Mowing pastures regularly could prove to be a useful strategy for reducing the risk of insulin resistance and associated laminitis developing in horses, the findings of an American study suggest.
Excess consumption of the non-structural carbohydrates (NSCs) found in cool-season grasses has been linked to pasture-associated laminitis in horses.
While the exact mechanism is unknown, it is possible that an increased intake of these carbohydrates increases the flux of glucose into the bloodstream, resulting in a heightened insulin response.
Researchers from North Carolina State University found that decreasing the height of pasture through frequent mowing reduced concentrations of NSCs in the grass.
These lower concentrations resulted in a decreased insulin response in horses after eating the shorter grass compared to those that had grazed on the taller grass.
“These findings may be important in developing strategies aimed at preventing insulin resistance in grazing horses,” Paul Siciliano, Jennifer Gill and Morghan Bowman reported in the Journal of Equine Veterinary Science.
“For example, mowing pasture during seasons of the year when cool-season grass NSC concentrations are highest (e.g., spring and fall) could maintain forage in a ‘re-growth’ phase that consumes stored NSC, thereby decreasing overall NSC concentration.”
Another possibility was to employ a “leader-follower” rotational grazing system, whereby other farm animals with higher requirements, such as lactating sheep, grazed a defined area for a time to reduce grass height before allowing horses to graze.
The study team’s experiment involved six stock-type geldings, aged 6-12, with maintenance-only requirements in terms of their diet.
The researchers set up a series of grazing areas of 0.37 hectares (just under an acre) each in which a tall fescue grass, Lolium arundinaceum, was grown. It was a non-toxic endophyte-infected variety known as Max-Q.
All the grazing areas were mowed to 15cm 32 days before the grazing experiment began. Half were not mown again and were left to grow long, to a height of 30 to 40cm, before the horses were introduced.
The other half were mown to 15cm again 11 days before the start of the experiment, and again the day before.
The researchers used a randomized cross-over design, in which the horses were variously assigned to graze either the tall areas or the shorter mown areas for 10 hours a day in seven-day blocks. They were housed in covered pens on a crushed stone surface when not grazing.
Herbage mass was determined for each pasture to be sure that all horses had adequate dry matter to meet their needs.
Blood samples were taken at regular intervals, before and after turnout, to check insulin and glucose
Pasture samples were analyzed for levels of water soluble carbohydrates, ethanol soluble carbohydrates and starch. The sum of water soluble carbohydrates and starch were used as an estimate of NSCs.
Average pasture plant carbohydrate concentrations were found to be lower in the mown pastures compared to the tall plots, although starch concentrations did not differ between the two.
Average plasma glucose concentrations were similar in all the horses before grazing, as were average serum insulin concentrations.
However, plasma insulin levels were found to be greater when horses grazed the tall grass, as compared to the mown areas.
The researchers found that there was a weight decrease among the horses when on the short-grass treatment, but they suspect this was probably due to a reduced gut fill rather than an actual loss of body mass.
The study had financial backing from the North Carolina Horse Council and the North Carolina Agricultural Research Service.
Siciliano PD, Gill JC, Bowman M, Effect of sward height on pasture
non-structural carbohydrate concentrations and blood glucose/insulin profiles in grazing horses, Journal
of Equine Veterinary Science (2017), doi: 10.1016/j.jevs.2017.06.004.
The abstract of the study can be read here.