Digestive Capacity in Weanling and Mature Horses Studied

Equine researchers have evaluated common horse feeds' digestibility (the percentage of the digestion and absorption of various nutrients present in a feed source) primarily in mature horses, but little is known about the digestive capacity of young, growing horses.

Previous research suggests mature and young horses utilize high-fiber diets differently than adults, mainly because younger horses have a shorter retention time in the large intestine. A team of University of Kentucky (UK) researchers recently set out to compare the digestibility of a high-forage diet when fed to weanling versus mature horses.

The research team paired six weanling colts with six mature geldings (with an average age of 13.2 years) and allowed them to adapt for 21 days to a diet comprosed of 67% alfalfa cubes and 33% commercial concentrate.

On Day 1 of the study's five-day collection period, Laurie Lawrence, PhD, professor of equine nutrition at the University of Kentucky, and colleagues fed each pair the same amount of two indigestible markers used to measure the average period of feed retention in the digestive tract (also called the mean retention time, or MRT)—called Co-EDTA and Yb—mixed with molasses and a portion of their concentrate for palatability.

The researchers then fitted the horses with fecal collection harnesses and offered the remainder of the concentrate and alfalfa cubes. The team collected feces from the harness every one to two hours throughout the entire collection period.

They analyzed all the fecal samples for the presence of the indigestible markers, as well as for dry matter (DM, calculated by the weight of the feed minus the water content) digestibility; organic matter (OM, feed minus the mineral content) digestibility; and neutral detergent fiber (NDF, an indicator of feed digestibility comprised of the least digestible parts of the plant) digestibility. Researchers also measured the horses' water intake during the collection period.

The researchers based daily feed amount offered and calculated DM intake on each individual horse's metabolic body weight, which refers to the percentage of metabolically active tissue in the horse’s body, such as muscles. This calculation allowed the team to compare the digestive capacity of two horses on an equal basis, even if they weren't the same weight or height.

In the current study, the team concluded that colts and geldings had similar daily DM intake, and age did not appear to affect either DM or OM digestibility. They also found that NDF digestibility did not differ between weanlings and mature horses, although the team noted that different results could occur with a larger number of animals, lower-quality diets, long-stemmed forage, or different feeding rates.

The team used the two indigestible markers, Co-EDTA and Yb, to estimate the MRT of the two phases of digestion: particulate phase (digesting fiber components) and fluid phase (digesting water). Although they hypothesized that the MRT of both phases would be shorter for weanlings than mature horses, the researchers found no difference in MRT between the colts and geldings. They found that the MRT for the particulate phase was significantly longer than that of the fluid phase, but age was not a factor in these differences. The researchers concluded that when consuming a good-quality diet, weanlings and mature horses have similar MRTs for both phases.

One trend the team observed during the study was that colts appeared to consume more water than geldings (0.29 vs. 0.21 L/kg body weight). This finding could indicate that growing horses might require more water than mature horses.

The team noted that because the weanlings appeared to have similar digestive capacities as mature horses, further studies are needed to determine the exact age at which young horses' digestive capacity nears that of mature horses.

The study, "Digestive capacity in weanling and mature horses," will appear in an upcoming issue of the Journal of Animal Science

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