Experiment 1

A total of 14 New Zealand Red rabbits, 32 days old and with an initial weight of 649.5 ± 17.6 g were used to determine the digestibility coefficients and the nutritional and energetic value of forage cactus meal. The animals were distributed in a completely randomized design with two treatments, seven replications and one animal per experimental unit. The animals were housed in metal cages with nylon screens to facilitate the feces collection and allow the flow of urine.

The treatments consisted of a reference diet (Table 1), and another test showing 70% of the reference diet and 30% forage cactus meal. The reference feed was formulated to meet the nutritional needs for growing rabbits according to the recommendations of ^{Lebas et al. (1999)}. The diets were subsequently mixed and pelleted.

The experimental period was 12 days with seven days for adaptation of the animals to the facilities and diets, and five days for the feces collection using the total collection method.

Stool collections were carried out twice a day, once in the morning and once in the afternoon, and then individually packed in plastic bags and stored at -18°C. The feces from each experimental unit were homogenized at the end of the collection period. The material was weighed and placed in a forced ventilation oven at 55°C for 72 hours. In the next stage, the material was weighed and ground and samples of the dry material were taken to the laboratory for analysis.

The forage cactus meal, ration and fecal samples were analyzed to determine the dry matter (DM), crude protein (CP), mineral matter (MM), organic matter (OM), crude fiber (CF), detergent fiber neutral (NDF) and acid detergent fiber (ADF) values according to the AOAC (2005). The gross energy (GE) of the forage cactus meal, feed and feces was measured in a Parr model 6100 calorimetric pump.

Next, the digestibility coefficients of DM, OM, MM, CP, CF, NDF, ADF and GE were calculated based on the results of laboratory analyzes of the rations, feces and forage cactus meal, as well as the digestible energy and the respective digestible nutrients of the forage cactus meal diet using the equations proposed by ^{Matterson et al. (1965)}.

Experiment II

In order to evaluate the inclusion of different levels of forage cactus meal (0, 10, 20 and 30%), 40 male New Zealand red rabbits, 32 days old and average weight of 706.6g ± 1.65g were used. The animals were distributed in a completely randomized design with four treatments and five replications and two animals per experimental unit. The animals were housed in galvanized wire cages containing a feeder and drinker.

The percentage composition and nutritional values of the forage cactus meal were obtained in the digestibility test to formulate the experimental diets for the second test (Table 2). The diets were formulated in order to meet the nutritional requirements according to the nutritional recommendations for growing rabbits proposed by ^{Lebas (1999)}.

The diets were dry pelleted and the animals were provided with them ad libitum. The productive performance of the animals was monitored from 32 days to 67 days of age. The animals and the rations were weighed to determine the following performance variables: feed consumption, weight gain and feed conversion.

The animals were submitted to 12 hours of fasting at the end of the experimental period (at 67 days of age), being weighed again to obtain the slaughter weight that served as a reference for calculating the relative carcass weight. The slaughtering was performed by cervical dislocation followed by bleeding and evisceration.

The carcass was obtained and weighed shortly after the head and feet were removed. The relative weights of the viscera, liver and internal fat were determined after slaughter. The carcasses were cooled for 24 hours at a temperature of 1 to 2°C so that they could subsequently be subjected to quantitative evaluation according to the American NPPC method (1991).

Carcass yield (CY) was obtained using the formula: CY%=Carcassweight*100/Bodyweight ; and the cold (CCW) and hot (HCW) carcass weights. The cold (CCY) and hot (HCY) carcass yields were calculated using the following formulas: HCY=HCW/ALWx100andCCY=CCW/ALWx100 , with the animal’s live weight (ALW) being considered.

The prices of the ingredients described in Table 3 were used to check the economic feasibility of including different forage cactus meal levels in the diet of growing rabbits. These were the prices of the ingredients in the Northeast Region throughout the experimental period.

The following indices were determined for the economic viability calculations of experimental diets:

The average feed cost per kilogram of live weight (Yi) during the experimental period was calculated using the equation described by Bellaver et al. (1985):

In which:

Yi = Average feed cost per kilogram gained in the i-^th treatment;

Pi = Average price per kilogram of the feed used in the i-^th treatment;

Qi = Quantity per kilogram of the feed used in the i-^th treatment;

Gi = average weight gain of the i-^th treatment.

The economic efficiency index (EEI) and the average cost index (CI) were calculated using the adapted model by ^{Barbosa et al. (1992)}:

In which:

Mce = Lowest Yi between treatments;

CTei = Yi in the treatment considered.

The data obtained were analyzed using the Cramer Van-Misses test at 5% submitted to analysis of variance using the SAS statistical program (SAS version 9.1) and multiple linear regressions were performed up to the third degree. The treatment means were compared with the control treatment (0% FC) by the Dunnet test at 5%.

Experiment I

The nutritional composition values obtained for the forage cactus meal indicate that it has high fiber content, even so it presented high digestibility coefficients of dry and organic matter (92.48 and 91.92%, respectively), which can be related to forage cactus presenting a high content of soluble fibers and possibly due to the high fermentability of these compounds (Table 4).

The values ​​found in relation to the low crude protein content (5.63%) are within the results obtained by ^{Menezes et al. (2007)} (5.30%) and those found by ^{Lira et al. (2006)} (5.00%). Cacti have low levels of crude protein in their constitution, and require strategies for nutritional enrichment even with its adaptive characteristics, especially regarding crude protein levels (Araújo et al., 2008).

In this context, ^{Syomiti et al. (2014)} evaluated the chemical composition of the forage cactus in different parts of the plant according to the cladode maturity stage, finding that nutritional values ​​vary according to the part and degree of maturity, thus indicating that nutritional value varies according to maturity. The study in question used forage cactus meal, and it was prepared with different parts of the plant without considering the degree of maturity of the plant, which may have contributed to the high levels of CF, NDF and ADF.

There was also a reduced digestible protein content (3.18%) of the ingredient for rabbits, with this effect being possibly due to the interaction of the fiber on the protein. The protein results in less nutrient availability when it is complexed to insoluble fiber, in addition to hindering its degradation in the animal’s digestive tract. Soluble fibers also impair the digestibility of proteins because they increase the viscosity of the intestinal contents, reducing the action of proteolytic enzymes. In addition, they play a role as a substrate for microbial fermentation at the cecal level and contribute to endogenous nitrogen losses (^{Jansmann, 2002}).

Digestibility coefficients of 51.83%, 63.24% and 51.18% were respectively observed regarding the digestibility of fibrous fractions (crude fiber, neutral detergent fiber and acid detergent fiber) contained in forage cactus meal, possibly due to the presence of cellulose and lignin since these compounds are used in lower quantities by rabbits. However, it was observed that the digestibility coefficient of the fiber in acid detergent found was higher than that reported in the literature of 25.77% by ^{Tosto et al. (2007)}.

Regarding digestible energy, the forage cactus meal presented 2,347.77 kcal of DE/kg, being lower than that of corn (3260 kcal of DE/kg) and close to that of wheat bran (2190 kcal of DE/kg).

In evaluating the chemical composition of forage cactus in the diet of rabbits, ^{Ferreira et al. (2009)} found similar dry matter (90.93%), organic matter (84.28%) and acid detergent fiber (18.75%) values to those obtained in the present study, with only the crude protein (3.63%) and neutral detergent fiber (29.59%) values differing. Factors such as species, age of the plant and time of year can modify the chemical composition of the forage cactus (^{Santos, 2006}), in addition to the type of soil in the region and the variety of the forage cactus.

Experiment II

There was no effect (P>0.05) of the inclusion of forage cactus meal at the levels of 0%, 10%, 20% and 30% on the following productive performance variables: average daily feed consumption, average daily weight gain, feed conversion and final weight in diets for rabbits in the growing phase (Table 5).

The absence of influence of including the forage cactus meal on the productive performance variables suggests that the nutritional quality of the diets were maintained as the inclusion level of forage cactus meal increased, suggesting good acceptance of the ingredient in the diets by the animals.

Unlike what was observed in this study, ^{Zabut et al. (2007)} evaluated the consumption of forage cactus cladodes for rabbits, reducing the supply of pelleted feed by 20, 40 and 60%, respectively, and verified worsening in the animals’ performance as it was offered the forage cactus cladode in natura; the difference in the results found by the authors when compared to this study being that we used forage cactus meal and tried to formulate the diets so that they met the nutritional requirements so that the animals were able to maintain performance parameters.

Diets for non-ruminants present satisfactory amounts of non-starch polysaccharides in their composition which increase the viscosity of the intestinal contents with damage to the digestive and absorptive processes (^{Retore et al., 2010}). However, the inclusion of up to 30% of forage cactus meal for growing rabbits did not compromise the digestion and absorption processes of nutrients.

Similar results were observed by ^{Arruda et al. (2003)} in testing different sources of fiber (alfalfa hay or soy husk), and did not find any influence on the performance variables of growing rabbits.

In a study evaluating the performance of New Zealand White rabbits fed with alfalfa hay and manioc branch hay, the authors observed that animals which received the reference diet showed greater weight gain and better feed conversion from 35 to 50 days (^{Faria et al., 2008}).

There was no effect (P>0.05) of the inclusion of different levels of forage cactus meal in the diet of rabbits for the hot carcass weight (HCW), cold carcass weight (CCW), carcass yield (CY), viscera weight (VIW) and liver weight (LIVW) variables (Table 6).

In supplying forage cactus cladodes in natura and ad libitum to reduce the supply of pelleted feed, ^{Zabut et al. (2007)} found changes in organ weights and carcass weight. The authors justify this influence due to the metabolic process of ingesting forage cactus in natura.

Similarly, no influence of fiber sources on carcass yield was found in a study in which different fiber sources (alfalfa hay or soybean husks) were tested for growing rabbits (^{Arruda et al., 2003}).

Corroborating these results, it was observed that the substitution of 50% and 100% of the alfalfa hay with the soybean husk had no effect on the carcass yield or the liver weight of rabbits in the growing phase (^{Toledo et al., 2012}).

The nutritional role of including fiber from the forage cactus meal in the rabbits’ diet probably consisted of the fermentation process, the selective retention of particles in the cecum-colon and cecotrophagy. Such processes provide greater digestive efficiency of the fiber, use of volatile fatty acids, essential amino acids, water-soluble vitamins and certain electrolytes or minerals given the physiological processes of these animals (^{Arruda et al., 2003}).

Economic indices were not influenced (P>0.05) by including different levels of forage cactus meal in the diets of growing rabbits (Table 7). Although forage cactus meal has a lower kilogram value than corn (which was the ingredient with the greatest proportional substitution), no statistical differences were observed.