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Grazing Spring Pastures


By Jennifer Earing, Ph.D.
Nutritionist, Tennessee Farmers Cooperative

With spring time come warm sunny days and an abundance of lush, green pasture – as well as an increased probability of laminitis and colic in horses if not managed properly.  As much as we’d like to turn our horses out on that pasture to graze to their hearts’ content, for the sake of their health, we know better. 

Fresh pasture can be a great source of relatively inexpensive nutrition for horses.  A well-managed pasture (in which measures have been taken to fertilize, control weeds, and prevent of over-grazing) can cost as much as one-third less than feeding hay.  Access to pasture also provides horses with an opportunity to partake in natural grazing behaviors. 

However, fresh pasture can also pose a few problems.  The switch from hay to pasture can represent a relatively dramatic change in the horse’s overall diet, with the potential to cause digestive upset.  The nutrient composition of hay and pasture can be quite different, particularly in terms of non-structural carbohydrate content. 

The term non-structural carbohydrate (NSC) is used to describe the non-fibrous carbohydrates found in plant material.  In equine nutrition, NSC includes simple sugars, starch, and fructans (a complex sugar) and is generally calculated as starch + water-soluble carbohydrates (Water-soluble carbohydrates = simple sugars + fructans).  Alternatively, the term structural carbohydrate is used to describe the fibrous carbohydrates that are more slowly fermented in the hindgut of the horse. 

Research has shown that NSC accumulates in plants throughout the course of a day as a result of normal plant metabolism.  Plants utilize photosynthesis to produce simple sugars to supply their energy requirements for growth and development.  When the plant’s energy requirement has been met, any additional simple sugars produced via photosynthesis are stored as fructan or starch.  Once the sun sets, photosynthesis ceases, forcing the plant to rely on stored NSC for energy.  This reliance gradually depletes the plant’s energy reserves over the course of the night.  Consequently, NSC levels are highest in late afternoon and lowest in early morning hours. 

A variety of factors affect NSC content in pasture, including plant species and plant maturity.  Generally speaking, cool-season grasses (i.e., tall fescue, orchardgrass, and timothy) tend to have higher NSC content than warm-season grasses (i.e., bermudagrass, dallisgrass) and legumes (i.e., alfalfa, clover).  Likewise, immature forage will have higher NSC levels than mature forage because, as the plant matures, nonstructural carbohydrates are gradually replaced with structural carbohydrates. 

Sudden increases in intake of nonstructural carbohydrates may lead to increased incidence of laminitis and colic.  Consequently, it’s important to transition horses slowly from hay to pasture; start by allowing horses access to pasture for 30-45 minutes a day for the first few days and gradually increase the duration of their grazing time over the course of 2-3 weeks.  One way of preventing horses from gorging themselves when turned out to pasture is to feed them hay first; this way they’re already full and physically limited to how much grass they can consume. 

Extra precautions should be taken with horses that are particularly sensitive to NSC levels (i.e., those that have previously battled laminitis, or deal with insulin resistance, Cushing’s disease, polysaccharide storage myopathy [PSSM], or equine metabolic syndrome [EMS]).  For these horses, it is recommended to eliminate grazing time altogether or reduce grazing to a few hours per day until pasture forages have matured and NSC content has decreased.

Unfortunately, both laminitis and colic have been associated with the lush, spring pasture we look forward to each year.  Consider your current management strategies, and the horses under your care.  Do you need to make adjustments to your management program to help prevent these potentially devastating conditions on your farm?

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