1. Introduction
It is common practice to use maize as a major source of dietary energy in poultry and human nutrition, as well as for industrial use [
1]. The multiple uses of maize create stiff competition among humans, livestock, and industry, leading to its price in the market becoming unpredictably high, and therefore becoming unsustainable for small and medium-scale poultry farmers. However, to save the collapse of the poultry sector, there is a need to search for and evaluate alternative energy sources such as sorghum.
In comparison to maize, sorghum can be grown successfully on relatively poor soils with lower moisture characteristics, such as in semi-arid environments [
2,
3]. This is due, among other things, to superior leaf physical and physiological attributes when exposed to stressful conditions [
4]. The metabolizable energy (ME) and crude protein (CP) content of sorghum are 13.4 MJ/Kg and 9.5%, respectively, compared with 14.2 MJ/Kg ME and 10.1% CP, respectively, for maize [
5]. However, the major constraint on the efficient utilisation of sorghum in poultry diets is the presence of high amounts of anti-nutritional factors (ANFs) such as tannins that depress poultry performance [
6]. Tannins present in chicken diets reduces feed intake due to reduced palatability, resulting in low live weight gain, low digestibility, and poor feed conversion efficiency [
7]. However, white sorghum varieties contain lower polyphenol concentrations, which should be advantageous as concentrations of total phenolic compounds are negatively correlated with ME:GE ratios [
8], opening an opportunity to utilize white sorghum in poultry feeding.
Sorghum varieties are continuously being improved by plant breeders and reduced tannins greatly improving nutrient digestibility for poultry [
3]. Recently varieties released in Botswana [
9] and analysed for tannins by Badubi [
10] may fit this purpose. In addition, physical, chemical and biological treatments can be used to upgrade cereals for different uses, including use as animal feed. Medugu et al. [
11] have reported that the fermentation process improves feed utilisation in poultry. A study by Fafiolu et al. [
12] concluded that up to 300 g/kg of malted sorghum sprout can be fed to growing pullets without any adverse effect. It is for these reasons that treated sorghum which, at present, has limited alternative uses, is being proposed by the present study for application in the feeding of broiler chickens. Therefore, it is imperative that, when fed to poultry, malted sorghum diets are assessed, not only for feed intake or conversion efficiency but also for carcass and meat quality. This is because the ultimate test for food animal products is their acceptability by consumers. Therefore, the present study was undertaken to evaluate the effects of malted sorghum-based diets (malted red or white grains) on the carcass characteristics and meat quality of broiler chickens.
3. Results
Table 3 shows that there were significant dietary effects on the carcass traits of the broilers with the exception of pH
U. The broilers fed the control diet had the highest (
p ˂ 0.05) slaughter, HCW, and CCW, followed by those fed MWSBD, and lastly those fed MRSBD. Higher (
p ˂ 0.05) HCY and dressing percentage were observed in broilers fed the control diet compared to those fed the malted sorghum-based diets (MSBD), which did not differ (
p > 0.05) from each other. Broilers fed MSBD had heavier (
p < 0.05) wings compared to those fed the control diet. However, those fed the control diet and MRSBD had similar (
p > 0.05) breast weights, while MRSBD also had similar (
p > 0.05) breast weights to those fed MWSBD. The broilers fed MSBD had a heavier thigh–drumstick weight (TDW) %, higher thigh–drumstick weight ratio (TDWR), and higher vertebrae weight compared to those fed the control diets. Significantly higher pH
i (
p < 0.05) of muscle was observed in the broilers fed the control diets and MWSBD compared to those fed MRSBD.
Apart from small intestine and caeca, all internal organs were affected by experimental diets (
Table 4). Chickens fed the control diet had a shorter (
p ˂ 0.05) large intestine and lighter liver weight compared to those fed malted sorghum-based diets, which did not differ (
p > 0.05) between themselves. The broilers fed MRSBD had heavier (
p ˂ 0.05) gizzards followed by those fed MWSBD, and lastly those fed the control diet. A heavier heart (
p < 0.05) was observed from the broilers fed MRSBD compared to those fed the control diet and MWSBD, with the latter two being similar (
p > 0.05).
Diet did not affect the chemical composition of the breast muscle of the broilers, except for crude fat content (
Table 5). Breast muscle from broilers fed MRSBD had the lowest (
p ˂0.05) crude fat content compared to those fed the control diet. However, there were no significant (
p > 0.05) differences observed in the crude fat content of the breast muscle of broilers fed MWSBD and the control diet.
Table 6 shows that there were significant dietary effects on the mineral concentration of breast muscle. The breast muscle of chickens fed MRSBD had a lower (
p ˂ 0.05) P content compared to those fed the control diet, but no difference in P concentration was observed between broilers fed the malted sorghum-based diets. The calcium levels in breast muscle of broilers fed MWSBD were the lowest compared to those fed the control diet and MRSBD. The broilers fed MRSBD had the highest (
p ˂ 0.05) content of Na in their breast muscle, followed by those fed MWSBD and lastly those fed the control diet. The potassium and Mg content of the breast muscle of the broilers fed malted sorghum-based diets was significantly higher compared to those fed the control diet.
4. Discussion
Research exploring the effects of malted sorghum on the carcass traits and meat quality of broilers are limited, making it difficult to compare the present results with any studies carried elsewhere. However, Adeboye et al. [
17] and Oke et al. [
18] noted a reduction in the final weight and weight gain of broiler birds and turkey poults, respectively, because of an increase in the concentration of malted sorghum in chicken diets. The reason for such poor performance was attributed to the presence of tannin and non-starch polysaccharides (NSP) in the malted sorghum sprouts [
19]. The above studies [
17,
18,
19] did not specify what type of malted sorghum meal they investigated. The present study investigated the effects of malted sorghum meal from red and white varieties on the performance of broilers.
It is known that red sorghum contains more tannin than white sorghum [
20], which may explain the current results on HCW, CCW, HCY, and dressing out percentage, whereby broilers fed on white sorghum-based diets performed better than those fed red sorghum-based diets. The reason why hot carcass weight, cold carcass weight, hot carcass yield and dressing out percentage were low in broilers fed malted sorghum-based diets could be due to the increased partitioning of energy to the gastrointestinal organs (i.e., gizzards and large intestine) and other internal organs (i.e., liver and heart), consequently increasing the heat increment and the total cost of maintenance in broilers fed these diets. These are relatively energetically active organs and their growth results in an increase in heat production, and consequently reduces the energy needed for production (NEp) [
21,
22]. However, these results contradict the findings by Yaşar et al. [
23], who observed that broiler broilers fed fermented cereals (barley, wheat and oats) produced higher carcass yields than those fed unfermented cereals.
In the present study, broilers fed malted sorghum-based diets had heavier wings, TDW, and TDWR, but it is not clear how wings, TDW, or TDWR were favoured under the present feeding regime that included malted sorghum meal. Similar reports [
24,
25] have reported higher mean wing and TDW in broilers fed fermented diets. Broilers in the current study fed malted sorghum diets had lower and/or similar breast weight, implying that there was a re-direction in the partitioning of nutrients away from the breast muscle, perhaps to other parts of the body such as the wings, thigh–drumsticks, and gastrointestinal organs.
The biological criteria for the discriminatory partitioning of nutrients to different organs are not understood in the present study. Similarly, Zhai et al. [
25] found no significant differences in the breast weight of ducks fed fermented liquor distiller’s grains. The pH
u was within a normal range (5.5–6.5) for chickens [
26], suggesting that the inclusion of malted sorghum in a broiler diet did not affect glycogen levels. The pH range is an indication of how much glycogen was in the breast muscle prior to slaughter, and how rapidly the remaining glycogen was converted to lactic acid after slaughter [
27].
It is known that feeding high fibrous diets increases the volume of GIT and organ development in chickens, ducks, and geese [
28]. This explains why broilers fed the malted sorghum diets had longer large intestines, heavier livers, gizzards and hearts. The enlargement of the organs is an adaptive mechanism to deal with high amounts of fibre [
29,
30] in a monogastric animals. The crude fibre of malted sorghum was found to be 28.6% [
31], while [
13] recorded NDF concentration in malted Segaolane to be 52.3%. These results are supported by studies in the literature [
30,
32] who reported enlarged and thicker intestines in broilers fed diets with high fibre.
The liver detoxifies any toxins in the diet, and in the present study the larger livers of broilers fed MRSBD could have been triggered by a high concentration of tannin or other anti-nutrients in the MRSBD. In contrast, Silva et al. [
33] observed higher relative liver weights when ground maize was fed compared to ground sorghum. In another study, Ahmed et al. [
34] observed that the replacement of maize with 100%, 75%, 50%, 25%, and 0% sorghum in broiler diets did not show any significant effects on liver weights. Since the study by Ahmed et al. [
34] did not specify the type of the sorghum (i.e., red or white) or processing (malting/fermenting or not), it would be difficult to tell if anti-nutrients, especially tannins, were absent or not.
In a recent study, Al Mashhadani and Al-Rubaie [
35] found that, in Ross 308 broiler chicks fed from age 0 to 10 days, raw or germinated red sorghum did not elicit a difference in performance, while at age 11 to 24 days, raw red sorghum resulted in superior body weight and weight gain compared to the germinated red sorghum diet. These conflicting results from various studies point to differences in the composition of, processing of, and rate of feeding of sorghum, as well as the rearing environment and management of broilers.
In our unpublished research [
31], it has been observed that the malting of sorghum does not completely remove condensed tannin, and hence the liver may be over-worked to detoxify phyto-chemicals [
30]. Generally, an increase in liver and heart size could be indicative of the need to deal with toxic substances in the feed. Larger gizzards could be explained by the higher structural wall components of malted sorghum meal (28.6% crude fibre reported by Moses et al. [
31] or course particles in the diet, causing the gizzard muscle to enlarge as an adaptive measure to create a suitable texture for digestion. Similar results were reported by Manyelo et al. [
36], who found heavy gizzard weights in broiler chickens fed 75% and 100% sorghum-based diets.
The similar length of the small intestine (SI) and caeca of all broilers across the experimental diets implies that the crude fibre contents may have been effectively digested prior to arriving at the SI and caeca, curtailing the accelerated development of these organs. This finding is in line with a study by Silva et al. [
33], who compared ground maize, sorghum, and whole-grain sorghum in broilers and found similar SI and caeca across the treatment diets. Similarly, the graded level of white sorghum meal did not result in any difference in the weights of the liver or intestine, or the intestinal length when included in the broiler chicken diets to replace maize [
37]. However, Manyelo et al. [
36] indicated that longer small intestines in broiler chickens are associated with a strategy to provide a greater surface area for nutrient absorption.
The excess supply of carbohydrates and proteins in the diets would be stored as crude fat in the muscle [
38]. It is, therefore, not surprising that in the present study lower crude fat was found in the breast muscle of broilers fed MRSBD. This could be due to the efficient utilization of nutrients due to the antioxidant characteristics of the diets, such as the lowering of cholesterol and other fatty acids. Indeed, Saleh et al. [
39] noted that feeding low-tannin sorghum decreased plasma triglycerides and total cholesterol concentration in broilers. Similarly, Marcinčák et al. [
40] found that the breast muscle of Cobb 500 broilers fed with cornmeal fermented with filamentous fungi
Umbelopsis isabellina had lower crude fat content when compared to the commercial diet.
Recent research [
39] investigating the effect of substitutions of yellow corn diets with low-tannin sorghum on growth performance, plasma lipid profile, and gene-expression-related growth and antioxidative properties in broiler chickens recorded positive results for the metabogenomics of chickens. The mRNAs of genes related to growth and antioxidative properties, insulin growth factor (IGF), β-actin, fatty acid synthesis (FAS), glutathione peroxidase (GPX), and superoxide dismutase (SOD) were significantly increased due to low-tannin sorghum substitutions [
38], implying health benefits to the birds. Thus, the results from the current study imply that malted sorghum-based diets reduce the fat content in chicken breast muscle, which is a good meat attribute, since the high consumption of fat is associated with nutritional diseases such as high blood pressure and obesity in consumers.
As for the other parameters, the lack of difference in the dry matter, ash, organic matter, energy, crude fibre, and crude protein content of the chicken muscle was in line with a study by Kim and Kang [
41] who investigated the effects of diets containing fermented barley or wheat on the proximate analysis of the breast meat of broilers and did not notice any differences. This implies that malting did not lower or change the quality of the meat of the broiler chickens in relation to the yellow maize diet, and thus could be used as a substitute for yellow maize in broiler diets.
The challenge of mineral nutrition in sorghum diets is unavailability due to anti-nutrients, i.e., tannin, phytic acid, oxalate, polyphenol, and trypsin inhibitors [
42]. However, as in contemporary Australian sorghum crops, Botswana white varieties such as BSH1, Mmabaitse, Segaolane, and Phofu [
9,
10] almost certainly have no or little tannin; nevertheless, the likelihood is that other “non-tannin” phenolic compounds may negatively influence energy utilization [
43] and certain minerals. The observation that malted sorghum diets had lower muscle P contents as compared to the control could be due to the lower concentration and/or supply of this mineral from the experimental diets.
The breast muscle of the broilers fed MWSBD had low Ca and Na contents, which could be due to the antagonistic relationship with other minerals or anti-nutrients such as phytic acid, leading to the poor absorption of minerals from the diet. Phytic acid was not quantified in this study, but our laboratory previously recorded 11 and 17% differences between malted and un-malted sorghum grains for tannins in red and white sorghum types, respectively [
31]. Indeed, Al-Yasiry et al. [
44] reported a relationship between fat and calcium level in the muscle. A high calcium level is usually accompanied by a reduced fat content in meat [
45].
The broilers fed MSBD had higher contents of K and Mg in their muscle, suggesting the availability of these minerals in the malted sorghum grains and the ability of the chickens to absorb and utilize them. Additionally, these results are supported by Mohammed et al. [
46], who reported that, generally, sorghum grains contain a higher concentration of K but red sorghum contains higher K than white and yellow sorghum.