Palatability and digestibility of horse diets containing increasing levels of citrus pulp


Palatabilidad y digestibilidad de las dietas que contienen pulpa de cítricos para caballos


Camilla G Moreira,1 M.Sc, Ives CS Bueno,1 Ph.D, Madalena L Menezes,1 M.Sc, Thaís P Mota,1 Zoot, Aline D Souza,1 Zoot, Amanda F Tavares,1 Zoot, Laise S Augusto,1 Zoot, Roberta A Brandi,1* Ph.D.

1Universidade de São Paulo, Departamento de Zootecnia, Faculdade de Zootecnia e Engenharia de Alimentos - FZEA/USP, Pirassununga, SP. Brasil. Av. Duque de Caxias norte, 225. Campus USP, CEP: 13635-900.

*Correspondence: robertabrandi@usp.br

Received: May 2014; Accepted: January 2015.


Objective. To evaluate the impact of citrus pulp on the palatability and digestibility of horse diets and the physicochemical characteristics of the feces. Materials and methods. The diets were composed of 60% of the energy from the forage and 40% from the concentrate, with increasing levels of citrus pulp. Two different tests were performed. The first test assessed the palatability of concentrates. Using a randomized experimental design, 15 horses were observed for 10 days. The variables recorded were first action, first choice and intake ratio (IR). Five horses were arranged in 5x5 Latin Square design for the test assessing digestibility and fecal physicochemical characteristics. The apparent digestibility of the nutrients and the color, consistency, pH and buffering capacity (BC) of the feces were evaluated. Results. The addition of increasing levels of citrus pulp had an effect (p≤0.001) on first action. A difference was also observed in first choice, and the addition of 0, 7 or 14% of citrus pulp was preferred. A difference between treatments (p≤0.001) was also observed for IR, and the control concentrate was consumed the most. The amount of citrus pulp included had no effect (p>0.05) on the digestibility of nutrients, fecal consistency and color, and there was no effect (p>0.05) on fecal pH and BC. Conclusions. Horses can identify the presence of citrus pulp in concentrates but prefer concentrates without added citrus pulp. Citrus pulp does not negatively affect the digestibility of concentrates or the physicochemical characteristics of the feces; thus, citrus pulp is a viable alternative ingredient in the formulation of horse diets.

Key words: Fecal pH, pectin, preference, super fiber (Source: CAB).


Objetivo.Evaluar la inclusión de pulpa de cítricos en la dieta de los caballos a través de la evaluación de la de palatabilidad, digestibilidad y características fisicoquímicas de las heces. Materiales y métodos. El valor energético de las dietas fue aportado en el 60% por la energía del forraje y en el 40% a partir del concentrado con niveles crecientes de pulpa de cítricos. En la primera prueba se evaluó la palatabilidad de los concentrados. Se utilizó un diseño experimental aleatório. Se observaron 15 caballos durante 10 días. Las variables registradas fueron la primera acción, la primera opción y la relación de la ingesta (IR). Cinco caballos fueron dispuestos en un diseño cuadrado latino 5x5 para la evaluación de la digestibilidad y las características fisicoquímicas fecales. La digestibilidad de los nutrientes y el color, la consistencia, el pH, y se evaluó la capacidad de tamponamiento (CT) de las heces. Resultados. El nível de inclusión de pulpa de cítricos tuvo un efecto (p<0.001) en la primera acción. Una diferencia también se observó en primera elección. Una diferencia (p<0.001) entre los tratamientos también se observó para IR. El nível de inclusión de pulpa de cítricos no efectó (p>0.05) la digestibilidad, consistencia fecal y el color de las heces, pH fecal y CT de las heces. Conclusiones. Los caballos pueden identificar la presencia de pulpa de cítricos en los concentrados, pero prefieren concentrados sin pulpa de cítricos. La pulpa de cítricos no afecta la digestibilidad de los nutrimentos en el concentrado, ni las características fisicoquímicas de las heces; la pulpa de cítricos es un ingrediente alternativo viable en la formulación de dietas para caballos.

Palabras clave: Pectina, pH fecal, preferencia, super fibra (Fuente: CAB).


The horses can use large quantities of forage to meet their nutritional requirements (1). However, diets with a high percentage of grains are used to maximize productivity (2). Currently, researchers are searching for alternative ingredients that can replace traditional ingredients such as corn, soybean meal and wheat bran for use the alternative ingredients, easily fermentable fibers, or “super fibers”, such as citrus pulp, seem especially promising (3).

However, only a few studies have been published regarding the use of citrus pulp in horse feed (4-7). Ott et al (4) investigated the acceptability and digestibility of diets with increasing levels of citrus pulp (0, 15 and 30%) and observed that diets with 30% citrus pulp were rejected by horses. Nevertheless no difference was observed in the digestibility of the nutrients. Manzano et al (5) observed an increase in the digestibility of dry matter and crude protein with the addition of citrus pulp; these authors using no more than 15% citrus pulp to prevent a decrease in palatability.

Another important factor to determine the digestibility of horse diets is the physicochemical characteristics of the feces, as:

Feces parameters (8) such as color (9), consistency (10), pH (8) and buffering capacity (BC) (11) are important to evaluate the gastrointestinal tract health (10).

The aim of this study was to evaluate the effect of increasing levels of citrus pulp in concentrate on the palatability and apparent digestibility of horse diets, as well as the physicochemical characteristics of the feces.


Study site. This study was performed in the Equine Production section of the Pirassununga Administrative Campus of Animal Science and Food Engineering School, University of São Paulo.

Two experiments were conducted: a palatability test (test 1) and a test of apparent digestibility and physicochemical characteristics of feces (test 2).

Animals and experimental design. Fifteen horses, 10 years old, and 400 kg body weight, were used in the study. The adaptation period lasted five days and the experimental period lasted 10 days; protocol was performed once a day totalizing 150 observations. The animals were observed by instantaneous focal sampling for three consecutive minutes, and an ethogram to organize the dates. It was tested concentrates with 0, 7, 14, 21 or 28% citrus pulp (Table 1).

Table 1

Before starting the trial, the animals received 1 kg of concentrate (different from concentrates to be tested) in stall one hour before the test and were then led individually to the arena to conduce the trial. The troughs containing 200 g of each concentrate were arranged in five equidistant points, and the treatments and animals were randomized each day.

The preference test was used to assess the palatability of the concentrates (12). The first action (1 - sniffed, 2 - ate), first choice (first concentrate actually consumed) and intake ratio (amount of each concentrate ingested) were evaluated.

The leftovers were removed and weighed, and the intake ratio (IR) was calculated using the equation below:

IR = concentrate (% citrus pulp) / (intake of 0% concentrate + intake of 7% concentrate + intake of 14% concentrate + intake of 21% concentrate + intake of 28% concentrate).

Statistical analysis. The experimental design was a split pot. A chi-square goodness of fit test was used to evaluate the first choice and first action, under the null hypothesis that all treatments would occur with equal probability.

A linear mixed model with a 5x10 factorial scheme was used to analyze the IR, taking into consideration the fixed effects of the treatment and the day, as well as the random effect of the animal, in addition to the effect of the residue. In the case of a significant interaction, Student’s t test was used to assess each treatment within each evaluation day. All analyses were performed using the Statistical Analysis System software, version 9.1.3. (13).

Animals and experimental design. Were used five nonpregnant mares, Brasileiro de Hipismo breed, weighing 406.6±76.86 kg and aging six years were housed in individual stalls and arranged in 5x5 Latin Square design. The experimental period included seven days of adaptation followed by three days of data collection and analysis.

The experimental diets were formulated to meet the maintenance requirements for horses, as recommended by the National Research Council, 2007 (14). The diet were composed by 60% of the energy coming of forage (Coast Cross hay) and 40% came from concentrate with increasing levels of citrus pulp (0, 7, 14, 21 and 28%), (Table 1 and Table 2).

Table 2

To evaluate the physicochemical characteristics of the feces, the samples were collected for three days in plastic bags. Subsequently, the pH was measured using a bench pH meter, and the BC was measured as described in Zeyner et al (11).

The consistency of the feces was evaluated as described by Berg et al (9), and the feces color was evaluated as described by Godoi et al (8).

Laboratory methods. A composite sample from each animal was obtained at the end of the experiment and sent to the Bromatology laboratory at the FZEA/USP. The dry matter (DM), mineral matter (MM), ether extract (EE) and crude protein (CP) contents of the concentrates, hay and feces were determined according to the procedures described by the Association of Official Analytical Chemistry (15). The acid detergent fiber (ADF), neutral detergent fiber (NDF), cellulose (CEL) and hemicellulose (HEM) contents were analyzed as described by Van Soest et al (16). The gross energy (GE) was determined using a bomb calorimeter.

We further determined the digestibility coefficients of DM, OM, CP, EE, NDF, ADF, CEL, HEM and GE.

Statistical analysis. To evaluate the digestibility, we used the 5x5 Latin Square design using five animals fed with five diets with increasing levels of citrus pulp, repeated in five consecutive periods. The quantitative parameters were analyzed according to the statistical model:

Yijk = µ + Ai + Tj + Pk + εijk,

Yijk represents the dependent variable
µ, the overall mean
Ai, the effect of the animal (i = 1 to 5)
Tj, the effect of the treatment (j = 1 to 5)
Pk, the effect of the period (k = 1 to 5) and
εijk, the residual error.

The PROC GLM function of the SAS statistical package for Windows (13) was used for statistical analysis. The means were compared by Tukey’s test with a probability level of 5%.

The consistency and color of the feces were assessed using the chi-square goodness of fit test. The BC and fecal pH were assessed using the mixed linear model function of the SAS (13) program.


Palatability. There was a difference in first action (p≤0.001), 66.91% of the animals sniffed the concentrates upon initial exposure to them, while 33.09% of the animals ate the concentrates. There was a difference (p<0.001) between treatments for the first choice variable, and treatment with 0, 7 and 14% citrus pulp were preferred (Table 3). There was a difference (p≤0.001) in IRs, as the control treatment (0%) was consumed the most, followed by treatments with 7, 14, 21 and 28% citrus pulp (Table 3).

Table 3

Apparent digestibility of the diets and physicochemical characteristics of the feces. The addition of citrus pulp had no effect (p>0.05) over the digestibility coefficients of DM, CP, OM, EE, GE, CEL, HEM, NDF or ADF (Table 4).

Table 4

Treatment with citrus pulp had no effect (p>0.05) on fecal consistency or color. The color of the feces remained greenish, and the consistency was classified as normal (score of 3). Adding citrus pulp had no effect (p>0.05) on BC or fecal pH. The mean values observed for BC at pH 5, BC at pH 6 and pH are shown in table 5.

Table 5


Palatability. The first action performed by the animals upon to the initial exposure to the concentrates was sniffing (66.91%), which demonstrates that horses can select the preferred concentrate through smell (17).

This selection ability was reiterated by observation of the first choice, which demonstrated that lower levels of citrus pulp (0, 7 and 14%) were preferred (Table 3). The concentrates containing 21% and 28% citrus pulp were chosen less frequently, most likely because citrus scents are not preferred by horses (18).

In an investigation of the acceptability and digestibility of diets with increasing levels of citrus pulp (0, 15 and 30%), Ott et al (4) observed that six out of eight horses refused a diet containing 30% citrus pulp. It was not observed effect when 28% citrus pulp was added to the concentrate in this trial.

In contrast to our results, Tribucci et al (18) found no effect of different levels of citrus pulp on the first choice when studying the palatability of diets with increasing levels of citrus pulp (7, 14, 21 and 28%) in horse diets. However, they did observe an effect on the first action. The methodology used in these studies may have influenced the results, as Tribucci et al (18) used continuous troughs in a stall, while our study used separate troughs in a round pen (open area). In addition, the authors of the previous study noted a strong citrus pulp odor, which we did not observe in this study.

Assis et al (19) reported that the palatability and composition of citrus pulp can be altered by factors such as: fruit varieties, the presence of seeds, whether essential oils are removed and the processes that the fruit and fruit residues were subjected to. This variation in the composition of citrus pulp may change the taste of the final product, and could therefore account for the disparities between studies of this subject.

Reinforcing the distinct selection capacity of horses, we observed a higher IR for concentrates with lower levels of citrus pulp (Table 3), and the control concentrate was preferred by animals.

Considering only the IR, the concentrate containing 0% citrus pulp was the favorite, with an IR=0.3137 (the reference value for preference was IR>0.25). This reinforces the preference for this level (0%) of citrus pulp in the concentrate.

Manzano et al (5) observed different results in their study of growing fillies when three levels of citrus pulp (0, 7.5 and 15%) were added to the diet. The diet containing 15% citrus pulp was consumed more by the fillies. The animals in our study received experimental concentrates only during the evaluation, whereas in the protocol developed by Manzano et al (5), the animals received hay and the test concentrates.

The use of different tests to assess palatability and acceptability greatly influences experimental results. Ott et al (4) and Manzano et al (5) concluded that horses had a preference for certain diets by assessing intake and leftovers, whereas Müller and Udén (20) used the first choice test. In these studies, the duration of testing and the form of assessment were the primary variables. Our study analyzed multiple factors to explain preference; thus, it is difficult to compare our results with the literature. To better understand preference and feeding behavior, it is necessary to standardize the tests used in these studies.

Apparent digestibility of the diets and physicochemical characteristics of the feces. We observed different values for the nutrient digestibility coefficients (Table 4) compared to other studies (5-7), which may have been affected by factors such as: the forage:concentrate ratio, the composition of the diet ingredients and the quantity of food provided.

The total amount of DM provided per animal in this experiment was 2% of body weight, including at least 1% of the body weight of the animal as forage (Coast Cross hay), as recommended by the NRC (14). The forage:concentrate ratio was 60:40, based on the energy supply, representing a 75:25 DM ratio. Miraglia et al (6) analyzed the digestibility of different forage:concentrate ratios and observed that, as the percentage of concentrate increased, the apparent digestibility coefficients of the DM, organic matter, GE and CP also increased. When these authors used a 75:25 diet ratio, as we did, they observed similar apparent digestibility values. The differences between the results can be attributed to different qualities of forage used in the studies. Manzano et al (5) provided food with forage:concentrate ratio of 40:60 ad libitum and observed higher nutrient digestibility coefficients than we did.

Miraglia et al (6) note that when a diet comprised of concentrate and lower quality forage favors nutrient digestibility.

The forage used in the present study had a chemical composition similar to that the Bermuda Grass hay recommended by NRC (14) and superior to the hay used by Miraglia et al (6). The hay used in our study is considered a good quality hay for horses, and accounted for a greater proportion of the total diet, which may have contributed to the consistency of the digestibility coefficients.

The amount of citrus pulp added to the concentrate did not affect the digestibility of the fiber fraction. These results are consistent with a study by Oliveira et al (2), in which pectin was added to horse diets. According to the authors, the rapid rate of food passage in horses may not have allowed sufficient microorganism activity to degrade the fiber, which resulted in no change in the digestibility of NDF and ADF. A similar observation was made by Weyenberg et al (21), who noted that the higher forage:concentrate ratio in the diet, the faster the rate of passage through the entire gastrointestinal tract.

According to Miraglia et al (6), the digestibility of the fiber fraction decreases with the addition of concentrate to the diet. We observed no decrease in fiber digestibility in this study, most likely due to the low percentage of concentrate in the diet and the quality of the hay, which provided a substrate for microorganism activity in the large intestine of horses.

We observed a GE digestibility coefficient of 46.15%, which is close to that found in diets composed solely of forage (7). This coefficient is lower than that observed by Ott et al (4) and Manzano et al (5), which could be explained by the higher amount of concentrate provided in the earlier studies.

Although the GE digestibility coefficient we observed was lower than that reported in other studies using citrus pulp (5-6), the values did not differ depending on the levels of citrus pulp included (Table 4), indicating that the inclusion of this ingredient did not negatively alter the digestibility of GE. The consistent GE digestibility coefficients can also be explained by the consistent levels of NDF and ADF digestibility (2), which provided a constant substrate for degradation by microorganisms, which produces energy.

Adding fiber to the diet increases intestinal motility, which may impair the digestibility of nutrients in the small intestine. This could explain the low value of the GE digestibility observed in this study.

The presence of fiber as well as the quality and quantity of food intake, particle size and speed of transit of the digesta can affect both nutrient digestibility and feces quality in horses (21). Studying nutrient digestibility in conjunction with the physicochemical characteristics of feces is important for evaluating digestive tract function, as it allows the diagnosis of gastrointestinal disorders.

We observed no effect (p>0.05) of diet on the color of the feces, which had a greenish color. Feces color is primarily influenced by the forage used (22), and the color observed in this study was due to the characteristics and the amount of the hay provided (75% Coast Cross hay on a DM basis).

The consistency of the feces observed in this study was normal, suggesting that adding citrus pulp to the diet does not promote production of high levels of lactic acid and consequently feces fluidization, which can be proven by maintaining the pH and buffering capacity (Table 5).

The fecal pH was not influenced (p>0.05) by treatment (Table 5). The mean pH value we observed (6.62) is considered appropriate for the activity of cellulolytic bacteria that ferment cell wall carbohydrates, thereby producing short chain fatty acids (SCFA) that are absorbed in the large intestine (23,24).

The buffering capacity conferred the feces, may be related to ion exchange at the SCFA absorption time, which occurs through the absorption of sodium ions (Na+) at the apical membrane of the absorptive cells in exchange with hydrogen ions (H+), this process can be directly linked to the exchange of chlorine (Cl) and bicarbonate (HCO3) (21), process according to Van Soest et al (15), is essential for maintaining proper pH levels, as evidenced in this study.

The forage:concentrate ratio (75:25, based on DM) may have created favorable conditions in the large intestine for cellulolytic bacteria, data that corroborate with Hoffman (25) findings, which cites that diets with higher amounts of roughage, the main SCFA produced is the acetate.

This indicates that the main substrates for fermentation in large intestine were rapidly and slowly fermentable carbohydrate, with predominance production of weak acids.

The quantity and the quality of fiber in the diet may modify intestinal transit, leading to changes in fecal characteristics. The treatment tested in this present study provided a high amount of fiber as a substrate for the microorganisms. The fiber also has high BC due to the exchange of cations and metallic ions (16). Consistent with this concept, we observed no reduction in BC in this study (Table 5), this demonstrates that animals fed diets with increasing levels of citrus pulp –an ingredient that is considered a “super fiber”– maintained a fecal pH of above 6.0, which is optimal for fermentation by cellulolytic bacteria.

In conclusion horses can identify the presence of citrus pulp in concentrate and prefer concentrates without citrus pulp.

Adding up to 28% citrus pulp to horse diets does not negatively affect the digestibility of nutrients and the physicochemical characteristics of the feces; thus, it is an alternative energy ingredient in the formulation of horse diets.


1. Brandi RA, Furtado CA. Nutritional and metabolic importance of fiber in the horse diet. Rev Bras Zootec 2009; 38:246-258.

2. Oliveira GJC, Lima JAF, Araújo KV, Fialho ET, Bertechini AG, Pérez JRO, Freitas RTF. Effect of Pectin and Soybean Meal Addition on the Apparent Digestibility of Nutrients, in Horses. Rev Bras Zootec 2002; 31(3):1184-1192.

3. Furtado CE, Brandi RA, Ribeiro LB. Utilização de coprodutos e demais alimentos alternativos para dietas de equinos. Rev Bras Zootec 2011.; 40:214-223.

4. Ott EA, Feaster JP, Lieb S. Acceptability and digestibility of dried citrus pulp by horses. J Anim Sci Champaign 1979; 49(4):983-987.

5. Manzano A, Freitas AR, Esteves SN, Novaes NJ. Pelleted Citrus Pulp in Equine Feeding. Rev Bras Zootec 1999; 29(6):1327-1332.

6. Miraglia N, Bergero D, Polidori M, Peiretti PG, Ladetto G. The effects of a new fibre-rich concentrate on the digestibility of horse rations. Livest Sci 2006; 100:10–13.

7. Peiretti PG, Meineri G, Miraglia N, Mucciarelli M, Bergero D. Intake and apparent digestibility of hay or hay plus concentrate diets determined in horses by the total collection of feces and n-alkanes as internal markers. Livest Sc 2006; 100:189–194.

8. Richards N, Hinch GN, Rowe JB. The effect of current grain feeding practices on hindgut starch fermentation and acidosis in the Australian racing Thoroughbred. Aust Vet J 2006; 84(11): 402-407.

9. Godoi FN, Almeida FQ, Guarienti GA, Santiago JM, Guedes Júnios D, Nogueira YC, Brasileiro LS. Blood profile and characteristics of the feces of horses fed with high fat diets. Cien Rural 2009; 39(9):2571-2577.

10. Berg EL, Fu CF, Potter JH, Kerley MS. Fructooligosaccharide supplementation in the yearling horse: effects on fecal pH, microbial content, and volatile fatty acid concentrations. J Anim Sci 2005; 83(7):1549-1553.

11. Zeyner A, Geibler C, Dittrich A. Effects of hay intake and feeding sequence on variables in faeces and faecal water (dry matter, pH value, organic acids, ammonia, buffering capacity) of horses. J Anim Physiol Anim Nutr 2004; 88(1):7-19.

12. Moreira CM. Palatabilidade e digestibilidade aparente de dietas com níveis crescentes de inclusão de polpa cítrica para equinos [Dissertação de Mestrado]. Pirassununga. Faculdade de Zootecnia e Engenharia de Alimentos, São Paulo, Brasil. 2014.

13. Statistical Analysis System - SAS. SAS OnlineDoc. Version 9.1.3. Cary: SAS Institute, 2004.

14. NRC. Nutrients requirements of domestic horses. 6.ed. Washington, D.C.: National Academy of Science 2007; 341.

15. AOAC. Official Methods of Analysis. 19. ed. Association of Official Analytical Chemistry, Arlington, VA. 2012.

16. Van Soest PJ, Robertson JP, Lewis BA. Methods for dietary fiber, neutral detergent fiber, and nonstarch polysaccharides in relation to animal nutrition. J Dairy Sci 1991;74: 3583-3597.

17. Cairns MC, Cooper JJ, Davidson HPB, Mills DS. Association in horses of orosensory characteristics of foods with their post-ingestive consequences. Animal Science 2002; 75: 257-265.

18. Tribucci AMO, Brandi RA, Balieiro JCC, Titto EAL, Bueno ICS. Palatability of horse diets containing citrus pulp (Citrus sinensis) through the preference test. Ital J Anim Sci 2013; 12(33):204-207.

19. Assis AJ, Campos JMS, Filho SCV, Queiroz AC, Lana RP, Euclydes RF, Neto JM, Magalhães ALR, Mendonça SS. Citrus pulp in diets for milking cows. 1. Intake of nutrients, milk production and composition. Rev. Bras. Zootec 2004; 33(1):242-250.

20. Müller CE, Udén P. Preference of horses for grass conserved as hay, haylage or silage. Anim Feed Sci Technol 2007; 132:66-78.

21. Weynberg SV, Sales J, Janssens GPJ. Passage rate of digesta through the equine gastrointestinal tract: A review. Livest Sci 2006; 99:3-12.

22. Gonçalves S, Leblond A, Drogoul C, Julliand V. Using feces characteristics as a criterion for the diagnosis of colic in the horse: a clinical review of 207 cases. Revue de Méd Vét 2006; 1(157) 3-10.

23. Filgueiras JM, Melo UP, Ferreira C, França AS, Shimoda E. Feces characteristics and faecal sand excretion in equine keep at pasture in Cachoeiro Itapemirim city, Espírito Santo, Brazil. Cienc Anim Bras 2009; 10(4):1200-1206.

24. Brokner C, Austbo D, Naesset JA, Knudsen KEB, Tauson AT. Equine pre-caecal and total tract digestibility of individual carbohydrate fractions and their effect on caecal pH response. Arch Anim Nutr 2012; 66(6):490-506.

25. Hoffman RM. Carbohydrate metabolism and metabolic disorders in horses. Rev Bras Zootec 2009; 38:270-276.