The continuous growth of the population in the world has a great challenge for the supply of proteins. According to Evaris et al. (2020), the poultry industry is the largest sector of agriculture, representing the main source of animal protein for many countries. In recent years, consumer interest in chicken meat that comes from alternative rearing systems, such as free rearing has grown. These animals, which are generally from local lines, exclusively intake food from plant origin, and the use of growth promoters is prohibited in their rearing. In this type of system, the stress of the growing birds is reduced, which contributes to improving the quality of meat (Faria et al. 2012).

The use of feed with the use of alternative raw matters is a way to adequately and economically replace the traditional foods used in poultry farming. Legumes, such as mucuna (Mucuna pruriens), could make chicken production viable in free-range rearing systems. These plants have an acceptable nutritional quality, in addition to the possibility of using different methods to reduce their antinutritional factors (ANFs) (Sarmiento-Franco et al. 2019). The objective of this research was to study digestive and carcass indicators in Rhode Island Red chickens, which intake a diet with 15 % processed Mucuna pruriens, in two rearing systems.

A total of 120 one-day-old male Rhode Island Red chickens were used. They were housed in 10 metal cages on the floor (2 m2), with a wood chip bed, with 12 birds each. The first four weeks, the animals were kept under artificial light for 24 hours. From the seventh, the birds from five cages had free access to the grazing area during the day (08:00 - 17:00 h), while the rest remained in captivity. The dimensions of the paddock were 24 m2, there where shrubs and native vegetation, typical of warm, subhumid climate conditions (AW0), in Yucatan, Mexico. The chickens were vaccinated against Newcastle, pox and Gumboro.

The experimental treatments consisted of rearing the animals in captivity and free, with grazing on native vegetation. Diet and water were supplied in an ad-libitum feeding system. Circular feeders and automatic plastic drinkers were used, respectively.

The grains of M. pruriens, ash variety, were processed according to Sarmiento-Franco et al. (2019) methodology: soaking in water for 24 h in plastic trays (1:10) (L/W) and then cooking at 100 °C for 60 min. Then, they were dried in an oven at 60 °C for 72 h and milled in a hammer mill (JF 2D, Brazil) until reaching a particle size of 3 mm. They were stored in nylon bags until their use. The chemical composition of the processed mucuna was: dry matter (DM) 90.53 %, crude protein (CP) 27.24 %, gross energy (GE) 16.45 MJ/kg, acid detergent fiber (ADF) 13.05 %, neutral detergent fiber (NDF) 27.22 %, total phenols 2.46 % and tannins 0.37 %.

The experiment was carried out for 17 weeks and was divided into stages. At the starting (one to six weeks of age), commercial balanced starter food was offered: crude protein (CP) 20 %, ether extract (EE) 2 %, crude fiber (CF) 7 %, moisture 12 %, ashes 6 % and nitrogen free extract (NFE) 53 %). From seven weeks of age and up to 14 weeks, a diet that included processed grain meal of M. pruriens was prepared. Its formulation was carried out according to the NRC (1994) requirements. Table 1 shows the chemical composition of the meal. From this stage, and up to 17 weeks of age, finished commercial balanced food was offered for chickens (CP 18 %, EE 2 %, CF 6 %, humidity 12 %, ashes 9 % and NFE 53 %).

Table 1

Chemical composition of the diet corresponding to the period from 7 to 14 weeks of age for Rhode Island Red chickens

¹Content kg^-1: vitamin A 8000 UI, D 2500 UI, E 8 UI, K 2 mg, B₁₂ 0.002 mg, riboflavin 5.5 mg, calcium pantothenate 13 mg, niacin 36 mg, choline 500 mg, folic acid 0.5 mg, thiamin 1 mg, piridoxine 2.2 mg, biotin 0.05 mg. ²Content kg^-1 of diet: manganese 65 mg, iodine 1 mg, iron 55 mg, copper 6 mg, zinc 55 mg, selenium 0.3 mg

For the analysis of the digestive and carcass indicators, a total of 30 animals of 17 weeks of age per treatment were selected. They were sacrificed by the jugular vein incision method. From each cage, three birds were random selected and the animal represented the experimental unit.

After slaughter, the abdominal cavity was opened and the gastrointestinal tract (GIT) organs were removed. The digestive content was manually removed and subsequently, the entire GIT, gizzard, empty small intestine (SI) and large intestine (LI) were weighed. Results were expressed as relative to live weight (LW, g/kg LW).

The carcass was separated from the feathers, head and legs. It was weighed on a digital scale (SARTORIUS, Germany). With these data, the yield was calculated, according to the equation:

Analysis of variance was performed according to a completely random design. The t Student’s test was used for p<0.05. The statistical package INFOSTAT (Di Rienzo et al. 2012) was applied for data analysis.

Table 2 shows the relative weights of the empty digestive organs of the GIT and those of the carcass of Rhode Island Red chickens in the two rearing systems. There were not differences between treatments for the indicators under study, except in the LI weight, which was higher for the birds that were free.

Table 2

Digestive and carcass indicators of Rhode Island Red chickens, which intake processed Mucuna pruriens, in two rearing systems

Values between columns significantly differ at p < 0.05

The higher LI weight for chickens raised free could be related to the intake of native vegetation. This has in its cell walls cellulose, non-starch polysaccharides, pectin and lignin, which are part of the fibrous fraction in vegetables. Birds have adaptive characteristics that allow them to make use of the fibrous components, digested mainly in the caecum of the animals, since the caecal flora is stimulated.

According to Elling-Staats et al. (2022), in birds there are two sac-shaped caecum, which are at the junction of the ileum with the colon. At least 13 different types of bacteria live in them: 90 % is represented by Firmicutes, Bacteroidetes and Proteobacteria. As a final product, short-chain fatty acids are generated, mainly acetate; in addition to propionate and butyrate. These contribute to cover between 11-18 % of the energy needed for avian basal metabolism. They are regulators of the immune system and cell proliferation. Butyrate also acts as an energy source for colonocytes (Williams et al. 2019). As a result, the walls weight of this section of the digestive tract eventually increases (Adebowale et al. 2019).

The rest of the relative weights of the different sections of the GIT did not vary between treatments. Akinmutimi and Okwu (2006) did not observe differences for the weights of the digestive organs, when using cooked mucuna in the finishing stage of broiler chickens. This comparison suggests that the residual FANs and the fibrous fraction of mucuna do not exert observable effects on the digestive organs, which occurred thanks to the processing to which the seed was subjected. According to Sarmiento-Franco et al. (2019), when mucuna grains are soaked in water and subsequently cooked, the concentration of L-DOPA and other toxic metabolites is reduced to 41 %.

Carcass yield did not differ between treatments. Similar results were obtained by Evaris et al. (2020), when they used Rhode Island Red chickens in a free system with corn-soybean-based diets. Free-range systems are known to improve poultry welfare (Chen et al. 2018). However, the results regarding the quality of the final products and productivity are still contradictory. Additional studies in this area are needed to obtain more details.

It is evident that, in the diets of this genetic line, the use of alternative foods, such as processed mucuna, does not have a marked influence on carcass indicators. Similar results were obtained by Faria et al. (2012) in chickens with the naked neck gene. These authors used 10 % leucaena, rice bran and sweet potato in free-range systems.

It is concluded that the free range rearing of Rhode Island Red chickens, which intake processed Mucuna pruriens, modifies the GIT section related to the digestion of the fiber that comes from the intake of native vegetation. However, this does not influence on carcass yield.

ACKNOWLEDGMENTS

Thanks to TWAS-CONACYT, Mexico, for the partial financing of the research.

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Author notes

^*Email:mademar@ica.co.cu

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