The Journal of Nutrition Vol. 128 No. 12 December 1998, pp. 2704S-2707S

Exercise Affects Digestibility and Rate of Passage of All-Forage and Mixed Diets in Thoroughbred Horses¹

Joe D. Pagan², Pat Harris^*, Tammy Brewster-Barnes, Stephen E. Duren, and Stephen G. Jackson

Kentucky Equine Research, Versailles, KY 40383 USA and ^* Waltham Centre for Pet Nutrition, Waltham-on-the-Wolds, Leicestershire, LE14 5RT UK

KEY WORDS: horses · exercise · digestibility · rate of passage · forage

	INTRODUCTION

Introduction References

Most digestibility studies in horses have been conducted with idle horses confined to metabolism stalls. The values obtained from such studies are used for all classes of horses, including the performance horse. It has not been clearly established whether exercise affects digestibility. Olsson and Ruudvere (1955), summarizing a number of earlier studies, suggested that digestion may be affected in horses by work or exercise in such a way that it is improved by light exercise and inhibited by heavy work.

Orton et al. (1985) reported reduced retention time of a particulate marker in yearling horses trotting 12 km/d. However, apparent digestibility and retention of a fluid marker were increased with exercise. These discrepancies raise questions concerning the effect of exercise on digestibility and rate of passage in older, physically fit horses. Therefore, it was the purpose of this study to measure the effect of 8 km daily trotting and galloping exercise on the digestibility and rate of passage of either an all-forage or a mixed forage/grain ration in trained Thoroughbred horses.

Materials and methods. 

Horses.  Four Thoroughbred geldings (aged 3-8 y; 508 ± 15.9 kg, mean ± SD) owned by Kentucky Equine Research, were used in the experiment. For at least 6 mo before the study, the horses underwent a regular training program and were judged to be healthy and physically fit at the beginning of the experiment. The experimental protocol and procedures used in this study were in accordance with the Guide for the Care and Use of Agricultural Animals in Agricultural Research and Teaching (1988).

Treatments.  The horses were used in a 2 × 2 factorial design to investigate the effect of exercise and diet on apparent nutrient digestibility and rate of passage. The experiment consisted of four periods that were each 4 wk in length. During each period, the horses were fed either an all-forage diet (4.54 kg alfalfa cubes + 5.45 kg alfalfa-grass hay)(FORAGE) or a mixture of forage and grain (3.63 kg grain + 2.72 kg alfalfa-grass hay + 2.27 kg alfalfa cubes)(MIXED)(Table 1). During each period, one horse from each diet was exercised daily on a high speed treadmill (EX) (Table 2). The non-exercised horses (NON-EX) were turned out daily in paddocks with muzzles to prevent grazing. The treatments were assigned in a 4 × 4 Latin-square design so that each horse received every combination of diet and exercise (FORAGE/NON-EX, FORAGE/EX, MIXED/NON-EX, and MIXED/EX).

View larger version (16K): [in this window] [in a new window]   Fig 1. (A) Ytterbium excretion per 12-h sampling period expressed as a percentage of the total dose administered during a 120-h complete collection digestion trial. Values are the least-square means ± SEM of eight observations. Solid circles represent FORAGE-fed horses and open circles represent horses receiving the MIXED diet. FORAGE-fed horses excreted significantly more Yb at 12 (P < 0.05) and 24 h (P < 0.01). The MIXED diet horses excreted significantly more Yb at 48, 60 and 72 h (P < 0.01). (B) Cumulative ytterbium excretion during the entire 120-h collection period. Values are least-square means ± SEM of eight observations. Solid circles represent FORAGE- fed horses and open circles represent horses receiving the MIXED diet. The FORAGE-fed horses had excreted more Yb at 12 (P < 0.05), 24 (P < 0.01), 36 (P < 0.01) and 48 h (P < 0.01) than those receiving the MIXED diet.

View larger version (17K): [in this window] [in a new window]   Fig 2. (A) Ytterbium excretion per 12-h sampling period expressed as a percentage of the total dose administered during a 120-h complete collection digestion trial. Values are the least-square means ± SEM of eight observations. Solid circles represent exercised (EX) horses and open circles represent non-exercised (NON-EX) horses. EX horses excreted significantly more Yb at 24 h (P < 0.05. (B) Cumulative ytterbium excretion during the entire 120-h collection period. Values are least-square means ± SEM of eight observations. Solid circles represent EX horses and open circles represent NON-EX horses. The EX horses had excreted more Yb at 24 (P < 0.05), 36 (P < 0.05) and 48 h (P <0.05) than the NON-EX group.

During wk 4 of each period, a complete collection digestion trial was conducted. Each morning during the collection period, the EX horses performed an exercise bout on the treadmill (inclined to 3°), which consisted of a 5-min walk, 1.6 km at 4 m/s, 1.6 km at 7 m/s, 1.6 km at 9 m/s, 1.6 km at 7 m/s, 1.6 km at 4 m/s and a 5-min walk. Each afternoon, the EX horses were also hand-walked 1.6 km. The NON-EX horses were hand-walked 1.6 km twice daily.

Digestion trial.  During the 5-d collection period, the horses were housed in metabolism stalls that allowed the complete and separate collection of urine and feces. During the collection period, daily feed intake (Table 3) and total fecal and urine output were measured. Salt was offered free choice to the horses in 1-kg salt blocks during both the 3-wk adjustment period and the collection period. The horses were given free access to water in their box stalls during the first 3 wk of each period. During the collection period, the horses were offered water hourly from 0700 to 2300 h daily, and water consumption was recorded.

Feed and fecal analysis.  Both feed and feces were analyzed (AOAC 1983) for dry matter (DM) (AOAC 930.15), crude protein (CP) by Kjeldahl (AOAC 984.13), acid detergent fiber (ADF) and neutral detergent fiber (NDF) (ANKOM A200 Filter Bag technique, Ankom Technology, Fairport, NY), ether extract (Tecator Soxtec System HT6, Foss North America, Eden Prairie, MN), ash (AOAC 942.05), calcium, phosphorus, magnesium, sodium, potassium, zinc, copper and manganese. Minerals were analyzed using inductively coupled plasma spectroscopy (ICP). Hemicellulose was calculated as NDF  ADF, and nonstructural carbohydrates (NSC) were calculated as 100  (CP + NDF + ether extract + ash).

Rate of passage.  At the beginning of each collection period, 2 g of ytterbium chloride (YbCl₃ · 6H₂0) was administered to each horse by voluntary intake with their morning meal. Total fecal excretion was measured every 12 h for 120 h, and Yb concentration was measured in each 12-h fecal collection (Hart and Polan 1984). The percentage of the total Yb dose excreted during each 12- h collection period was calculated.

Calculations and statistical analysis.  Apparent digestibility of each dietary constituent was calculated as follows: [(daily intake  daily fecal excretion)/daily intake] × 100. The mean retention time (MRT) of Yb in the gut was calculated according to the equation of Blaxter et al. (1956). Least-squares ANOVA were conducted with general linear models procedures. Data were analyzed using a model that included diet, exercise, and the diet × exercise interaction as well as pertinent sources of variation such as horse and period.

Results. 

Digestibility.  The apparent digestibilities of the diets with and without exercise are shown in Table 4. Dry matter and NSC digestibility of the MIXED diet were significantly higher than those of the FORAGE diet (P < 0.01). ADF, NDF and hemicellulose digestibility were significantly higher in the FORAGE diet (P < 0.05). Exercise resulted in a small, but significant decrease in dry matter digestibility (57.8 vs. 58.9%)(P < 0.05). This decrease in DM digestibility was primarily from a reduction in ADF digestibility. Potassium digestibility was also significantly reduced in EX horses (66.3 vs. 74.3%)(P < 0.02), and sodium was increased (83.6 vs. 48.9%)(P < 0.001).

Rate of passage.  The pattern of Yb excretion in the FORAGE vs. MIXED diets is shown in Figure 1. Significantly more Yb was excreted at 12 (P < 0.05) and 24 h (P < 0.01) in the FORAGE group. The horses excreted significantly more Yb at 24 h (P < 0.05) when they were exercised (Fig. 2). Table 5 shows the MRT of the Yb in the various treatments. The MIXED diet had a MRT that was 9.3 h longer than that for the FORAGE diet (P < 0.01). Exercise decreased MRT by 3.3 h (P < 0.05). There was no diet × exercise interaction with MRT.

Discussion. 

FORAGE vs. MIXED.  The MIXED diet was more digestible than the FORAGE diet because it contained a higher concentration of NSC (37.6 vs. 20.5%) and a lower level of fiber (38.3 vs. 52.7% NDF). NSC is composed of a mixture of starch, soluble sugars and soluble fiber that is not recovered in NDF. NSC is much more digestible by horses than NDF (Pagan 1994). The FORAGE fiber digestibility was higher than that of the MIXED diet; this was probably a reflection of the quality of the fiber in the hay compared with the grain. NSC, however, was significantly less digestible in the FORAGE diet (74.4 vs. 87.7%)(P < 0.01). This may have resulted from a lower retention time for the FORAGE diet along with a greater proportion of the NSC in the FORAGE diet being soluble fiber.

The rate of passage in the FORAGE diet was faster than in the MIXED diet for a number of possible reasons. First, the intakes of the two diets were fed at levels that were calculated to be isoenergetic, resulting in a 19% higher DM intake in the FORAGE diet (9.12 vs. 7.62 kg/d). An increase in DM results in a faster rate of passage in cattle and pigs, and an increase in fiber intake decreases retention time in humans and pigs (Warner 1981). In addition, several studies have shown that increasing the amount of concentrates in the diet increases mean retention time (Warner 1981). Second, horses produce about twice as much saliva per unit of DM intake when eating hay compared with grain (Meyer et al. 1985). Finally, the horses drank more water when consuming the FORAGE diet than when they received the MIXED diet (35.4 vs. 28.0 L/d)(P < 0.01). A combination of increased saliva flow and greater water intake probably increased rate of passage because fluids travel faster through the equine digestive tract than particles (Orton et al. 1985, Uden et al. 1982).

Exercise vs. non-exercise.  Exercise resulted in a small but significant decrease in DM digestibility. Exercise also reduced mean retention time. Exercised horses consumed more water (34.4 vs. 29.4 L/d)(P < 0.02) than when not exercised, suggesting that water intake may have affected rate of passage. Warner (1981) stated that exercise in humans increased rate of passage, and Orton et al. (1985) showed that particulate retention time was reduced in yearling horses that were trotted for 12 km/d. Besides affecting rate of passage, exercise may have reduced blood flow to the digestive tract. Duren et al. (1992) reported that ponies exercised at 75% of maximal heart rate had reduced blood flow to the digestive tract. In this study, heart rate at the gallop averaged 150 and 170 beats/min at 7 and 9 m/s, respectively. These intensities of exercise would be equal to ~65-75% of maximal heart rate.

The decreased potassium digestibility in the exercised horses may have been related to an increased rate of passage. Most of the potassium was contained in the forage portion of the diet. Sodium, on the other hand, was supplied mainly from supplemental salt intake, and its digestibility increased with exercise. It is possible the horses increased sodium digestibility to compensate for the sodium lost through sweating during exercise.

The reduction in DM digestibility in this study is similar to that reported by Olsson and Ruudvere (1955) for horses that were trotted with and without a load. Although this reduction in DM digestibility caused by exercise was statistically significant, its practical significance is questionable. Of more interest are the large differences in rate of passage between the FORAGE and MIXED diets. More research is required to determine what factors affect rate of passage in horses and how these differences affect feed utilization and performance.

	FOOTNOTES

	LITERATURE CITED

Introduction References

• Association of Official Analytical Chemists (1983) Official Methods of Analysis, 14th ed. AOAC, Washington, DC
• Blaxter K. L., Graham, N. Mc., Wainmain F. W. Some observations on the digestibility of food by sheep and on related problems. Br. J. Nutr. 1956; 18:153-162
• Duren S. E., Manohar M., Sikkes B., Jackson S., Baker J. Blood flow distribution in fasted and fed ponies at rest and during endurance exercise. Pferdeheilkunde 1992; 9:24
• Guide for the Care and Use of Agricultural Animals in Agricultural Research and Teaching (1988) Consortium for Developing a Guide for the Use of Agricultural Animals in Agricultural Research and Teaching, Champaign, IL.
• Hart S. P., Polan C. E. Simultaneous extraction and determination of ytterbium and cobalt ethylenediaminetetraacetate complex in feces. J. Dairy Sci. 1984; 67:888-892[Abstract/Free Full Text]
• Meyer, H., Coenen, M. & Gurer, C. (1985) Investigations on saliva production and chewing effects in horses fed various feeds. In: Proceedings of 9th ENPS, pp. 38-41. East Lansing, MI.
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• Pagan, J. D. (1994) Nutrient digestibility in horses. In: Proceedings KER Short Course: Feeding the Performance Horse, pp. 127-136. Lexington, KY.
• Uden P., Rounsaville T. R., Wiggans G. R., Van Soest P. J. The measurement of liquid and solid digesta retention in ruminants, equines and rabbits given timothy (Phleum pratense) hay. Br. J. Nutr. 1982; 48:329-339[Medline]
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