Study area

This study was from July 2016 to May 2017 conducted, in the experimental unit of the Academic Body "Sustainable Tropical Animal Production" of the Autonomous University of Chiapas, located in the Ejido Loma Bonita of Tuxtla Gutiérrez municipality, Chiapas. It is located at geographical coordinates 19° 8.64´N and 98° 16.55´W, at an altitude of 522 m a.s.l. The region has a warm subhumid climate with rains in summer Aw2 (^{García, 2004}). The mean annual temperature and total annual precipitation vary between 20-28 ° C and 800-1200 mm, respectively (^{INEGI, 2017}). This study was carried out in accordance with the standards for the use of research animals of the Faculty of Veterinary Medicine and Zootechnics of the Autonomous University of Chiapas and in accordance with the Official Mexican Standards NOM-024-ZOO-1995 and NOM-033 -ZOO-1995 (NOM-024-ZOO-1995; NOM-033-ZOO-1995).

Experimental animals

The animals used in this study came from eggs from a flock of 209 guinea fowls in the laying stage, which were under natural environmental conditions kept. Egg collection was carried out in the first hours of the day for a week, recording the collection date, the egg weight (g) and the total number of eggs collected/day. The eggs were in special 30 x 30 cm containers collected, previously disinfected, and they were stored at room temperature. A total of 200 eggs with an average weight of 38.5 g were artificially incubated for 25 days using a Texotronics^® incubator (model CM108V1, Mexico), adjusted to a temperature and relative humidity range of 37.5-37.8 ºC and 70-98%, respectively (^{Eleroğlu et al., 2016}), managing to obtain a population of 120 chicks.

Evaluation of productive behavior

For this phase of the study, 100 chicks were without considering sex selected, due to the difficulty in sexing caused by the sexual monomorphism of Guinea fowls in the first weeks of age (^{Abdul-Rahman et al., 2015}). Subsequently, they were randomly into 10 groups divided with 10 individuals (repetitions) each. Each chick was with a tag attached to the wing identified. The birds were in conventional 2x1 m pens housed, made with chicken mesh, concrete floor and chip bed. Rustic drinkers and feeders made with polyvinyl chloride (PVC) tubes were used. From week 0 to 3 they were offered commercial chicken feed with a crude protein content of 21%, while from week 4 to 14 the commercial feed offered had a crude protein content of 13% (National Research Council, 1994). Both food and water were offered ad libitum throughout the experimental period. Every 15 days the body weight (BW) of the birds was using a Medidata^® electronic scale measured (model PS-5, Mexico) to determine the weight gain (WG) using the following formula:

WG = BW final (g) , BW initial (g) 

The feed consumption (FC) was also determined for each group using the following formula:

FC=food of fered-food refusednumber of birds

Likewise, the feed conversion efficiency (FCO) was determined with the formula used by ^{Sebola et al. (2015)}, which is shown below:

FCO=food consumtionweigh gain

Determination of carcass traits

This phase of the study was carried out in the Animal Nutrition Laboratory of the Postgraduate College, Montecillo Campus, Texcoco, Mexico. To evaluate the characteristics of the carcass, an individual from each group was randomly selected, considering five females and five males previously sexed through visual and morphometric evaluation (^{Arhin et al., 2018}). The BW of each bird was measured and later they were slaughtered in a conventional manner (stunning, bleeding, plucking and evisceration). Carcass yield (CY) was determined through the relationship of the weight of the hot carcass (gutted and without considering the head, neck and legs) and the WB of the animal*100 (^{Barbosa-Filho et al., 2017}). To determine the water retention capacity (WRC), pH and color, the methodologies suggested by ^{Guerrero et al. (2002)}, which are described below:

To evaluate the WRC two samples of 5 g/bird of the pectoralis major muscle were used. Each sample was placed in a special mini blender, 8 mL of cold 0.6 M NaCl solution was added and subsequently they were ground for 30 s. The obtained mixtures were transferred to centrifuge tubes and they were placed in an ice bath for 30 min, shaking them periodically every 10 min. All tubes were then centrifuged for 15 min at a speed of 11,200 × g and 4 °C using a Beckman Coulter^® centrifuge (Avanti J-E model, Jersey City, CA). The supernatant was decanted and measured using a 10 mL graduated cylinder. The amount of mL of solution retained in 100 g of sample was recorded to determine the WRC using the following formula:

WRC=Va-Vssample weightx100

Where: Va is the volume of saline added to the centrifuge tube; Vs: is the volume of the supernatant.

The pH was determined with a penetration electrode in samples of the pectoralis major muscle immediately after the slaughter of the birds and 24 h later (postmortem) using a portable Hanna^® potentiometer (model HI 99163, Bogotá, Colombia).

Color was measured 4 h after sacrifice in pectoralis major muscle samples approximately 1 cm² thick using a Konica Minolta^® brand colorimetry meter (model CR-200, Osaka, Japan). The samples were to light for 30 min before the readings exposed. The values of luminosity (L*), redness (between green and red; a*) and yellowness (between blue and yellow; b*) were recorded in the CIE Lab system. The average values of L*, a* and b* were calculated from three readings in different positions considering the average of the readings for the statistical analyzes.

The analysis of the proximal composition of the meat was carried out considering the analytical methods of the ^{AOAC (2000)}, determining the humidity and dry matter by the gravimetric method by drying in an oven at 110 °C for 24 h. Also was determined the ash content through from the oxidation of organic matter by incineration, the total protein by nitrogen determination using the micro Kjeldahl method and the ethereal extract by extraction with Soxhlet-type solvents. All analysis were done in duplicate.

Statistical analysis

All statistical analysis were performed using the SAS program, see 9.4 (^{SAS, 2016}). The data set was analyzed by descriptive statistics using the means procedure (PROC MEANS). To differentiate between sexes, the data set of the variables on the carcass traits and proximal composition of the meat, the generalized linear model procedure (PROC GLM) was used, the model used was:

𝑌_𝑖𝑗 = 𝜇 + 𝑆_𝑖 + 𝑖𝑗

Where: Yij refers to the observation of animal j and treatment i for each of the dependent variables; µ is the general mean; Si is a fixed effect due to the sex of the guineas; and ij is the random residual effect associated with observation ij. When significant differences (P <0.05) appeared when performing the F-test (ANOVA), the Tukey test was performed to differentiate the means from each other.

Characterization of productive behavior

The results of the productive indicators of the growth of the guinea fowl are shown in table 1. Each bird consumed daily 62.04±2.48 g of commercial feed, achieving an average final weight at week 14 of 1161.56±94.82 g, with a minimum and maximum of 905 and 1365 g, respectively. A daily weight gain of 10.98±0.95 g/bird was obtained. These data were superior to those reported by ^{Dahouda et al. (2008)}, who showed that guinea fowl reared under intensive conditions have an average feed intake of 41.8 g/day/bird in month 4 of age, a body weight of 831±141.5 and 846±146.7 g in females and males, respectively, and a daily weight gain of 4.16 g/bird. The variation in the data could be due to the type of production system used in the research, since the animals confined in very intensive conditions have higher levels of stress, which causes a lower consumption of feed and with it a lower gain in body weight ^{(Lara and Rostagno, 2013)} .

In the study, an efficiency of feed conversion of 5.65 ± 0.57 g was obtained. In this regard, ^{Seabo et al. (2011)} showed that this parameter improves by increasing the levels of crude protein in the diets of guinea fowl, since they determined a feed conversion of 6.71, 6.37 and 6.23 g by increasing the level of crude protein by 14, 16 and 18%, respectively. In Creole chickens, a feed conversion of 3.41±0.27 and 3.34±0.25 g has been reported in females and males, respectively ^{(Paredes et al., 2019)}. ^{Rezaei et al., (2018)} point out that the main factor of variation of feed conversion in poultry is the type of breed, line or genetic line used.

The growth curve of the guinea fowl during the first 14 weeks is in figure 1 shown. A slight decrease in weight gain can be between week 4 and 6 of age observed, possibly due to the adaptation stage caused by the change of feed offered to the birds, which contained a lower percentage of crude protein. The data obtained were superior to those found by ^{Houndonougbo et al. (2017)} when evaluating the growth of five genetic varieties of guinea fowl, since they found a range of variation between body weights at week 16 of 876.70±36.10 g to 965.00±22.00 g of the varieties known in the study as "black" and "common", respectively. However, they were lower than the reported ones by ^{Nahashon et al. (2006)} in French guinea fowls for 8-week-old females (1138.9 ± 118.9 g) and males (1145.6 ± 114.7 g). According to ^{Seabo et al. (2011)}, the variation in the growth of Guineas is mainly due to factors associated with nutrition.

Figure 1 Growth curve of the guinea fowl raised under tropical conditions in Mexico 

Another of the associated factors is the genetics of the birds, since the body weight in poultry is moderately to slightly heritable, which would imply that the selection of heavier individuals in a population of guinea fowl, should result in a genetic improvement of the trait (^{Oke et al., 2004}). However, environmental conditions can affect or improve growth ^{(Porter et al., 2010)}, since in production systems the increase in body weight is of economic importance (^{Aggrey, 2009}).

Analysis of carcass traits

No statistical differences (P> 0.05) were found in carcass yield due to the effect of sex; however, it was higher in males (79.3%) (Table 2). These results are consistent with those reported by ^{Houndonougbo et al. (2017)} in different varieties of guinea fowl: gray (85.1%), black (79.2%), bonaparte (78.5%), white (78.2%) and common (77.4%). For their part, ^{Mareko et al. (2006)} found carcass yields> 90% in African guinea fowls at different ages. Consumer demand for poultry meat is on high-weight carcass traits focused, with the muscle pectoralis major or commonly known as the breast being the most important (^{Faria et al., 2010}). In this study, sex had no effect (P> 0.05) on any of the traits evaluated in the pectoralis major muscle of the guinea fowl (Table 2). A water retention capacity of 14.46 and 14.47% was obtained for females and males, respectively.

The pH is an important factor that affects the quality of meat, for example, a high pH shortens the shelf life of meat as it creates a more favorable environment for bacteria ^{(Sarica et al., 2019)}. In the present study, a pH of 6.04 and 6.03 was in females and males, respectively found. These results are similar to those found by Sarica et al. (2019) in the pectoralis major muscle of 14-week-old guinea fowls (6.56-6.79), also reported that the type of production system, age at slaughter and sex have an effect on this parameter. Another important factor that affects the sensory quality of poultry meat is color, since consumers associate this characteristic with the freshness of the meat. In general, primary pigments consisting of myoglobin, hemoglobin, and cytochrome C ^{(Boz et al., 2019)} control the surface colors of meat. The values of luminosity (44.40 and 45.04), redness (15.58 and 14.95) and yellowness (7.51 and 7.11) found in this study for females and males, respectively, are similar to those reported by ^{Tufarelli et al., (2015)} in hens from Guinea 12 weeks old. ^{Sarica et al. (2019)} found that the yellowness values of Guineas breast were significantly affected by sex, with higher values for females than for males (8.21 vs 5.93), for which they attributed this difference to the higher fat content that females have. The color values obtained in the present study are consistent with the demands of consumers, who prefer white meat, particularly breast (^{Sarica et al., 2019}).

Proximal meat composition

The results of the proximal analysis of the pectoralis major muscle or breast of the guinea fowl are shown in table 3. None of the evaluated nutrients presented statistical differences (P> 0.05) due to the effect of sex. The results of the moisture content (45.33 and 49.18%), crude protein (19.95 and 19.98%) and ether extract (2.59 and 2.61%) obtained in this study for females and males. Respectively, they were different from those reported by ^{Premavalli et al. (2015)} on meat from the breast of guinea fowl raised under traditional management in India; these authors also found that the value of these nutrients is significantly by the age of the birds affected. For their part, ^{Musundire et al. (2017)} found that the species (Guinea fowls vs chickens) and age of the birds had a significant effect on the content of dry matter and crude protein in breast meat samples, with higher values for Guineas. Likewise, they found that the content of the ether extract was affected by species, age and sex, while the ash content differed with the age of the birds. In the literature, it has been reported that the Guineas fowls´ breast cooking method has a significant effect on the moisture, protein and ash content ^{(Hoffman and Tlhong, 2012)} .

CONCLUSION

Guinea fowl production is an alternative meat supply being a protein source of animal origin, with carcass yields superior to other domestic birds.