Picking out stall

We horse owners associate an array of scents with barns. Some—freshly cut hay, clean leather, or a new bag of grain—never get old. Others—such as that burning ammonia odor—are not only foul-smelling but can also harm horse and human health.

Ammonia results from the breakdown of undigested nitrogen from protein in feces and urine, and much research has focused on how it impacts air quality. Recently, a group of scientists took a closer look at how dietary protein and bedding type impact ammonia emissions.

A group of researchers led by Jessie Weir, PhD, of the University of Florida’s Department of Animal Sciences, and Hong Li, PhD, of the University of Delaware, randomly assigned nine mature geldings to one of three dietary treatments supplying crude protein at 10.6% (LOW-CP), 11.4% (MED-CP), or 12% (HIGH-CP) of dry matter intake (DMI) per day. This would amount to roughly 11.5% to 15.5% crude protein in the total diet on an as-fed basis. During each test period, the horses had 11 days to adapt to the dietary treatment before a three-day fecal and urine collection phase. Every eight hours, the researchers emptied the horses’ waste collection harnesses, recorded the urine and fecal water pH, weighed total feces and urine, and froze a representative sample of each. Then, the team rotated the horses through the treatments so all horses consumed each diet.

Upon analysis, the team found that horses consuming the HIGH-CP diet excreted more total feces and urine, but also consumed more dry matter per day compared to the two other diets. When assigned to the LOW-CP diet, horses excreted 10% less total nitrogen in feces and 45% less in urine compared to the HIGH-CP diet.

With the HIGH-CP diet, fecal pH rose to 6.8 compared to 6.62 and 6.70 on the LOW-CP and MED-CP diets, respectively. Horses consuming the HIGH-CP diet had the highest urine urea nitrogen concentration, followed by the MED-CP, and lowest when consuming the LOW-CP diet. Because urea nitrogen is the main source of nitrogen volatilization (a process of vaporization) from urine, higher levels of urea nitrogen could contribute to increased ammonia emissions.

To simulate the environment in a stall, the team measured ammonia concentration in the laboratory by placing fecal and urine samples into separate 5-gallon emission vessels for seven days. They mixed urine samples with 250 grams of wheat straw or 500 grams of wood shavings and feces with the wheat straw only. They calculated ammonia emissions using intermittent samples of exhaust air from each vessel and determined that:

  • The largest increase in ammonia emissions from either feces or urine occurred between Days 3 and 5 of incubation;
  • The highest cumulative ammonia emission rate (sum of the emission changes over the seven-day period) occurred in the feces when horses consumed the HIGH-CP (0.071 parts per million [ppm]) and lowest with the LOW-CP (0.34 ppm), although those figures were not statistically different; and
  • Urine ammonia emissions were greater with wheat straw than wood shavings (97.33 vs. 73.53 ppm, respectively).

Take-Home Message

Feeding crude protein at 12% of total dry matter intake caused horses to excrete more nitrogen in their urine and feces, leading to a greater risk for ammonia emissions. Also, wheat straw produced a greater amount of urine ammonia emissions than wood shavings when tested in the laboratory.

“This research is a step toward demonstrating a direct relationship between dietary protein and nitrogen excretion that may lead to a greater risk for loss of ammonia, which can potentially have harmful effects on the environment,” said Weir.

Will this research help us further understand and quantify equine facilities’ ammonia emissions and their effects on human and horse health? Study team member Carissa Wickens, PhD, PAS, the Extension equine specialist at the University of Florida, thinks so. “The findings will help guide future research and the development of equine operation best management practices for air quality protection,” she said.

The study, “Characterizing ammonia emissions from horses fed different crude protein concentrations,” was published in the Journal of Animal Science.