10/17/2024
Pastures, nonstructural carbohydrates, and frost 🌱🍂❄️
Because it is that season, I want to discuss horse pastures and nonstructural carbohydrates, specifically as they relate to the beginning and end of a grazing season. Let’s begin by breaking this down by each topic before we add them all together:
Nonstructural carbohydrates, often abbreviated as NSCs, are comprised of water soluble carbohydrates (simple sugars and fructans) and starch. This class of carbohydrates is an important source of energy for the horse and they are digested and absorbed in the foregut, leading to an increase in blood glucose and insulin. As a result, high levels of NSCs in the diet can be an issue for horses with metabolic concerns specifically related to insulin dysregulation such as equine metabolic syndrome (EMS), Cushing’s (PPID), and laminitis/founder. While NSCs can be more consistent and documented in manufactured concentrates, more variation is observed in pasture or hay due to outside factors such as plant maturity, time of day, and weather conditions.
In pasture plants, NSCs are produced via photosynthesis, a process in which sunlight, water, and carbon dioxide generate glucose and oxygen. A separate but complementary process known as respiration is responsible for breaking down the glucose created during photosynthesis and releasing that energy for cellular use. As a result, increased photosynthesis leads to an accumulation of NSCs and increased cellular respiration leads to a decrease of NSCs. There are many weather conditions that can alter the balance between these processes in plants.
So how are these two factors connected to frost and the swing in temperatures observed in the spring and fall when horses are either being introduced to or removed from pasture? Well, when temperatures fall below 40°F, the enzymes involved in cellular respiration become significantly less active. And when temperatures fall below freezing, the enzymes can become denatured, stopping respiration altogether. This means low temperatures can lead to an accumulation of NSCs in the forage which can be problematic for grazing horses.
As a result, it is important to look at the nightly lows, and if these temperatures are sustained for more than a few hours, change your management practices accordingly, especially for glucose-sensitive horses. Furthermore, a hard frost occurs when temperatures drop below 28°F for more than four consecutive hours and will not only result in NSC accumulation but also marks the end of the growing season. While current recommendations allow grazing to resume 7 days following a hard frost, pastures require a minimum height of 3” to 4” going into the winter to support pasture productivity the following grazing season. Since grasses will no longer grow after a hard frost, continuing to graze can put undue stress on the pasture.
Additionally, sudden changes in the diet that would occur from removing horses from pasture abruptly, keeping horses on dried forage for 7 days, and then reintroducing horses to pasture, could lead to digestive upset or other health problems and the process will be time intensive for owners. When shifting the diet, even between different forages, it is critical to plan intentional transitions that allow the gut microbiota to sufficiently adapt. While the transition back to pasture can be accelerated in the fall compared to beginning grazing in the spring, it should still be done with care.
I hope this information is helpful as you make decisions on horse management and grazing this season!
- Dr. DeBoer
