*Dr. Joe D. Pagan is president and founder of Kentucky Equine Research Inc., which, through consultation and research, aims to bridge the gap that may exist between basic research and horse production.
THE Equine Science Society recently held its biennial symposium in Ruidoso, N.M., and researchers from around the world presented the results of more than 150 studies from a wide range of disciplines, including nutrition, exercise physiology, reproductive physiology and genetics.
This article will summarize several studies of relevance to the horse feed industry. The proceedings from this symposium are available from the Journal of Equine Veterinary Science at www.j-evs.com.
Furosemide, mineral balance
Kentucky Equine Research (KER) presented findings from a series of studies involving furosemide (Lasix), a diuretic commonly administered pre-race to racehorses to curb exercise-induced pulmonary hemorrhage (EIPH) or bleeding.
While furosemide certainly helps bleeders, it remains controversial whether furosemide improves performance independent of its effect on EIPH. Additionally, there is concern that chronic use of furosemide may adversely affect mineral balance in racehorses, leading to increased lameness and skeletal fragility.
Six Thoroughbreds were used in a 3 x 3 Latin square design study to test the effect of furosemide on energetic efficiency during a standardized exercise test (SET) on a high-speed treadmill. The treatment groups consisted of an untreated control and two groups that received furosemide four hours before the SET.
Weight loss during the four-hour period before the SET was higher in furosemide-treated horses than controls.
During the SET, heart rate was higher in controls compared to furosemide-treated horses. Absolute maximum oxygen consumption and carbon dioxide production were higher in controls than furosemide-treated horses.
Lactate accumulation was higher in controls compared to furosemide-treated horses.
The incidence of EIPH was very low and unrelated to treatment. Furosemide administration reduced energy generation from both aerobic and anaerobic pathways during exercise, and this improvement was probably due to a reduction in bodyweight.
The effect of furosemide on 72-hour urinary and fecal mineral excretion was also studied. Urinary calcium, phosphorus, sodium and chloride excretion significantly increased 24 hours post-treatment. Urinary magnesium and potassium excretion was unaffected by treatment. Seventy-two-hour calcium and chloride balance was decreased in furosemide-treated horses.
This study suggests that racehorses receiving furosemide may have increased calcium requirements. Further research is needed to assess the effect of chronic furosemide administration on mineral excretion and balance in racehorses.
Dextrose is often added to commercial electrolytes based on the assumption that sugar improves electrolyte uptake in horses. KER presented results from a two-part study designed to evaluate the effects of dextrose on the uptake and retention of electrolytes and water in horses.
In the first part, Thoroughbred geldings were dosed with electrolytes either alone, with 10 g of dextrose or with 100 g of dextrose. A fourth treatment of no electrolytes served as the control. Plasma electrolyte levels were determined before dosing and every 30 minutes after dosing for four hours. Free-choice water was offered, and intake was measured.
Plasma electrolytes and osmolality rose significantly in all treatment groups — as did increased voluntary water intake — compared to the control group, but dextrose did not affect the rate or duration of increase.
In a second study, Thoroughbred geldings were administered 1 mL of distilled water per 100 g of bodyweight plus 0.15 g/kg of deuterium oxide either: (1) alone (control), (2) with 100 g of electrolytes, (3) with electrolytes plus 10 g of dextrose or (4) with electrolytes plus 10 g of corn starch.
Plasma sodium ion and osmolality were significantly elevated post-dosing in all three electrolyte treatments compared to the control, but neither dextrose nor starch affected the rate or duration of increase.
Plasma deuterium oxide was elevated to a greater extent in the control versus the three electrolyte treatments at 30 and 60 minutes post-dosing, suggesting that isotonic electrolyte solutions delay water uptake compared to pure water. Total body water was unaffected by treatment.
Urinary and fecal electrolyte excretion did not differ among the three electrolyte treatments.
These studies suggest that the inclusion of sugar in electrolyte products does not improve electrolyte uptake or retention.
Hay net design
Researchers from the University of Minnesota investigated whether hay net design affects the rate of consumption of hay.
Eight horses were fed hay at 1% of bodyweight either on the ground or in hay nets with either 15.2 cm openings, 4.4 cm openings or 3.2 cm openings for four-hour periods twice per day.
There was no difference in the rate at which the horses completed their hay or the total amount consumed when the hay was fed on the ground or in hay nets with the largest opening (15.2 cm).
There was, however, a significant effect on the rate of intake and total amount consumed when openings were 4.4 cm and 3.2 cm; in fact, not all of the horses were able to finish the hay within the four-hour period from either opening size. The rate of consumption was slowest for the smallest opening, at 0.88 kg per hour, down from the control, which was 1.49 kg per hour.
The researchers concluded that offering horses hay in hay nets with a reduced opening size is an effective way to slow horses' consumption of hay.
Steaming vs. soaking hay
Horse hay is often soaked or steamed to remove dust and mold and to reduce its sugar content. KER conducted a trial in a 3 x 3 Latin square design to compare the intake and digestibility of timothy hay either dry, soaked or steamed in a Haygain hay steamer.
Steaming increased the free-choice intake of hay compared to soaking, but the rate of intake was not affected when expressed on a dry matter basis.
The fiber components — acid detergent fiber and neutral detergent fiber — of the soaked and steamed hays were greater than the dry hay. This was due to a loss of other components from the soaked and steamed hays.
If these compositions were recalculated, assuming that the neutral detergent fiber content of the hays were equal, then about 9% of the original dry matter of the soaked hay and 4% of the dry matter of the steamed hay were lost as a result of soaking and steaming. Much of this loss was from water-soluble carbohydrates. Soaking or steaming did not affect apparent nutrient digestibility.
Concern over the effect of high dietary starch has been the topic of many research projects recently. One performed at Texas A&M University focused on the acute effect of feeding a high-starch versus low-starch meal on cecal microbial populations.
Seven cecally cannulated geldings were randomly assigned to a twice-daily feeding regime of a commercial concentrate supplying either low starch intake (0.9 kg of starch per feeding) or high starch intake (1.8 kg of starch per feeding) during two 28-day periods with a 21-day forage-only washout period between periods.
On the first day of feeding concentrate, samples of cecal fluid were collected at three-hour intervals after the morning feeding for 12 hours to look at the acute effect of starch ingestion and at six hours post-morning feeding on days 2, 3 and 7 to look at adaptation of the cecal microbial population to starch ingestion.
Samples were analyzed for pH, and the next-generation sequencing technique was used to determine changes in cecal populations.
As expected, the high-starch treatment had the greatest effect on cecal pH during the acute feeding phase, and this trend continued in the six-hour samples over the entire seven-day feeding period.
In both treatment groups, cecal pH remained lower than pre-concentrate feeding levels. There were shifts in microbial populations in both treatment groups in response to the lower cecal pH, but they were more notable in the horses with the higher starch intake and were characterized by an increase in potentially pathogenic proteobacteria as well as an overall decrease in the diversity of the microbial populations.
While feeding starch is often necessary to meet the energy requirements of performance horses, the amount of starch in the meal may negatively affect hindgut microbial populations, warranting the use of smaller, more frequent meals.