1. Introduction
The primary aim of animal production is to provide high nutritional and dietary value of meat, which mainly depends on its chemical composition. Currently, attention is paid not only to the content of essential nutrients but also to the content of minerals and biologically active substances [
1,
2]. The quality of final products, like meat quality, in large-scale poultry production is ensured mainly through appropriate optimization of the composition of poultry diets [
3]. Addition of full-fat seeds into diets for broiler chickens can modify the content of nutrients in birds’ muscles to a certain extent, yielding a product with desired nutritional and dietary properties [
4,
5,
6]. In this respect, the seeds of local oil-bearing plants, such as flax, camelina, or sunflower, seem to be extremely valuable [
7,
8,
9].
The major focus in the literature on the supplementation of birds’ diets with flax, camelina, or sunflower seeds is placed on reduction of the ratio of n-6/n-3 fatty acids in poultry products [
6,
10,
11,
12]. Simultaneously, researchers report various effects of these seeds administered in diets on production efficiency. Rama Rao et al. [
11] and Alagawany et al. [
13] observed that administration of oil seeds contributed to higher body weight gains, reduced feed intake, and better slaughter analysis parameters.
Oilseeds also influence the digestive tract and digestion processes in poultry. This is connected with the relatively large amounts of dietary fiber in oilseeds, a significant part of which (ca. 1/3) dissolves in water, forming highly viscous solutions. The high viscosity of digesta may limit the digestion and absorption of nutrients in young birds [
14,
15,
16]. Camelina, flax, and sunflower seeds also contain anti-nutritional components: linatin, cyanogenic glycosides, phytones, trypsin inhibitors, lignans, and saponins [
17,
18]. These factors can limit the potential of these seeds to be used in poultry production to a certain extent. Such doubts were raised by Ryhänen et al. [
12] and Pekel et al. [
19]. They reported a negative effect of oilseeds in poultry diets on production effects, especially when used in high doses. They noted impaired feed conversion and decreased feed intake during the starter period.
Given such diverse information on the potential nutritional application of camelina, flax, and sunflower seeds in poultry production, it seems advisable to conduct further research in this field. Moreover, there is little information in the literature on the effect of seeds administered in feed mixes to chickens on the content of macronutrients and micronutrients in their meat.
Therefore, the aim of the study was to analyze the impact of the addition of raw full-fat camelina, flax, and sunflower seeds to diets for broiler chickens on the basic production parameters, nutrient digestibility, hematological profile, and the nutritional and dietary quality of meat and the basic nutrient and mineral profile.
4. Discussion
The inclusion of 15% of full-fat camelina, flax, and sunflower seeds to the experimental diets for broilers at 21–42 days of age resulted in an improvement of some production parameters, e.g., body weight in the finisher period of rearing, average daily feed intake, weight of breast and drumstick muscles and gizzard. This phenomenon is very interesting, especially given the significantly lower fat digestibility noted in chickens fed the experimental camelina- and flax-supplemented diets. When energy- and protein-balanced diets are provided to animals, it is difficult to obtain varied production effects expressed in basic parameters, such as body weight or daily feed intake. However, the extremely healthy fat composition of oilseeds, i.e., camelina, flax, and sunflower seeds, can probably stimulate the bird’s organism to intensified growth and development of tissues and organs [
36]. A positive effect of the use of camelina seeds in the nutrition of broiler chickens was noted by Ciurescu et al. [
3] in their investigations of broiler chickens. The highest body weight gains were recorded in birds receiving mixtures with 5% camelina addition, but the most effective reduction in the feed intake and the best slaughter parameters were observed in chickens receiving these seeds at a dose increased to 10%. In contrast, Aziza et al. [
37] administered 2.5%, 5%, and 10% of camelina meal to broiler chickens and reported no differences in the body weight gain or feed efficiency, compared with the control. In turn, as reported by Gonzalez and Leeson [
14], already a 20% dose of camelina in the diet can induce negative production effects. In the present study, the flax seeds proved to be equally beneficial in the experiments. Similarly, Apperson and Cherian [
6] reported a multidirectional positive effect of 10% and 15% addition of flax seeds to diets on the gastrointestinal health status in broiler chicken, e.g., improved intestinal morphometric parameters and reduced viscosity of feces. However, they did not detect significant differences in the average body weight, average daily gain, or feed consumption. Similarly, Konieczka et al. [
10] did not observe a significant effect of the use of flax seeds in broiler chicken nutrition on rearing parameters. However, they reported a beneficial impact of this additive on the quality of poultry meat and improvement of slaughter analysis parameters accompanied by a significant decrease in the content of abdominal fat in carcass. As demonstrated by Waititu et al. [
38] and Attia et al. [
15,
39], sunflower meal can replace up to 50% of soybean meal without depressing growth performance in either the starter or the finisher period. The authors suggest that the inclusion of sunflower meal in broiler diets does not affect productivity or barn hygiene management. Slightly different results were presented by Amerah et al. [
40], who supplemented broiler chicken diets with 5–6%, 8–10%, and 8–12% of sunflower meal. They observed no significant differences in the average body weight, feed consumption, feed efficiency mortality, and organ weight.
In the present study, the fat digestibility and energy parameters were reduced. This phenomenon was especially intensified in the groups of broiler chickens fed mixtures containing camelina and flax seed. The reduction in the efficiency of digestion of oilseed fat in the chicken intestine is associated with the high content of non-starch polysaccharides (NSPs). Flax seeds contain water-soluble and insoluble NSP forms. In these seeds, over 46% of NSP are represented by the water-soluble fraction [
41]. As reported by Rebol’e et al. [
42] and Alzueta et al. [
43], the high levels of water-soluble NSP in diets for chickens are associated with increased digesta viscosity and decreased digestibility of all nutrients, particularly fat [
40]. Whole flax seeds also contain other anti-nutritional substances, such as linamarin, mucilages, or cyanogenic glycosides, which can exert a negative effect on nutrient digestion [
10,
40]. Young birds are particularly sensitive to these factors; therefore, high doses of oilseeds are not recommended in the first period of rearing [
6,
37]. In these studies, these concerns were a basis for inclusion of full-fat oilseeds in broiler chicken diets only in the second and third rearing periods. Rodríguez et al. [
44] analyzed the effect of diets for male broilers containing 0, 80, 120, and 160 g/kg of flaxseed on the digestibility of nutrients. The authors observed a decrease in nitrogen retention and digestibility of amino acids, ether extract, and fatty acids in diets with an increasing share of flax seed. The viscosity of jejunal digesta was markedly increased by each increment of linseed in the diets. This is attributable to the presence of mucilage in linseed. Jia and Slominski [
41] used carbohydrase enzymes to increase the digestibility of flax seeds. Similar studies were conducted by Apperson and Cherian [
6], who reported increased digestibility of not only fat but also ALA and n-3 fatty acids. The enzyme supplementation also influenced the expression of genes involved in lipid metabolism.
It was found in the present study that the inclusion of oilseeds into broiler chicken feed mixtures reduced the fat content in their muscles (breast and drumstick). This may have been caused by the reduced digestibility of fat in the diet. The lower fat accumulation in the muscles was also observed by Rebolé et al. [
42] in the muscles of chickens fed with high doses of sunflower in the diet (10%, 15%, and 20%). They explained this phenomenon by an increase in the lipid oxidation rate and lower synthesis of endogenous fatty acid. As suggested by Sanz et al. [
45], the metabolic utilization of energy from fat in broiler chickens is determined by fatty acid saturation. The authors propose that circulating fat can be absorbed by muscle tissue and used as a direct source of energy rather than stored in adipose tissue. There are also other literature reports on the effect of flax in diets for poultry on the chemical composition of meat. No significant effect on the fat content in muscles was observed by Jankowski et al. [
46] in their research on turkeys fed a diet of linseed oil and by Rahimi et al. [
47] in investigations of broilers fed flax seeds. However, the authors emphasize the beneficial dietary modifications of fat quality, i.e., a significant increase in the concentrations of n-3 polyunsaturated fatty acids (PUFAs). Similarly, in the present study, a beneficial effect of the nutritional use of oilseeds on the dietary properties of poultry meat was found. The breast and drumstick muscles of broiler chickens fed with the mixtures containing camelina, flax, and sunflower seeds exhibited significantly lower fat and energy contents than the control birds. Reduction of the calorie content in meat is desirable in terms of the proper composition of a human diet and is being increasingly expected by consumers.
There is no information in the literature about the impact of nutrition supplemented with full-fat seeds on the mineral composition in chicken muscles. The higher Fe concentrations detected in the muscles of the FLA broiler chickens can be associated with the high blood level of the element, which to some extent reflects its concentration in the organism. The level of iron in the body largely depends on the diet and gastrointestinal absorption [
48]. In the present study, the content of the other minerals in the breast and drumstick muscles did not differ significantly, regardless of the energy source in the diet (camelina, flax, and sunflower seeds).
The present study also involved analysis of the relationships between basic nutrients and mineral elements in the oilseeds in mixtures for broilers and their breast and drumstick muscles. The values and trends of the correlation coefficients obtained for the components are probably associated with their physicochemical properties, antagonistic or synergistic interactions, bioavailability, as well as coexistence and co-involvement in metabolic and physiological processes with other ingredients.
Hematological indices are useful in assessment of the potential toxicity of feed, especially in the case of feed components that can affect blood parameters and animal health. The values of the analyzed blood indicators corresponded to the reference values specified for this species [
49,
50]. Similarly, other authors reported a negative effect of the addition of flax seeds into the diet on the hematological indicators in animals. Investigations consisting in the supplementation of diets for rabbits with two flaxseed varieties demonstrated a decline in the values of RBC, HgB, MPV, and PCV in the blood of the animals [
51]. Rajesha et al. [
52] reported reduced PCV and RBC levels in the blood of hens correlated with the level of flax seeds in the diet. However, they did not observe an effect of the flax seeds on the HgB level.
In the present study, the reduction in the HgB level induced by the FLA treatment may have been associated with the content of cyanogenic glycosides (linamarins, linustatins, and neolinustatin) and enzymes (β-bis-glucosidase, β-monoglucosidase, and α-hydroxynitrile lyase) involved in the hydrolysis of cyanogenic glycosides and release of hydrocyanic acid [
53]. Hydrogen cyanide is toxic to animal organisms, as it contributes to inhibition of cytochrome oxidase activity in the mitochondrial respiratory chain. It is also an effective inhibitor of other enzymes due to its ability to bind with iron, manganese, or copper ions, which are functional groups of many enzymes. Hydrogen cyanide can also bind to hemoglobin to form cyanohemoglobin, which does not dissociate to HgB [
54,
55].
Blood serum and plasma often serve as a biological matrix for assessment of the proper supply of elements and their transformations in the organism. Their content in the body is a result of a dynamic balance of the demand, supplied quantity, and availability [
56,
57]. The concentration of iron in blood depends on, e.g., its content in the diet, gastrointestinal absorption, and the intensity of hemoglobin breakdown and synthesis [
48]. Probably, the presence of cyanogenic glycosides in flax seeds and their effect on the formation of cyanomethemoglobin mobilizes iron stores, increasing its level in the blood plasma.
Oilseeds contain a substantial amount of minerals, in particular phosphorus, magnesium, iron, zinc, and calcium, which, however, are characterized by reduced bioavailability. These seeds also contain substances with anti-nutritional properties, which may exert a negative effect on the organism, e.g., by limitation of the absorption of nutrients or even toxic effects. The main anti-nutrients found in oilseeds are phytic acid, tannins, cyanogenic glycosides in flax seeds, as well as glucosinolates and sinapine mainly in seeds of
Brassicaceae plants [
58,
59]. The available literature presents the results of numerous nutritional studies suggesting poor bioavailability of such minerals as phosphorus, zinc, calcium, magnesium, or even iron from phytic acid-rich diets [
60,
61]. The content of phytic acid in the oilseeds was not analyzed in the present study. However, there are many reports confirming considerable amounts of this compound, especially in camelina seeds [
58,
62], which may explain the low plasma magnesium level in the broiler chickens fed the diet with camelina seeds.