Obesity and altered insulin dynamics are associated with laminitis in grazing ponies. These metabolic predispositions may interact with seasonal changes in pasture carbohydrates increasing, the risk for laminitis. To evaluate potential seasonal interactions, we sampled ~30 ponies and their pasture bimonthly on one farm in northern Virginia in 2007. Pasture was sampled at 1400 h on 4 consecutive days each month for measurement of water-soluble (WSC) and ethanol-soluble (ESC) carbohydrates and starch. Blood was collected between 0800 and 1100 h the following week to measure plasma glucose, non-esterified fatty acids (NEFA), triglycerides (TG), insulin and ACTH. Nine ponies developed clinical laminitis in late April/May and were categorized as CL. Data are shown in the table below as means ± SEM.

Means in the same row without the same subscript differ (P < 0.05) according to within-subject ANOVA with Fisher-Hayter pairwise comparisons. Over all months, ponies which developed laminitis in May (CL) had higher insulin (P = 0.002) and TG (P = 0.027) than ponies which did not display clinical laminitis in 2007 (NoCL), so data are reported separately.

In all ponies, plasma glucose and insulin were highest in spring (May) in association with the highest values for pasture WSC and ESC. Conversely, plasma TG and NEFA declined around May. Therefore carbohydrate metabolism appears to be up-regulated and fat metabolism down-regulated during spring when pasture starches and sugars are highest. These changes superimposed on already elevated insulin concentrations or insulin resistance in high-risk ponies may contribute to the development of laminitis in the spring.