Day and Time Effect on Pasture NSC and Insulin in ID Horses

Managing horses that are at risk for laminitis so they can have pasture turnout has been an important question in the equine industry.

A study looked at horses with insulin dysregulation (ID) grazing on Kentucky cool-season pasture in early summer to help explore this question. University of Kentucky researchers worked on the research in conjunction with others from Waltham Petcare Science Institute and Lincoln University.

The research was titled “Effect of Day and Time on Pasture Nonstructural Carbohydrates and Insulin in Horses with Insulin Dysregulation.” It was presented at the 6^th Global Equine Endocrine Symposium 2025—brought to you by Boehringer Ingelheim. The presenter was Morgan Askins, a graduate student in the Department of Veterinary Science, Gluck Equine Research Center, University of Kentucky.

Other authors of this research included Pat Harris, Equine Studies Group, Waltham Petcare Science Institute, Waltham-on-the-Wolds, Leicestershire, England, United Kingdom; G. Palmieri and A. Gregory of the Richard A. Gillespie College of Veterinary Medicine, Lincoln Memorial University, Ewing, Virginia; M. McClendon of the UK Department of Veterinary Science; and Amanda Adams, Mars Equestrian Research Fellow, M.H. Gluck Equine Research Center, Department of Veterinary Science, University of Kentucky.

Study Aims

Previously, the general recommendation for grazing horses at risk of laminitis on cool season grasses after the spring flush was to limit them to morning hours. This is when non-structural carbohydrates (NSC) should be at their lowest. However, there had not been research to correlate early summer pasture NSC and insulinemic response in insulin dysregulated (ID) horses.

(Researcher recommended not grazing at-risk horses on lush, growing pasture at any time of the year.)

Study Hypothesis

The researchers hypothesized that NSC content in pasture changes from day to day and by time (a.m. and p.m.) daily. This results in changes in circulating insulin concentrations, which is especially important in ID horses.

Study Methods

The study carried out in June 2024 used seven mixed-breed, mixed-sex horses (mean 16 +/- 3.8 years). They were group housed on the same paddock growing Kentucky cool season grass. All horses were classified as being insulin dysregulated.

The same horses and paddock with green, active growth vegetation were utilized in each of the study’s two phases.

In Phase 1, the researchers collected peripheral blood and pasture samples. They also recorded daily environmental temperatures (DT). These were taken during the same morning hours (between 8 and 9 a.m.) for five consecutive days.

The following week in Phase 2, peripheral blood and pasture samples were collected. The environmental temperature (ET) was recorded on one day. Blood and pasture samples were taken on the same day at two different times—8 a.m. and 3 p.m.

Blood samples were analyzed for insulin. Forage samples were immediately stored at minus 20°C (-4°F) prior to being shipped on ice to be analyzed by Equi-Analytical.

The researchers reported that all study horses remained clinically healthy and had no clinical signs of laminitis throughout the trial.

Phase 1 Results

In Phase 1, they found no correlation between daily environmental temperature and pasture NSC.

For example, the highest temperature was on Day 3 at 22ºC (71ºF), when NSC was 11.5% on a dry matter basis (DMB).

The lowest temperature happened on Days 1 and 2 at 19ºC (66.2ºF), when NSC was 15.4% (the highest recorded in this phase) and 12.5% DMB, respectively.

Day 4 had a temperature of 20ºC (68ºF) and 11.4% NSC (DMB). Day 5 had a temperature of 21ºC (69.8ºF) and 10.1% NSC (DMB).

As expected, the decreases in NSC dry matter over the five days was associated with changes in serum insulin concentrations. As NSC went down, so did serum insulin levels.

Phase 2 Results

As expected, in Phase 2 both environmental temperatures and NSC increased from the morning to the afternoon. Serum insulin concentrations also increased significantly from the morning to the afternoon samples.

The morning temperature was 24ºC (75.2ºF), when NSC was 9.5% (DMB) and insulin concentrations were 78.17 +/- 44.94mIU/mL. The afternoon temperature was 32ºC (89.6ºF), when NSC was 13.4% (DMB) and insulin concentrations were 101 +/- 50.97mIU/mL.

Study Conclusions

The study confirmed that non-structural carbohydrate (NSC) changes can occur rapidly in pasture grasses. Importantly, the study showed that NSC increases can occur between the morning to mid-afternoon in cool season pastures even under warm/hot conditions.

The researchers noted that these NSC increases were reflected in increased insulin concentrations.

They also highlighted that were was tremendous individual variability in the insulin dysregulated horses’ responses to the pasture and the changes in its NSC content.

Final Thoughts

Just because it is morning doesn’t mean that NSC will be low. The researchers noted, “Morning pasture NSC can change significantly from day to day. Even in early summer, the morning NSC percentage in the grass may result in undesirable insulin concentrations in some grazing ID animals.”

That means owners and managers have to be careful assuming that all morning grazing after the spring flush will be sufficiently low in NSC to be suitable for those equids prone to laminitis.

The researchers also reminded horse owners and managers that it is important to frequently monitor individual insulin responses. This is because NSC can change rapidly, and the insulin response to NSC is individual. This is especially true in high-risk, more severely insulin dysregulated animals.

(Editor’s note: See related article Effect of seasonal changes in pasture nonstructural carbohydrates on 24-hour insulin responses in horses with insulin dysregulation.)