to the development of metabolic disorders.

Other pasture management strategies to consider

Rotational or cell grazing should also be implemented. This can reduce the area you make available (at once) for your horses to graze and aids grass recovery (and pasture/soil health). Horses should ideally be removed from cool-climate pastures to allow for rest and regrowth when the forage is grazed down to between 7.5-10cm (never shorter than 5cm), and horses can be returned to graze when it reaches 15-20cm.

Cross grazing can also promote vegetative regrowth (if you also avoid over-grazing). For example, you can allow cattle to graze the pasture first before reintroducing the horses.

While cool-climate grasses have been mostly linked to metabolic disorders, such as laminitis, the condition can also occur on tropical/warm-season grasses whose growth period is during Summer.

Cool-season grasses can accumulate higher amounts of carbohydrates, because they store their fructans outside of the chloroplast (the part of plant cells that conducts photosynthesis) in vacuoles that don’t limit their storage.

In warm-season grasses, starch production and storage is limited to within the chloroplast where it is synthesised. Still, warm-season grasses (depending on the species) and environmental conditions can accumulate high levels of NSC.

Something else to think about is the importance of biodiversity in pastoral systems. Having a variety of grasses - both cool-climate and warm-season - will spread the growing season and, by selecting warm-season or low-NSC cool-season grasses (such as natives) when seeding pastures, we can also minimise the risk of NSC overconsumption. This, as well as many other benefits, explain why we should manage for biodiversity in pastures and is the focus of the Pasture Pharmacy series - check it out!

Take-home message

Not all pasture was created equal and even within the same species we are constantly confronted with fluctuations in sugars, secondary compounds and other nutrients from day to day and season to season.

This knowledge could, however, help hay growers whose aim is to produce ‘low sugar’ hay, for example, growing the right grass species then cutting and baling the grass during the elongation phase. However, laboratory analysis will be the only way to prove the results. 

While it would be great if we could solve all of our ‘sugar’ problems by regularly slashing pastures, in reality, we will have to continue adopting a wide variety of grazing and horse management strategies to keep both our horses and our pastures healthy and productive.

References:

Siciliano, P. D., Gill, J. C., & Bowman, M. A. (2017). Effect of sward height on pasture nonstructural carbohydrate concentrations and blood glucose/insulin profiles in grazing horses. Journal of Equine Veterinary Science, 57, 29-34.

FACT BOX:

NSC vs WSC and ESC

The non-structural carbohydrate (NSC) fraction has been the main focus when reviewing digestible carbohydrates and simple sugars in horse forage and feeds. This part of the analysis is of particular interest to those horse owners managing horses that are sugar sensitive or have metabolic disorders.

However, NSC appears to not be the best measure to evaluate the carbohdrates in your horse’s diet. One major reason is the large variation in the way different laboratories measure NSC components and calculate its value.

In addition, NSC doesn’t provide you with the complete picture of the types of carbohydrates in feed or forage that can affect your horse’s condition. NSC includes nearly all non-fibre carbohydrates that originate from the plant cell content and are easily digested, which are usually calculated as water-soluble carbohydrates (WSC) plus starch or ethanol-soluble carbohydrates (ESC) plus starch.

Some of the carbohydrates are digested in the small intestine (hydrolysable carbohydrates; CHO-H) and others pass on to the hindgut (rapidly fermentable carbohydrates; CHO-FR). Thus, they affect the horse’s blood sugar and gastrointestinal health differently, and it’s important we evaluate them separately.

Water-soluble carbohydrates and ethanol-soluble carbohydrates

By reviewing the ESC and WSC fractions instead of NSC, we get a better picture of the digestible carbohydrates and simple sugars, and their effect on the horse’s metabolic and digestive system.

WSC (water soluble carbohydrates): These include carbohydrates that are extracted from a sample by dissolving them in water. Simple sugars and fructans make up this measure, which is sometimes simply termed ‘sugar’. Interpreting and using this value depends on the proportions of sugars and fructans in the sample.

Simple sugars are digested and absorbed in the small intestine and have a significant impact on blood sugar (glycaemic response), while fructans are fermented in the large intestine and induce a much smaller response. Nevertheless, when eaten in large amounts, some fructans have been shown to cause laminitis, due to a disruption of the bacterial population in the large intestine. Fructans are rarely analysed separately from other WSC.

ESC (ethanol soluble carbohydrates): These carbohydrates are soluble in 80% ethanol; they are a subset of WSC that is primarily digestible in the small intestine and includes much fewer fructans. As such, this fraction is generally used to evaluate one set of carbohydrates in a feed that will induce a high glycaemic response.