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Keywords:

  • horse;
  • fluid balance;
  • metabolic response;
  • bodyweight;
  • urea;
  • insulin

Summary

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. Conflicts of interest
  9. Manufacturers' addresses
  10. References

Reasons for performing study: A higher forage allowance to athletic horses might be an alternative to increase health and the gut fluid reservoir. However, more forage might increase bodyweight (bwt) and could therefore be a limitation during competition.

Objectives: To investigate the effect of a forage-only diet (FD) compared to a 50:50 (dry matter basis) forage:oats diet (OD) on bwt, plasma protein concentration and some metabolic plasma parameters during 12 h of feed deprivation.

Methods: Twelve adult Standardbred horses in training were used. The 2 diets were fed in 2 experimental periods of 3 weeks each in a crossover design. The last day of each period the horses were fasted for 12 h. The horses were weighed and their water intake measured every day during the trial and every hour during the 12 h feed deprivation. During feed deprivation total plasma protein (TPP), insulin, nonesterified fatty acids (NEFA), urea, glucose and acetate concentrations were analysed.

Results: Bwt and water intake was higher on FD compared to OD. Bwt loss was higher during feed deprivation on FD compared to OD. TPP was lower before and during the last 8 h of feed deprivation on FD compared to OD. Plasma insulin was lower on FD than on OD at feeding and for 5 h during feed deprivation. Plasma NEFA and urea increased on both diets during feed deprivation. Plasma glucose was not affected by diet or feed deprivation.

Conclusion: High energy forage diets could be an alternative to high grain diets for athletic horses. The small increase in bwt on FD diminished with feed deprivation and the low TPP concentration indicate a greater potential to use an internal fluid compartment to maintain plasma volume.


Introduction

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. Conflicts of interest
  9. Manufacturers' addresses
  10. References

Horses are grass eaters and well adapted to high fibre diets (Janis 1976). Today, athletic horses are offered starch-rich diets although these are known to increase the risk for colic (Tinker et al. 1997), rhabdomyolysis (MacLeay et al. 2000) and stereotypic behaviour (Kusunose 1992; Gillham et al. 1994; Redbo et al. 1998). One reason for the use of high starch feeding strategies may be that forages with an energy density high enough for athletic horses are not readily available on the market. Moreover, in riding horses, a forage diet has been shown to increase bodyweight (bwt) and heart rate during exercise (Ellis et al. 2002). This suggests a higher workload during exercise. However, a recent study indicates that an energy dense forage-only diet will have no, or a limited effect, on the bwt of Standardbred horses in training, compared to a more traditional high concentrate diet (Jansson and Lindberg 2008). Observations on 4 Standardbred geldings also suggest that horses fed forage ad libitum rapidly lose bwt (mean 12 kg) during (8 h) feed deprivation (S. Muhonen and M. Connysson, unpublished data).

An increase in bwt on a high fibre diet might be due to an increased water holding capacity of the fibrous part of the digesta. The dry matter (DM) concentration in the hindgut digesta in forage fed horses is also considerably lower than in horses fed mixed diets (Hintz et al. 1971a). Dietary effects on the hindgut fluid volume could be of practical interest for the management of sport horses since the equine hindgut has been suggested to serve as a fluid reservoir to maintain fluid balance during dehydration (Meyer 1987).

Going to a race track might include long transportation for horses. Racetracks in Sweden are 60–1500 km apart and during transport between tracks, no water is available to horses and feed intake is limited. As a consequence, feed and water intake are generally low prior to competition. Limiting water and feed intake could also be a part of the trainers' management prior to competition (Pinchbeck et al. 2003; Burk and Williams 2008). As far as we know, there are no studies on the impact of the preceding feeding strategy on the physiological response of feed deprivation in trained horses.

The aim of this study was to investigate the effect of a forage-only diet compared to a 50:50 (DM basis) forage:oats diet on bwt, plasma volume and some metabolic plasma parameters during 12 h of feed deprivation. The hypothesis was that horses on a forage-only diet would be slightly heavier at the start of feed deprivation but that they will lose more weight and have a less variable metabolic plasma profile and plasma volume than horses fed a high oats diet.

Materials and methods

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. Conflicts of interest
  9. Manufacturers' addresses
  10. References

Horses and management

Twelve adult Standardbred horses (3 mares and 9 geldings) in training were used in this study. Their mean age was 8 years (range 5–12) and mean bwt at the beginning of the trial was 511 kg (range 474–560). All horses had slow exercise 1–3 times (approximately 1 h walk and slow trot [6–7 m/s]) every week and intensive exercise (approximately 1 h; heart rate between 100–200 beats/min) 1–2 times every week during the study. The horses were kept in individual stalls with sawdust bedding and they spent 5 h/day outdoors in sand paddocks.

Experimental design

Two diets were fed in 2 experimental periods of 21 days in a crossover design. Each experimental period was preceded by a 7 day transition period where the new diet was gradually introduced. The horses were divided into 2 groups and randomised on diets. Horses were fed 15, 20, 25 and 40% of the daily allowance at 06.00, 12.30, 17.00 and 21.00 h, respectively. The last day of each period the horses were subjected to 12 h feed deprivation after they finished their 06.00 h meal. During feed deprivation horses were kept in their ordinary stalls bedded with sawdust and had access to water, but all feed residues was removed. The Umeå local ethics committee approved this study.

Diets

One forage-only diet (FD) and one diet with 50% of the DM intake from forage and 50% from oats (OD) was fed (Table 1). Individual diets were calculated to fulfill metabolisable energy (ME) requirements for athletic horses (twice maintenance): 2 * (0.5 bwt0.75) (Jansson et al. 2004) and mineral requirements (Anon 2007). The diets were supplemented with 1.3 g/(10 kg bwt/day) of a commercial mineral and vitamin feedstuff1 (Ca 55 g/kg, P 65 g/kg, Mg 60 g/kg, NaCl 125 g/kg, Cu 900 mg/kg, Se 15 mg/kg, vitamin A 100,000 iu/kg, vitamin D 310,000 iu/kg and vitamin E 5000 mg/kg) and additional salt (total NaCl intake 1 g/(10 kg bwt and day)). Feed samples were collected every week during the trial. The forage used was a haylage (timothy and meadow fescue) with a DM content of 63% and a ME content of 9.8 MJ/kg DM. The ME content of the oats was 11.4 MJ/kg DM. The horses were offered water ad libitium in buckets.

Table 1. Mean ± s.e. daily intake of dry mattera and dietary componentsb from a forage-only diet (FD) and a 50:50 forage:oats diet (OD) (mineral feedstuff supplement not included)
 Diets
FDOD
  • a

    kg/100 kg bwt/day,

  • b

    MJ/100 kg bwt/day,

  • c

    g/100 kg bwt/day.

  • *

    *Significantly different from FD.

Dry mattera1.87 ± 0.021.92 ± 0.01
Metabolisable energyb18.5 ± 0.220.4 ± 0.1*
Crude proteinc229.3 ± 3.2234.5 ± 1.7
NDFc993.9 ± 11.1773.8 ± 3.4*
ADFc607.7 ± 6.7457.0 ± 1.8*
Free glucosec37.3 ± 0.422.0 ± 0.1*
Free fructosec86.3 ± 0.945.9 ± 0.3*
Sucrosec4.9 ± 0.18.4 ± 0.03*
Fructanesc22.6 ± 0.215.8 ± 0.1*
Maltodextrinesc0.07.8 ± 0.04*
Starchc11.8 ± 0.1341.1 ± 1.6*
Calciumc10.3 ± 0.26.2 ± 0.1*
Phosphorusc5.3 ± 0.16.3 ± 0.02*
Magnesiumc4.0 ± 0.13.4 ± 0.03*
Potassiumc47.9 ± 0.529.6 ± 0.2*
Ashc183.5 ± 2.4128.0 ± 0.9*

Measurements, sampling and analysis

The horses were weighed (E2000S)2 and water intake was measured through graded buckets (precision 0.5 l), every day during the trial and every hour during the 12 h feed deprivation. Rectal temperature was measured every day at 07.00 h. Time used to consume the 06.00 h meal was measured the day the horses were fasted. A faecal grab sample was collected during the 48 h preceding the feed deprivation. Faecal pH was measured on faecal fluid (squeezed from faeces) using an electronic pH meter (CD70 Portable pH-meter)3. Faecal DM was estimated by drying faeces for 48 h at 60°C.

During the 12 h feed deprivation, blood samples were taken every hour via a catheter inserted in the vena jugularis and collected in heparinised tubes (10 ml). Due to technical problems some blood samples (7–8 samples/horse in 5 horses) in Period 1 were collected by venipuncture from the jugular vein. The samples were kept on ice until centrifuged (10 min, 950 g) and then frozen (-20°C). Total plasma protein concentration (TPP) and insulin were measured on all plasma samples. Plasma nonesterified fatty acids (NEFA), urea, glucose and acetate were analysed in samples taken at 0, 6 and 12 h of feed deprivation. TPP was measured with a refractometer4. Plasma insulin was analysed using ELISA (Mercodia equine insulin kit)5. For quantitative determination of NEFA in plasma an enzymatic colorimetric method was used (ACS-ACOD method)6. Plasma urea, plasma acetate and plasma glucose concentrations were analysed with an enzymatic colorimetric/UV-method7.

Preparation and conventional chemical analysis of feeds were performed as described by Palmgren Karlsson et al. (2000). Minerals were determined by boiling samples in nitric acid (7 mol/l) and measurements were done with ICP8.

Statistical analysis

All data were subjected to analysis of variance (GLM procedure in the Statistical Analysis Systems package 9.1)9 using the following model:

  • image

Where Yij k is the observation, µ the mean value, αi the effect of animal, βj the effect of treatment, γk the effect of sample, εl the effect of period (βγ)j k the effect of interaction between treatment and sample and ei j kl the residuals: ei j kl∼IND (0, δ2). The P value for significance within and between treatments was <0.05. Values are presented as means ± s.e.

Results

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. Conflicts of interest
  9. Manufacturers' addresses
  10. References

General

Two geldings were excluded because of large feed residues on FD during the day preceding the feed deprivation (22 and 48% of the total allowance). The reason for the low appetite is unknown. Period 2 had to be shortened by 2 days due to limited forage availability.

Three week measurements

Daily DM intake was similar on both diets (Table 1). In OD, oats provided 48.6 ± 0.3% of total DM intake and 52.1 ± 0.2% of total ME intake. Bwt and daily water intakes were higher (P<0.05) on FD than on OD (495 ± 2 kg vs. 492 ± 2 kg and 23.0 ± 0.4 l vs. 20.0 ± 0.3 l). Faecal DM was lower (P<0.05) on FD than on OD (20.9 ± 0.9% vs. 25.2 ± 1.2%, respectively) and faecal pH was higher (P<0.05) on FD than on OD (6.7 ± 0.1 vs. 6.3 ± 0.1, respectively). There was no difference in rectal temperature for horses on the different diets (FD: 37.4 ± 0.01°C, OD: 37.4 ± 0.02°C).

Twelve hour feed deprivation

The time to eat the 06.00 h meal (15% of daily feed allowance) was longer on FD than on OD (60.8 ± 2.6 min vs. 40.4 ± 2.4 min). There was no difference in 12 h water consumption between diets (FD: 3.2 ± 1.0 l, OD: 2.0 ± 0.4 l). The total weight loss during feed deprivation was similar on FD and OD at 6 h of feed deprivation (7.0 ± 0.6 kg vs. OD: 7.6 ± 1.2 kg) and remained similar until 9 h of fasting. After 12 h of feed deprivation the weight loss was greater (P<0.05) on FD than on OD (10.8 ± 1.0 kg vs. 8.4 ± 1.0 kg) (Fig 1). After 5 h of feed deprivation there was no more significant weight loss on OD, but on FD a significant weight loss was observed for 9 h.

image

Figure 1. Weight loss during 12 h feed deprivation (FD, OD). * shows significant (P<0.05) difference between diets, unfilled marker shows significant (P<0.05) difference from 0 h of feed deprivation within treatment.

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Total plasma protein concentration was lower on FD than on OD before feeding and the last 8 h of feed deprivation (Fig 2). Plasma insulin concentration was lower on FD than on OD before, during and after feeding and remained lower during 0–5 h of feed deprivation (Fig 3). Plasma NEFA concentration was higher on FD than on OD at the start of and during feed deprivation (Fig 4). Plasma urea concentration was not different between diets but increased on both diets at 12 h of feed deprivation (Fig 4). Plasma acetate decreased by 12 h of feed deprivation on FD, but remained unchanged in horses receiving OD (Fig 4). Plasma glucose concentration was not affected by diet or feed deprivation (Fig 4).

image

Figure 2. Total plasma protein (TPP) concentrations before and during 12 h feed deprivation (FD, OD). 1hbf: 1 h before feeding; bf: before feeding; 0.5 haf: 0.5 h after feeding; 0:horses finished their feed allowances and feed deprivation starts; 1, 2, 3, etc. are hours of feed deprivation. * shows significant (P<0.05) difference between diets, unfilled marker shows significant (P<0.05) difference from 1 h before feeding within treatment.

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image

Figure 3. Plasma insulin concentrations before and during 12 h feed deprivation (FD,OD). 1hbf: 1 h before feeding; bf: before feeding; 0.5haf: 0.5 h after feeding; 0: horses finished their feed allowances and feed deprivation starts; 1, 2, 3, etc are hours of feed deprivation. * shows significant (P<0.05) difference between diets, unfilled marker shows significant (P<0.05) difference from 1 h before feeding within treatment.

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image

Figure 4. Plasma NEFA, urea, glucose and acetate concentrations during 12 h feed deprivation. (FD, OD). *shows significant (P<0.05) difference between diets, unfilled marker shows significant (P<0.05) difference from 0 h of feed deprivation within treatment.

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Discussion

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. Conflicts of interest
  9. Manufacturers' addresses
  10. References

Initial effects of diets on bodyweight

The significant increase in bwt (3 kg) after eating a forage-only diet in this study was similar to that recently reported in Standardbred horses in training (Jansson and Lindberg 2008). A larger increase (10.6 kg) has previously been reported comparing riding horses on concentrate-forage and forage-only diets (Ellis et al. 2002). The reason for the different responses between these studies is probably multifactorial. The forage chemical composition, total energy intake, as well as training intensity differed between these studies. The haylage used in the present study (and by Jansson and Lindberg 2008) was comparatively high in crude protein and low in fibre content (NDF and ADF) whereas the forage used by Ellis et al. (2002) was low in CP and high in NDF and ADF. Based on available data it can be estimated that the ME content was 40% higher (Anon 1989) in the forage of the present study compared to the forage of Ellis et al. (2002). This implies that the fibre fraction had different physical structure and physico-chemical properties that may have reduced both the undigested organic matter (‘bulk’) and the water volume of the digestive tract.

Increased bwt after feeding a forage-only diet has been related to higher gut fill and gut water content than when horses are fed a mixed diet (Meyer 1995). An increased gut water content, rather than organic fill, seems to be the major reason for the increased bwt on FD compared to OD, since the estimated difference in undigested organic matter in diets, using the digestibility coefficients of Ragnarsson and Lindberg (2008) for forage and of Olsson and Ruudvere (1955) for oats, was small (0.1 kg more on FD). The increased water content might also be reflected in lowered faecal DM concentration on FD. This has been shown in earlier studies, where DM content in the gut was not affected by diet (Meyer 1995) but DM concentration in the hindgut was lower in horses fed only forage (Hintz et al. 1971a; Meyer 1995).

The amount of forage DM intake per day does not seem to be a key factor for an increase in bwt on forage-only diets since DM intake was approximately the same in the present study and the study on riding horses (Ellis et al. 2002). This might, again, indicate that the chemical composition of the forage is of greater importance.

It is also possible that the level of training might affect the response in bwt. The horses in the present study were performing, and were, since they were aged 2 years, adapted to high intensity exercise (heart rate >200 beats/min) twice a week in addition to some submaximal exercise (approximately 1 h walk and slow trot [6–7 m/s] 1–3 times/week). The riding horses in the study by Ellis et al. (2002) performed only submaximal exercise (around 150 beats/min). Meyer (1995) showed that exercise reduced the amount of water in the hindgut and suggested that the water probably was absorbed and used to cover for evaporative losses that increase as exercise intensity and duration increase.

Another explanation for the limited increase in bwt on FD is the slightly lower calculated (Lindgren 1979) ME intake on this diet (FD: 18.5 ± 0.2 MJ/100 kg bwt and day vs. OD: 20.4 ± 0.1 MJ/100 kg bwt and day). The importance of this difference is difficult to interpret. No body condition score assessments were done during our study, but the general opinion among the staff was that the horses maintained body condition. If the horses were energy deficient on FD and used catabolic metabolism there should have been a difference in plasma urea concentration (Sticker et al. 1995) between diets, but there was not.

Initial effects of diets on water intake

The higher water intake on FD agrees with other studies comparing forage-diets with forage-grain diets (Fonnesbeck 1968). Fonnesbeck (1968) also showed that water intake was positively correlated to the fibre content of diet and this has been confirmed by Cymbaluk (1989) who also related water intake to ADF intake. Increased water content in the gut probably also contributed to weight gain as well as a larger plasma volume (lower TPP). Lower TPP on FD compared to OD, indicates that the plasma volume was larger, and implies that fluid balance was altered. A new equilibrium between fluid compartments of the body seems to have occurred, maybe due to the change in fluid volume and composition of the gut.

Effects of feed deprivation

The horses lost more weight during feed deprivation on FD than on OD, which can partly be explained by a larger faecal water weight loss on FD, since the faecal DM concentration was lower on this diet. However, the increased weight loss on FD was not significant until 9 h of feed deprivation, perhaps due to a slower passage rate on FD as suggested by Ellis et al. (2002). The larger weight loss on FD (2.4 kg more) was almost equal to the increase in bwt during the 18 days before fasting (3.0 kg) on this diet. This implies that a small weight gain caused by a forage-only diet would not be a limitation during exercise performance if the horse has limited access (or appetite) to feed before competition. It is also possible that faecal and bwt losses might be more rapid in a real competitive situation (including transportation and arrival to a race track). Therefore, we would not suggest a management routine prior to competition where horses on a forage-rich diet are fasted.

With the exception of a few hours post prandial, where the TPP was increased on both diets, the horses were able to maintain prefasting TPP until the last hour of fasting (i.e. 11 h) on FD, whereas on OD signs of dehydration (significant increase in TPP) were observed already at 8 h. These findings indicate that horses fed a forage-only diet could, during feed deprivation and almost no water intake, use some fluid compartment to maintain fluid balance. This also agrees with the theory of a larger fluid reservoir in their gut when fed a diet with a large forage (fibre) content as earlier suggested by Meyer (1987). This could be beneficial for the prerace fluid balance of the horse. These findings also agree with TPP measured during exercise that showed higher TPP during exercise in horses fed a ∼50/50 hay/grain diet compared to horses fed a diet that contained more hay (Danielsen et al. 1995).

The increase in plasma NEFA and urea concentrations during feed deprivation were consistent with earlier reports on horses (Baetz and Pearson 1972; Rose and Sampson 1982; Sticker et al. 1995; Christensen et al. 1997). In athletic horses, even short periods of feed deprivation, such as a feeding strategy with 2 feedings per 24 h have resulted in higher plasma urea concentrations than when horses were fed 6 times per 24 h (Jansson et al. 2006). The increased plasma urea concentrations during feed deprivation are probably due to increased catabolism of body protein for energy production and the increased plasma NEFA concentrations to an increased lipolysis of triacylglycerols from adipose tissues.

Plasma insulin concentrations remained almost unchanged at all times on FD but was elevated on OD both before and after the feeding and remained higher during the first 5 h of feed deprivation. This shows that the need for increased insulin secretion was limited on FD and that this diet caused a small alteration in the insulin response both in connection with feeding and during feed deprivation. Insulin decreases lipolysis in adipose tissue and thereby the release of NEFA. During the present study, the plasma NEFA concentration was higher on FD than on OD even in the fed state, which might be an effect of the low insulin secretion on FD. In addition to the low plasma insulin levels and increased plasma NEFA concentration on FD, the plasma acetate concentration was also increased prior to feed deprivation on FD. This indicates that the energy substrate profile was altered and that the metabolism could have been shifted towards a higher fat utilisation. The acetate in plasma originates from the fermentation of fibres and the concentration in colon and faeces fluid has been shown to increase on forage diets compared to grain (starch) diets (Hintz et al. 1971b). However, there was a significant drop in plasma acetate concentration during feed deprivation on FD indicating that to maintain the high concentration, a feeding interval shorter than 6–12 h is required.

In conclusion, feeding horses high-energy forage-only diets will result in a smaller increase in bwt than earlier reported for low-energy forage-only diets. This suggests that the physical properties and chemical composition of the forage is of greater importance for bwt changes than the forage DM intake per se. The impact of a bwt increase of 3 kg on performance remains to be investigated but observations from Jansson and Lindberg (2008) indicate that this will not affect heart rate and plasma lactate response following intensive exercise. However, the larger bwt loss on DC during feed deprivation (+2.4 kg) implies that a small weight gain caused by a forage-only diet might not be persistent if horses have a reduced feed intake prior to competition. Maintenance of TPP indicates that horses fed a forage-only diet prior to feed deprivation may use an internal fluid compartments to maintain plasma volume. This strategy could prove beneficial for the prerace fluid balance of exercising horses. The importance of the increased plasma acetate and NEFA concentrations on FD to metabolism remains to be investigated.

Acknowledgements

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. Conflicts of interest
  9. Manufacturers' addresses
  10. References

The authors thank Ulf Hedenström, Cajsa Löfqvist and all personnel and students at Travskolan Wången. We also thank Anna-Greta Haglund and the laboratory at Kungsängen SLU.

Manufacturers' addresses

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. Conflicts of interest
  9. Manufacturers' addresses
  10. References

1 Krafft AB, Falkenberg, Sweden.

2 Tru-test, Aukland, New Zealand.

3 WPA, Cambridge, UK.

4 Atago, Sur-Ne, Tokyo, Japan.

5 Mercodia, Uppsala, Sweden.

6 Wako Chemicals GmbH, Neuss, Germany.

7 Boehringer Mannheim/R-Biopharm, Darmstadt, Germany.

8 Ametec Spectro analytical instruments, Kleve, Germany.

9 SAS Institute Inc. Cary, North Carolina, USA.

References

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. Conflicts of interest
  9. Manufacturers' addresses
  10. References
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