Introduction

The pelleting process is the agglomeration of small particles by mechanical pressure combined with moisture and heat. The advantages of this process on poultry performance are well known, but it is challenging for feed factories to produce good quality pellets. Physical quality of pellets depends on their ability to resist fragmentation and abrasion during storage and transport to the feeding troughs without disintegrating, thus preventing a high proportion of small particles called "fines" (^{Lowe, 2005}; ^{Amerah, 2007}). Chickens given pelleted diets show higher feed consumption and conversion, hence greater weight gain (^{Freitas et al., 2008}; ^{Corzo et al., 2011}; ^{Abdollahi et al., 2013}). However, if the proportion of fines is high, the benefits offered by pelleted diets can disappear when compared to mash feed (^{Klein, 1996}; ^{McKinney & Teeter, 2004}). The benefits of pelleted feeds for fast- growing broilers have been demonstrated, but research is scarce on the effects of supplying pelleted feeds to slow-growing animals, especially when dealing with performance, metabolism, and nutrient digestibility. Breeding of slow-growing chickens in alternative systems -free-range or organic- is attracting more attention from farmers, not only as they require low initial investments but also due to higher sale price for the final product (Nascimento et al., 2009). In these systems, the birds are reared in confinement during the initial phase, lasting between 28 and 42 days, and then have access to pasture areas where they reach slaughter weight at an age greater than 85 days (^{Fanatico et al., 2008}; ^{Fantico & Born, 2002}).

Slow-growing chicken strains have different growth rates, nutritional demands, and development of the digestive system compared to fast-growing strains reared industrially in total confinement (^{Santos et al., 2015}a; ^{Mendonça et al., 2008}; ^{Santos et al., 2005}). Therefore, given the particularities of these free-ranging birds, they may transform pellet feeds into body mass differently than regular commercial strains.

The objective of this study was to evaluate performance, feed digestibility, serum biochemical profile, digestive morphometry, carcass and cut yields of slow-growing broiler chickens fed pelleted feeds with different concentrations of fines.

Materials and Methods

Results

No significant interaction was observed between proportion of fines and sex for performance variables, carcass yield, digestive organ biometry, serum biochemical profile, and nutritional efficiency (Tables 2, 3, 4, 5 and 6).

Greater mean weight (MW), weight gain (WG) and better feed conversion (FConv) were observed for male compared to female birds. However, the proportion of fines did not affect chicks during the initial rearing phase (Table 2).

Relative weight of the liver was higher for females than males (Table 4). The proportion of fines did not affect morphometry of digestive organs.

No significant correlation was found between proportion of fines in feed and sex of the birds for serum cholesterol, triglycerides, and total proteins at 28 days of age (Table 5).

The proportion of fines and sex of birds did not affect the AMEn and coefficients of apparent metabolization of nutrients in broilers at 14 to 18 days of age (Table 6).

Discussion

A difference in performance between male and female broilers was expected. According to ^{Santos et al., (2005)}, growth rate of males and females of slow-growing strains is similar up to 21 days of age, and starting at 28 days male broilers grow faster, indicating greater efficiency. ^{Api et al., (2017)}, working with three fast-growing strains (Cobb, Hubbard and Ross) also observed greater development in males than females in the third week of life. According to them, animal growth is determined by growth of the nervous system, bone, muscle and adipose tissues. Since females reach puberty sooner than males, growth of the adipose tissue occurs sooner, in detriment to muscle tissue, causing lower weight gain. It is known that fast-growing broilers fed diets with high-quality pellets perform better than those receiving mash feeds due to a greater ease of feeding, besides lower waste of feed and less energy expenditure during consumption (^{Abdollahi, 2011}; ^{Corzo et al., 2011}; ^{Zohair et al., 2012}). However, that effect was not observed in the present study with Isa Label strain. Slow-growing birds have specific traits, and their slower growth means lower nutritional demands compared to commercial broiler strains, in addition to greater weight of the gastrointestinal tract and organs with less occurrence of bone problems (^{Santos et al., 2005}; ^{Mendonça et al., 2008}; Santos et al., 2015a). These peculiar characteristics influence the nutritional efficiency and performance of these strains in relation to fast-growing birds.

Regarding carcass characteristics, as verified in the present study, ^{Lara et al., (2008)}, investigating on mash and pellet feeds, found that treatments did not influence yield of cuts and carcass. Similarly, ^{López & Baião (2002)} did not find differences when comparing carcasses of broilers slaughtered at 43 days of age fed mash or pellet diets.

Typically, consumption of good-quality pellets can influence the development of digestive organs, especially the size and weight of the gizzard. The study by ^{Freitas et al. (2008)} -working with mash, pellet, and ground feeds- did not find an effect concerning the physical form of feed on the relative weight of digestive organs, except for the gizzard, which was larger in the birds receiving mash feeds. The mechanism by which the physical characteristics of the feed affect gizzard size is not entirely understood. A leading hypothesis is that lower consumption of pelleted feed in relation to mash feed can cause lower passage rate. Thus, lower muscular stimulation occurs by the mechanical action promoted by the presence of feed in this organ (^{Engberg et al., 2002}; ^{López & Baião, 2004}). Therefore, the lack of effects of feed quality on consumption in this study can explain the similar relative weight of digestive organs, especially the gizzard. On the other hand, there is evidence that, irrespective of the physical form of feed, slow-growing birds have lower carcass weight and greater relative weight of digestive organs than conventional bird strains (^{Santos et al., 2015}a; ^{Santos et al., 2005}). Studies have related greater weight of the gastric compartment (proventriculus + gizzard) to better extraction of energy from feed (^{Rougiére et al., 2009}; ^{Verdal et al., 2010}). Therefore, high relative weight of the proventriculus and gizzard in slow-growing birds can enhance the energy extraction and consequently withdraw the positive effects pelleted feed may have concerning digestive development and nutrient digestibility, as seen in this study.

We stress that the increased performance of birds associated with consumption of pelleted feeds is mainly due to the greater ability for feeding, resulting in less energy expenditure for this activity, along with improved digestibility of nutrients -like starch and proteins- as well as energy utilization (^{Moran Jr., 1987}; McCraken, 2002). These positive effects on nutrient digestibility are attributed to the mechanical action, temperature, and moisture used during the pelleting process, which among other factors help to breakdown amylose and amylopectin granules and promote changes in the tertiary structure of proteins, facilitating the enzymatic action (^{López, 2007}). Therefore, although feeds with high proportions of fines can require greater energy expenditure for consumption, they benefited from the thermal processing, which can explain the absence of effects of diets with 0, 50, and 100% fines on nutritional efficiency and overall performance observed in this study. Additionally, according to ^{Santos et al. (2015b)}, Isa Label birds are more efficient in extracting energy from cornmeal and soybean meal than birds of the Cobb 500 strain. Therefore, the results obtained in this study indicate that due to a greater nutritional efficiency of slow-growing broilers, pellet quality was not a limiting factor in terms of performance during the initial rearing phase.

^{Santos et al. (2015a)}, in a study investigating the serum biochemical constituents of slow- growing broilers aged 21 days and given mash feed, observed worse results than those obtained in the present study for cholesterol (173.63 mg dL¹), and better for triglycerides and total proteins (244.35 and 5.72 mg dL¹). Laboratory analyses of blood samples are used to monitor broiler health by enabling diagnosis and subsequent treatment of diseases. However, few studies have been published on the levels of serum constituents to establish references for industrially-bred birds. Additionally, studies presenting serum biochemical parameters for slow-growing strains are also limited. One of them was conducted by ^{Barbosa et al. (2011)}, who postulated that age, strain, nutrition, and environmental conditions can affect serum parameters. Thus, the results obtained in the present study can serve as a reference for the Isa Label strain at 28 days of age fed pelleted feeds.

In conclusion, pellet rations with high proportion of fines do not affect the performance or metabolic parameters of slow-growing birds; therefore, the use of pellet rations for these strains in the initial phase is not justified.