Influence of lush pasture
Although the role of pasture carbohydrates in the pathogenesis of laminitis (disturbance to hindgut function) has not been
clearly demonstrated, some suggest that most laminitis cases occur because of excessive lush pasture intake, especially by
ponies. For horses and ponies with a history of pasture-associated laminitis, there is some rationale for restricting their
access to pasture to limit their intake of high levels of rapidly fermentable material, high levels of water-soluble carbohydrates
(simple sugars, glucose, fructose, sucrose and more complex carbohydrates such as fructan) and starch.
Pasture content of water-soluble carbohydrates is highest in the spring, lowest in mid-summer and intermediate in autumn.
Content also fluctuates at various times during the day. Sugar content coincides with patterns of energy storage (photosynthetic
activity) and utilization. Pasture water-soluble carbohydrates tend to rise during the morning, peak in the afternoon and
decline overnight. According to the NRC, a horse grazing in the afternoon, when compared with nighttime or morning feeding,
may ingest between two and four times as much water-soluble carbohydrates.
There is limited information on the quantities of pasture fructan or other storage carbohydrates required to cause significant
changes in hindgut function that may increase the risk of laminitis.
If pasture exposure is a concern, then it may be beneficial to restrict access to pasture and/or feed lower quality forage
alternatives (those that are low in rapidly fermentable material), especially to those animals at risk of developing the disease.
Laminitis could be largely avoided in predisposed animals by allowing grazing when the levels of starch, sugar and fructans
are low. However, this strategy is difficult to employ because these levels may vary season to season, location to location
and throughout a particular day. Also, plant species, field topography and grazing patterns of individual animals can influence
success with this strategy.
If some grazing is unavoidable, it has been advised to turn animals out very late at night or very early in the morning and
remove them from pasture by mid-morning, since fructan levels are likely to be at their lowest level at night through early
And since mature stemmy grasses can contain more fructan, it may be beneficial to avoid pasture that has not been managed
properly by regular grazing or cutting and to not allow animals to graze on recently cut stubble.3
Another consideration is that at the change from vegetative to reproductive development, water-soluble carbohydrate and fructan
levels are likely to be high, so it is advisable to avoid turning horses out to pasture during the spring when water-soluble
carbohydrate and fructan levels are rising. Furthermore, cold temperatures will reduce grass growth, resulting in the accumulation
of fructan. Horses at risk should not be turned out to pasture that has been exposed to low temperatures and bright sunlight.
These conditions are more typically encountered just after the autumn flush of growth or on bright, cool winter days.
In the United Kingdom, allowing animals to graze on pastures that contain timothy and cocksfoot is suggested because these
grass species tend to produce lower levels of water-soluble carbohydrates and fructans and are thought to be less likely to
cause hindgut acidosis than ryegrass.
"We're doing some work related to how structural carbohydrates might impact the hindgut," says Paul Siciliano, PhD, a nutritionist
at North Carolina State University. "Although a threshold for induction of laminitis using a nasogastric bolus of fructans
has been defined, the threshold for consumption of pasture nonstructural carbohydrates (of which fructan is often a major
component) through grazing is not clear. Grazing generally results in a clearly different intake pattern than a nasogastric
Siciliano is also working with endophyte-friendly tall fescue and its concentration of total nonstructural carbohydrates.
"We're seeing those concentrations at 13 to 14 percent, with about a 3 percent variation between morning and night," Siciliano
says. "Nonstructural carbohydrates increase in the fall and in the spring, when people tend to report pasture laminitis. If
you have a horse that is truly at risk, you might want to select hay with the lowest sugar content, but you want to make sure
that you're not suddenly feeding poor-quality forage and underfeeding the horse. In 1977, Butler and Hintz showed that energy
intake influences hoof growth, so you don't want to impede that. Good management and good feeding practices are what you're
Siciliano and his team also examined fecal pH as an indicator of hindgut pH. They compared horses on pasture that were consuming
higher nonstructural carbohydrates with those fed hay that had about half the amount. "Fecal pH tended to be a little bit
lower in the horses on pasture, but it was still above 7, so it didn't appear to be anything that upset them," says Siciliano.
Siciliano's team then did some in vitro fermentations using the feces from each horse. They took the feces, made a batch culture
for each horse, and then measured the volatile fatty acids that were produced. "We did see some differences in fermentation.
We haven't looked at lactate — that's the one we are really interested in, but I don't know how that will come out."