Associative effects on total tract digestibility in horses fed different ratios of grass hay and whole oats

Livestock Production Science 65 (2000) 143–153 www.elsevier.com / locate / livprodsci

Associative effects on total tract digestibility in horses fed different ratios of grass hay and whole oats C. Palmgren Karlsson*, J.E. Lindberg, M. Rundgren Swedish University of Agricultural Sciences, Department of Animal Nutrition and Management, P.O. Box 7024, S-750 07 Uppsala, Sweden Received 5 March 1999; received in revised form 18 October 1999; accepted 27 October 1999

Abstract The present experiment was performed according to a 4 3 4 Latin square design with diets composed of (on dry matter (DM) basis) the following ratios of grass hay to whole oats: 100:0 (O0), 80:20 (O20), 60:40 (O40) and 40:60 (O60). Four mature Standardbred geldings weighing 493 (S.D. 34) kg were used. The digestibility of DM, organic matter and energy (dE) increased in a curvilinear pattern with the increasing inclusion levels of oats. It is suggested that this was a reflection of the significantly lowered digestibility of nutrients and particularly of fibrous components for the O60 diet as compared with the other three rations, thus resulting in negative associative effects between grass hay and oats. On the basis of the present data, and excluding the results of the O60 diet, the digestibility of dE of oats was estimated to be 75%. As an expression of the associative effect at the O60 level, the dE of the total diet was estimated to be 58%, as compared with 63% when estimated on the oats value of 75% and the O0 diet, resulting in a depression of dE of the total diet by 8%, mainly affecting the utilization of fibrous components. Urinary energy losses decreased with increasing level of oats.  2000 Elsevier Science B.V. All rights reserved. Keywords: Horses; Hay; Oats; Digestibility; Associative effects; Fibre; Nutrients

1. Introduction In computing rations for horses it is assumed that the nutritive value of a diet corresponds to the sum of the individual feedstuffs included. According to

*Corresponding author. Tel.: 146-18-672-094; fax: 146-18672-995. E-mail address: [email protected] (C. Palmgren Karlsson)

Martin-Rosset et al. (1984), the digestibility of horse rations is equivalent to the weighted sum of the nutrients supplied as forages and the associated amounts of concentrates. This assumption is supported by other studies (Hintz et al., 1971a,b; Martin-Rosset and Dulphy, 1987; Vermorel et al., 1991), in which no associative effects were observed between dietary components and diet digestibility. However, it should be noted that the systematic effect of diet composition on the digestibility of fibrous components was not evaluated by these

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authors, with the exception of Hintz et al. (1971a,b). In contrast, Thompson et al. (1984) found associative effects for several nutrients, implying that the digestibility of the total ration was not equivalent to the weighted sum of the individual components, in agreement with Kienzle et al. (1999). The literature cited deals only superficially with the effect of diet composition on the digestibility of individual fibrous components in mixed rations fed to horses. There are several factors (i.e. diet composition, level of feed intake, feeding routines) that could influence ileal digestibility of nutrients in diets for horses, and thus indirectly also influence hindgut fermentation (Meyer et al., 1982; Schwabenbauer et al., 1982; Radicke et al., 1991). When comparing feeding with 4 g versus 2 g starch / kg body weight (BW), Meyer and Landes (1994) observed that the postprandial pH of the jejunal chyme decreased substantially. As demonstrated by Kienzle (1994) high levels of starch (oats and barley) in the small intestine decreased starch digestibility, a result of the intestinal pH dropping below the optimal pH of intestinal a-amylase. The caecum and colon are the predominant sites of fibre digestion in the horse. The microbial digestion in the hindgut depends on the nature of the ration (Kern et al., 1974; Tisserand et al., 1980; Meyer et al., 1995). The proportion of lignified products in plant cell walls generally influences the development of the cellulolytic activity and biomass in the large intestine (Tisserand, 1989). In addition, the proportion of roughage to concentrate in the ration increases VFA production (Schwabenbauer et al., 1982) and the excretion of H 2 and CH 4 in the exhalation (Nyari, 1992). The concentrate composition with different ileal starch digestibility might also influence the hindgut microbial activity (Radicke et al., 1991). Furthermore, by increasing feed intake Meyer et al. (1982) demonstrated an increase in the caecal VFA concentration, as well as a marked decrease in pH. The aim of this investigation was to study the effects of varying the dietary ratio of hay and oats on nutrient digestibility and urinary nitrogen and energy losses in mature Standardbred geldings, and to determine the influence of associative effects between feedstuffs on nutrient digestibility.

2. Materials and methods

2.1. Experimental design and animals Four hay-based rations with increasing proportions of oats were fed according to a 4 3 4 Latin square design. Four Standardbred geldings (4–11 years old) with an average body weight (BW) of 493 (S.D. 34) kg were used. The horses were weighed before and after each collection period. Each experimental period comprised 14 days, 9 days of adaptation to the new diet followed by a 4-day period of quantitative and separate collection of faeces and urine. The change in rations between experimental periods was made gradually in the first 3 to 4 days of the adaptation period. The collection period was divided into two sub-periods of 2 days each with 1 day of rest in between, to reduce the discomfort for the horses. During the adaptation period and the day of rest the horses were kept in individual pens and had access to a common grass free paddock 5 to 7 h per day. During the 4 days of faeces and urine collection the horses were kept in metabolism stalls equipped with rubber mats. The horses were not given any additional exercise.

2.2. Diets and feeding The horses were fed rations consisting of grass hay, harvested in a late state of maturity, and whole (unprocessed) oats. The chemical composition of the feedstuffs is shown in Table 1. Dietary treatments consisted of, on dry matter (DM) basis, the following ratios between hay and oats (O): 100:0 (O0), 80:20 (O20), 60:40 (O40) and 40:60 (O60). The aim was to provide the horses with the same daily amount of DM and gross energy (GE) on a BW basis in all treatment groups (Table 2). The daily feed intake varied depending on BW and was for ration O0: 7.4–8.9 kg hay; ration O20: 5.9–7.1 kg hay and 1.5–1.8 kg oats; ration O40: 4.4–5.3 kg hay and 3.0–3.6 kg oats and for ration O60: 3.0–3.6 kg hay and 4.4–5.3 kg oats. A slight difference in daily GE intake between O0 and O60 occurred. For all treatment groups the daily feed allowance was 16 g feed (air dry) / kg BW and covered the maintenance

C. Palmgren Karlsson et al. / Livestock Production Science 65 (2000) 143 – 153 Table 1 Chemical composition (g per kg DM) and gross energy content (MJ / kg DM) of hay and oats

Organic matter Crude protein Crude fat Starch a Sugars b Total fibre Neutral detergent fibre Acid detergent fibre Crude fibre Lignin Gross energy a b

Hay

Oats

952 65 8 1 96 782 617 354 311 75 18.5

966 130 40 418 12 365 268 128 106 47 19.3

Sum of starch and maltodextrins. Sum of fructose and glucose.

145

2.3. Collection of samples Feed samples were collected at every feeding occasion, and after each collection period, they were pooled, mixed, sampled and kept at 2 208C. Faeces were collected several times during the day directly on the rubber mats and stored at 1 108C. At the end of each collection day the faeces were mixed, sampled and frozen (2208C). Urine was collected with a plastic funnel attached to the horse with a girth, and connected with a tube to a can containing 1 l 10% sulphuric acid to keep pH below 3. Each collection day the urine was weighed, made up to constant volume with water, sampled and frozen (2208C).

2.4. Chemical analyses requirements for energy and protein, according to the Swedish standards (LBS, 1989; metabolizable energy (ME) MJ / day 5 0.55 3 BW 0.75 and 5.5 g digestible crude protein (CP) / MJ ME). The horses were fed twice a day at 08:30 and 16:30 h with the daily feed allowances evenly distributed. Hay was given 15 min before the oats. Water and salt blocks were offered ad libitum. A commercial mineral mixture (50 g / day; 14.6% Ca, 6.6% P and 7.0% Mg) was given once a day in the adaptation periods until 2 days before each collection period.

Feed and faeces analyses were performed on oven dried (658C, 24 h) samples after milling through a 1-mm screen. DM, ash and crude fibre (CF) were determined according to the official methods given by Jennische and Larsson (1990). CP was based on total nitrogen (N) content (also including urine analysis) determined according to Kjeldahl (Nordisk ´ 1976) and multiplied by 6.25. Metodikkommitte, Crude fat (EE) was analysed according to the EECmethod B (Larsson, 1989). Starch and sugars were

Table 2 Daily dietary intake of nutrients (g / kg BW) and gross energy (kJ / kg BW) Ratios (hay:oats)

Dry matter Organic matter Crude protein Crude fat Starch e Sugars f Total fibre Neutral detergent fibre Acid detergent fibre Crude fibre Lignin Gross energy a–d

O0 (100:0)

O20 (80:20)

O40 (60:40)

O60 (40:60)

a

b

b

b

15.5 14.7 1.0 d 0.1 d 0.0 d 1.5 a 12.1 a 9.5 a 5.5 a 4.8 a 1.2 a 287 b

15.3 14.6 1.2 c 0.2 c 1.3 c 1.2 b 10.7 b 8.4 b 4.7 b 4.2 b 1.1 b 287 b

15.3 14.6 1.4 b 0.3 b 2.5 b 1.0 c 9.4 c 7.3 c 4.0 c 3.5 c 1.0 c 288 ab

15.2 14.6 1.6 a 0.4 a 3.8 a 0.7 d 8.2 d 6.3 d 3.4 d 2.9 d 0.9 d 290 a

Least square mean values within rows with differing superscript letters differed significantly: P,0.05. Sum of starch and maltodextrins. f Sum of fructose and glucose. e

S.E.M. 0.04 0.04 0.01 0.003 0.02 0.01 0.07 0.05 0.03 0.02 0.01 0.7

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analysed according to Larsson and Bengtsson (1983). Neutral detergent fibre (NDF) was analysed according to Robertson and Van Soest (1977) with a modified procedure (Pettersson and Lindberg, 1997), acid detergent fibre (ADF) and lignin according to Goering and Van Soest (1970). Total fibre (TF) was computed by subtraction of the analysed contents of sugars, starch, CP, EE and ash from the DM content. Gross energy (GE) was determined using an isothermal calorimeter (dds CP 500 calorimeter, Northcliff, South Africa).

2.5. Calculations For calculating, by difference, the nutrient and energy digestibilities in oats in the present study and in the concentrate part used in the literature referred to in Table 5, the following equation was used: d nutrient oats 5 (d nutrient total 2 (a /T 3 d nutrient hay )) /(b /T ) where d is digestibility, a is content (%) in the diet of the nutrient from hay, b is content (%) in the diet of the nutrient from oats and T is the total content (%) in the diet of the nutrient.

2.6. Statistical analysis Analyses of variance were performed according to a Latin-square design (Patterson and Lucas, 1962) using the GLM procedure (SAS, 1990). The model used was: Yijkl 5 m 1 ai 1 bj 1 gk 1 e ijkl when Yijkl is the ijklth observation, m is the general mean, ai is the fixed effect of the ith diet (i 5 1, 2, 3, 4), bj is the fixed effect of the jth period ( j 5 1, 2, 3, 4), gk is the random effect of the kth animal (k 5 1, 2, 3, 4) and e ijkl is the residual variation. Effects of interactions between period and animal were tested and found non-significant (P.0.05). Differences between dietary treatments were tested with least squares (LS) means (SAS, 1990). Results are presented as LS means with their standard errors. The results were also tested as regressions in linear and quadratic models with the inclusion level of oats as independent variable.

3. Results

3.1. Intake of nutrients Daily nutrient intake was expressed as g / kg BW (Table 2). In comparison with the other three rations, diet O0 resulted in a slightly higher (P,0.05) intake of DM / kg BW, and diet O60 a slightly higher (P,0.05) intake of GE / kg BW than the others. With increasing proportion of oats in the rations, the intake of CP and starch increased (P,0.05), while the intake of CF, TF, NDF, ADF, lignin and sugar decreased (P,0.05), in agreement with that expected.

3.2. Total ration nutrient digestibility Due to unrealistic digestibility values for two of the horses in the first period (diet O40 and O60), these data were excluded from the calculations. However, this did not change any major trends in the results in the present experiment. The inclusion of oats (O20, O40, O60) resulted in a higher (P,0.05) digestibility of DM, organic matter (OM) and gross energy (dE) of these rations in comparison with hay alone (O0). However, digestibility did not differ significantly between oat substituted rations. The digestibility of CP in the rations was improved (P,0.05) linearly as the proportion of oats increased, but for the digestibility of EE the effect levelled off at the higher levels of oats inclusion (O40, O60) (Table 3). Digestibility of the fibre fraction decreased at the highest (O60) level of oat inclusion. For diet O60, the digestibility coefficients of the fibrous components (NDF, ADF, and CF) were lower (P,0.05) than for the other rations (Table 3). By testing the curvilinearity for NDF, ADF and CF as quadratic functions of the inclusion level of oats, the R 2 -values increased (R 2 50.26, 0.33 and 0.37, respectively), compared with the corresponding linear models (R 2 50.16, 0.21 and 0.23, respectively).

3.3. Calculated nutrient digestibilities of oats The apparent digestibility coefficients of nutrients and energy of total rations may be compared with those values calculated, assuming that the digestibili-

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Table 3 Total ration nutrient digestibility coefficients (%) Ratios (hay:oats)

n Dry matter Organic matter Crude protein Ether extract Total fibre Neutral detergent fibre Acid detergent fibre Crude fibre Lignin Energy a–c

O0 (100:0)

O20 (80:20)

O40 (60:40)

O60 (40:60)

S.E.M.

4 48 b 49 b 54 c 236 c 44 a 37 a 29 a 29 a 13 45 b

4 55 a 56 a 65 b 15 b 46 a 39 a 32 a 34 a 19 53 a

3 58 a 59 a 72 ab 42 a 43 a 35 a 24 a 25 a 12 56 a

3 58 a 60 a 76 a 51 a 36 b 26 b 12 b 12 b 8 57 a

1.3 1.3 2.0 6.6 1.9 2.3 3.2 3.5 4.6 1.3

Least square mean values within rows with differing superscript letters differed significantly: P,0.05.

ty of the nutrient in oats is either 0% or 100%, respectively (Fig. 1). The measured digestibilities of ADF and CF of the rations were lower than when assuming that the oat fibre digestibility was zero. Thus, when calculating by difference, on the basis of the pure hay diet, the digestibility coefficients of NDF, ADF and CF of oats for the levels of 20, 40 and 60% inclusion in each ration, the following values have been computed; NDF: 58, 40 and 13%; ADF: 62, 26 and ,0% and CF: 88, 26 and ,0%, respectively. For the estimation of dE of oats, a straight line extrapolation of the data was used, where the O60 results were excluded, since this level contributed to curvilinearity.

3.4. Urinary excretion of nitrogen and energy The urinary excretion of N increased, as expected, with increasing level of oats inclusion (Table 4). For the four treatment groups on average 64% of the digested N was retained. The urinary energy per g N was significantly higher for diet O0 and decreased with increasing oats inclusion.

4. Discussion This investigation demonstrates an enhanced digestibility of OM ( y) of the rations with increasing the percentage of oats (x) fed ( y 5 50.7 1 0.21x, P,0.01), which agrees fairly well with Martin-

Rosset and Dulphy (1987) ( y 5 51.7 1 0.32x). The higher regression coefficient of these authors is explained by differences in the composition of the concentrate mixture fed, i.e. maize–barley versus oats in our experiment. However, the curvilinearity of the diet digestibility of nutrients and energy with increasing level of oats inclusion (Fig. 1) indicates negative associative effects between the two dietary components. This associative effect was most apparent at the highest level of oats inclusion (O60) with the digestibility of fibrous components declining from O0 to O60 (Fig. 1). Similarly, by increasing the inclusion of oats Thompson et al. (1984) observed a significant, more linear and almost constant, negative associative effect on the digestibility of the fibrous components (Fig. 2a). The results of Hintz et al. (1971b) also seem to indicate curvilinearity for the digestibility of NDF (Fig. 2b), where a significant increase is levelled off by a drop in the digestibility (from 91 to 62% calculated by difference) at the highest level of cereal inclusion. This effect had not been commented on by the authors. In a subsequent investigation (Hintz et al., 1971a) a significantly linear increase in the digestibility of the fibrous components with increasing level of cereal inclusion (shelled maize) was reported (Fig. 2c). Difference calculations on the data from Hintz et al. (1971a) show a nearly complete digestibility of NDF and ADF of the cereal part of the diet. The tendency towards an improved digestibility of the fibrous fraction, noticeable in the present investigation at the lowest level of oats inclusion, and

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Fig. 1. The apparent digestibility coefficients (♦) of (a) organic matter, (b) energy, (c) crude protein, (d) neutral detergent fibre, (e) acid detergent fibre and (f) crude fibre found in the present study, compared with values predicted assuming that the digestibility of the nutrients in oats is 0% (s) or 100% (d).

which strengthens the curvilinearity of our results, agrees with the results of Pereira et al. (1989) (Fig. 2d) and Martin-Rosset and Dulphy (1987) when hay was used as forage and the horses were fed at maintenance and 1.43maintenance (Fig. 2e and f).

The slight increase in nutrient digestibility for the O20 diet may thus be explained by a stimulation of the hindgut microbial activity and hence fibre digestion by a small amount of concentrate (Kern et al., 1973). According to Bellet (1982) the hindgut flora

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Table 4 Urinary excretion of nitrogen, nitrogen retained, urinary energy loss and urinary energy percentage of digestible energy Ratios (hay:oats) O0 (100:0) Urinary N (g / day) N retained (g / day) N retained (% of dig.) Urinary energy MJ / day kJ / g N Percent of DE a–c

b

O20 (80:20) b

O40 (60:40) b

O60 (40:60)

18.2 25.3 c 56.8 b

22.1 40.4 b 63.7 ab

22.4 50.5 ab 68.2 a

31.8 a 64.6 a 65.8 ab

1.71 96 a 2.6 a

1.63 75 b 2.2 b

1.46 64 b,c 2.0 b

1.66 50 c 2.0 b

Least square mean values within rows with differing superscript letters differed significantly: P,0.05.

responds more positively to oats compared with maize. However, as demonstrated by Kienzle (1994) the maximum amount of starch provided at a feeding occasion should not exceed 2–2.5 g / kg BW to avoid digestive upsets. Although the amount of starch per feeding occasion did not exceed (1.9 g) this recommendation in the present study, a depression in digestibility occurred. To explain the influence of associative effects on nutrient digestibility, the type and quality of feedstuffs have also to be considered. Thus, the structure of fibrous components varies between roughages and concentrate feedstuffs, thereby affecting optimal conditions for hindgut fermentation (Tisserand, 1991). The availability of carbohydrates may also vary between cereals (Meyer, 1992; Potter et al., 1992; Meyer et al., 1993). Comparing the results from different studies of this type is complicated, since the conditions between experiments vary. Furthermore, often the numbers of observations per treatment group are limited and the proportion of fibrous components originating from the concentrate fraction is relatively small. Therefore, significant differences in the digestibility coefficients of fibrous components between different levels of concentrate inclusion are not always found. However, there seems to be a consistent negative trend in the studies of MartinRosset and Dulphy (1987), Pereira et al. (1989), Hintz et al. (1971b) and Thompson et al. (1984) and our results, as illustrated by the apparent digestibility coefficients of the different fibre parameters (NDF, ADF and / or CF) when compared with those values computed assuming the digestibility of the fibrous components of the cereal part to be either zero or

100% (Fig. 2a–f). In these figures the percentage of nutrients and energy, respectively, derived from the concentrate, have been used as independent variables. An overview of the background of the studies referred to is given in Table 5. The feeding routines of horses may also be of importance to explain differences between experiments and maybe the occurrence of associative effects. In the present study, the hay ration was fed 15 min prior to the oats, which is a common recommendation, in order to stimulate saliva production, to get a better mixing of the ingesta in the stomach as well as to optimise the rate of passage through the gastrointestinal tract (Meyer, 1992). Tisserand (1992) argued that for an optimal energy utilisation in diets with a predominant cereal component, horses should be fed cereals separately from hay or after the hay ration. In the present experiment 69% of the DE in the O60 diet was derived from oats, which is a predominant part of the total ration. Therefore, the feeding routine chosen by us may have been a contributing factor to the associative effect observed (Doreau, 1978). On the other hand one may argue, that the strategy recommended by Tisserand (1992), for predominantly cereal based diets, may result in a depressed precaecal starch breakdown (Kienzle, 1994) and hence transfer of too much starch into the hindgut, thereby lowering microbial fermentation of the fibrous components. With respect to the lowered digestibility of fibrous constituents observed in our study, this could have been a major factor explaining the associative effects. When feeding hay and cereals simultaneously using a pelleted diet, Hintz et al. (1971b) were unable to detect any associative effects. This could

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Fig. 2. The apparent digestibility coefficients (♦) of (a) acid detergent fibre (Thompson et al., 1984), (b) neutral detergent fibre — trial 1 (Hintz et al., 1971b), (c) neutral detergent fibre — trial 2 (Hintz et al., 1971a), (d) acid detergent fibre (Pereira et al., 1989), (e) crude fibre — hay: maintenance (Martin-Rosset and Dulphy, 1987) and (f) crude fibre — hay: 1.43maintenance (Martin-Rosset and Dulphy, 1987), compared with values predicted assuming that the digestibility of the nutrients in oats is 0% (s) or 100% (d).

be the result of a changed supply of the cereal part over time when providing pellets, but also by using maize, which is known to be less easily solubilised

in the small intestine compared with oats (Meyer et al., 1993). For the estimation of dE of oats a straight line

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Table 5 Information on the feeding background in the different studies referred to in the text Roughage

Oat forage hay Long grass hay

Maize silage

Long hay

Grass silage

Timothy hay (long) Alfalfa meal (pelleted with conc) Alfalfa hay (long stem) a b

Composition of concentrate a , %

Concentrate of total rations, %

Milo (95%) SBM (5%) Maize (59.5%) Barley (30%) Nut meal (8%) (pelleted) Maize (48%) Barley (25.2%) Nut meal (22.8%) (pelleted) Wheat offal (36%) Oatfeed (26%) Various (38%) (cubed) Barley (31%) Maize (31%) Oats (32%) Molasses (4%) Maize (shelled) Maize (78.2%) SBM (19.4%) Oats (crimped)

Feeding b

References

L

F

CM

0, 20, 40, 60

M

2

Total

Pereira et al., 1989

0, 30, 60

M 1.43M ad lib

2

Total

Martin-Rosset and Dulphy, 1987

0, 30, 60, 90

M 1.43M ad lib

2

Total

Martin-Rosset and Dulphy, 1987

29, 43, 71, 86

–

–

AIA

Frape et al., 1982

0, 33, 66

ad lib

–

Cr 2 O 3

Kerrour et al., 1997

0, 50, 80 0, 40, 80

? ?

2 2

Cr 2 O 3 Cr 2 O 3

Hintz et al., 1971a Hintz et al., 1971b

0, 20, 40, 60, 80

?

–

Total

Thompson et al., 1984

Minerals and vitamins up to 100%; SBM, soybean meal. L, level; F, frequency; CM, method of collection; M, maintenance; AIA, acid insoluble ash.

extrapolation has been chosen, excluding the data of the O60 diet. A value for the dE of 75% was obtained, which agrees fairly well with the dE of 73% and 71% given by NRC (1989) and INRA (1984), respectively. The difference between our value and those of NRC (1989) and INRA (1984) could be related to differences in the CF contents (10.6 versus 12.1 and 11.4%) between the oats. Based on a dE of oats of 75%, the expected dE of the O60 ration should have been 63% as compared with the value of 58% measured, corresponding to a difference of |8% in dE (Fig. 1). This means that the reduced digestibility of the dietary fibrous components affected total nutrient utilization negatively. No associative effects between hay and oats were found for the digestibility of CP, since the digestibility increased linearly (P,0.05) with increasing oat inclusion. The digestibility of CP in oats, calculated by difference, was on average 83% as compared to 79% in INRA (1984). The slightly higher digestibility values of CP observed by us should be explained

by the fact that the faeces samples were dried at 658C, which also resulted in an overestimation of N deposition. The estimates of total tract CP digestibility are also of limited value since they are greatly affected by the extent of hindgut fermentation and the microbial mass produced (Sauer et al., 1980; Low and Zebrowska, 1989; Meyer, 1992). The increasing excretion of urinary N with increasing level of oats inclusion is a result of the enhanced supply of cereal CP and its higher digestibility. The amount of N retained (g / day) was high for all diets, which is rather unexpected since the horses were adult. However, high N-retention corresponding to our results has also been noticed by other authors (Freeman et al., 1988; Lindberg and Jacobsson, 1992). The amount of urinary energy per g N was significantly highest for the pure hay diet (O0) and decreased with increasing inclusion level of oats. This could be an effect of the decreased amount of forage, resulting in a decline in the dietary supply of phenolic components voided via the urine,

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which is also reflected in the decline in percent of urinary energy of the DE (Nehring, 1972; Blaxter, 1989). An unexpected low energetic effect in feeding high concentrate rations to high performing horses, observed occasionally in practice, could possibly be related to a negative associative effect of the cereal part on nutrient digestibility of the total ration as found in this investigation.

5. Conclusion The present data clearly indicate negative associative effects between hay and oats on the digestibility of nutrients, mainly as a result of an impaired digestibility of fibrous components. As a consequence, horses performing at a high work intensity, and therefore being fed diets with a high proportion of cereal grains, could be expected to utilize the dietary energy less efficiently than assumed.

Acknowledgements The authors express their thanks to Dr. Sigvard Thomke for valuable discussions and criticism of the manuscript. This work was financially supported by the Swedish Farmers Foundation for Agricultural Research.

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