Turnout Time's Impact on Grass Consumption and Fecal pH

Many a horse or pony owner have restricted their overweight equid's turnout time in an effort to help him shed pounds. And while researchers know weight loss helps improve horses' overall health, until now they haven't known exactly what impact restricted grazing has on the equine gastrointestinal health or calorie intake. A group of North Carolina State University (NCSU) researchers recently set out to determine if restricted pasture access affected horses' intake rate, energy intake, and hindgut fermentation.
The research team, led by Paul D. Siciliano, PhD, associate professor in the NCSU Department of Animal Science, separated eight mature idle geldings into four groups and allowed each group pasture access for either 3, 6, 9, or 24 hours for seven days. After seven days, the team reassigned the groups to a different turnout treatment on an ungrazed pasture. By the end of the four-period study, each horse had been subjected to each pasture treatment. When not on pasture, the horses stayed in drylot pens with access to water and salt; horses in the three- and six-hour treatment groups had free-choice access to a low-quality grass hay. Throughout the study, the team recorded the amount of hay consumed and collected fecal samples from each horse on each Day 7.
The team also measured or estimated pasture plant composition, herbage mass (used to quantify pasture available to an animal), grazing height, and forage preference for each period. Pasture composition consisted of 54.7% crabgrass, 28.3% tall fescue, 8.2% fox tail, 1.14% dallisgrass, 0.86% weeds, and 6.6% bare ground. They noted no difference in digestible energy concentrations (DE) or initial herbage mass for each pasture. However, less pasture was available during periods 2 and 3 compared to 1 and 4.
Key study findings included:

• Horses' total daily dry matter (DM) intake (pasture plus hay, if hay was provided) was not affected by length of turnout time. In other words, horses consumed the same amount regardless of the amount of time they were allowed to graze.
• The team found the highest total daily DM intake—which equaled 1.4% of body weight (BW)—in horses on pasture for 24 hours, which is less than the 2-3% BW previous research suggested. Siciliano and colleagues suggested the lower DM intake in their study could be due to high temperatures seen during portions of the research: High temperatures have been shown to decrease forage intake in horses up to 15-20%.
• Pasture DM intake, total DM intake, and pasture DM intake rate were higher in periods 1 and 4 compared to 2 and 3, which correlates to the amount of pasture available during those periods.
• Pasture DM intake rate increased with restricted grazing. Horses grazing for only 3 hours had a higher intake rate compared to horses grazing for 9 and 24 hours, and horses grazing for 6 hours had a higher consumption rate than horses on pasture for 24 hours.

The researchers also learned that horses' fecal pH—which is both influenced by diet and used as an indicator of hindgut pH—decreased as time on pasture decreased; an acidic environment in the hindgut (termed hindgut acidosis) can lead to colic and other health concerns. This showed the team that length of time on pasture affected hindgut microbial fermentation. The authors believed the increase in rate of pasture intake could have played a role in the lower fecal pH, especially when the horses consumed high-quality pasture. However, they stressed, all fecal pH values were within the range considered to be normal.
Average DE intake was greatest when horses were on pasture for 24 hours, but total DE intake did not differ between treatments. During periods 1 and 4, average and total DE intake was greater than periods 2 and 3. The horses consumed 40%, 66%, 67%, and 94% of their total DE requirements with 3, 6, 9, and 24 hours of pasture access, respectively, which indicates that horses increase their rate of consumption with decreased time on pasture.
The team concluded that their findings support the belief that reduced time on pasture increases consumption rate and decreases fecal pH in horses.
“Simply reducing the time a horse spends at pasture may not always be an effective means of decreasing caloric intake,” said Siciliano.
The team noted that more work is needed to develop methods to accurately predict pasture intake of horses grazing for periods less than 24 hours, which could be of great value in preventing or managing obesity in the future.
The study, "Effect of Restricted Pasture Access on Pasture Dry Matter Intake Rate, Dietary Energy Intake, and Fecal pH in Horses," appeared in June in the Journal of Equine Veterinary Science