CHAPTER 5: GUT MORPHOLOGY AND HISTOLOGICAL PARAMETERS OF ILEAL MUCOSA IN
5.5 D ISCUSSION
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there is lymphocyte proliferation in order to kill bacteria and cause inflammation, and this increases muscularis thickness.
Intestinal surface area is a key determinant of the overall hydrolytic capacity of membrane bound digestive enzymes and absorptive capacity of the same (Zhang et al., 2015). Shorter and lighter intestines were observed under free-range rearing where birds fed mainly on fibrous material. The difference in intestine weight observed in Experiment 2 can be attributed to reduced muscularis externa thickness (Gunal et al., 2006). The morphology of the intestinal epithelium is particularly affected by both diet composition (Jiang et al., 2012) but also, the nature of the diet. A decrease in the relative length of all components of the GIT as grain particle size increased was reported by Amerah et al. (2007). Besides dietary influence on GIT development, the observed decrease in intestinal weight and/or length is thought to contribute to improved feed efficiency due to reduced maintenance cost (Xu et al., 2015). Incharoen et al. (2010) concluded that a reduction in dietary CP would also affect intestinal morphology. The diet composed by free-range chickens is likely to have been deficient in CP. The same study by Incharoen and co-workers (2010) noted an increase in duodenal and jejunal weight and length in response to a low-CP diet. This was attributed to a compensatory mechanism meant to increase absorptive capacity in an attempt to assimilate any available nutritional benefit from the hypo-protein diet. The absence of differences in AFP yield, in the current study, is consistent with findings of Santos et al. (2005).
Strain differences in final BW have been reported before (Chikumba and Chimonyo, 2014; Nthimo et al., 2004; Mikulski et al., 2011) but contradicts the findings of Zhao et al. (2015). Strain
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differences in selected parts weights e.g. liver and proventriculus were observed between 2 broiler strains (Santos et al., 2005). The observation that sex of bird influenced BW in this study agrees with earlier studies (Chabault et al., 2012; Zhao et al., 2015) where males were heavier than females. Cocks were 15 - 20 % heavier than hens at slaughter age (Mignon-Grasteau et al., 1998).
Similar sexual dimorphism was reported by Remesˇ and Sze´kely (2010). Sexual dimorphism in relation to BW can be ascribed to differences in feed intake as well as hormonal influence on tissue accretion. The observation on males yielding less abdominal fat than females was expected and is consistent with literature (Hrnčár et al., 2010). Higher fat yield in females was also observed in the study of Santos et al. (2005). Generally, females deposit more fat compared to males. Bigger organs in the heaviest bird is consistent with expectations. Gut and other organs are expected to grow proportionate to BW in order to support tissue and cellular demands for nutrients and oxygen, among other requirements.
Mean BW, heart, proventriculus, crop and intestinal weight observed in the study were higher in confined than free-range birds. Similar results on BW were observed by Wang et al. (2009) contrary to findings of Santos et al. (2005) who compared growth performance between confined and semi-confined birds. The former reported that BW and weight gain of free-range chickens were significantly lower than those of confined chickens. The differences observed may be due to the inherent variability typical of free-range systems. Birds are exposed to factors that are inherently variable including light intensity, photoperiod, and temperature (Li et al., 2016) and humidity.
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The exposure of birds to cyclic temperatures and humidity and increased exercise raises their energy requirement thus influencing their feed conversion and hence, overall growth performance (Li et al., 2016) resulting in inferior growth performance. The same was also echoed by Wang et al. (2009). In a separate study, rearing system had no effect on BW and fatty acid profile of lipids in abdominal fat (Mikulski et al., 2011). The absence of differences in gizzard weight between rearing systems was surprising. It was anticipated that free-range birds would yield heavier gizzards. Our observation contradicts that of Santos et al. (2005) where gizzard weight was higher in semi-confined compared to confined birds resulting from greater intake of fibre and grit. The difference could probably be a consequence of access to wood shavings by confined birds. The physical form of dietary structural components, such as dietary fibre, may affect the morphological and physiological characteristics, hence GIT development and function (Xu et al., 2015) and this influences the growth of chickens (Brunsgaard, 1998; Engberg et al., 2004). Also, gizzard weight was higher in males than females in the current study, contrary to findings of Santos et al. (2005).
Crop weight was significantly higher on the free-range system, a possible consequence of distension in order to increase holding capacity. Fibre digestion is slow and this may lead to diastrophy in the crop, an organ adapted for temporary feed storage. The abdominal fat yield of chickens in the free-range system was expected to be significantly lower than that of chickens raised in confinement. This was not observed in this study, contrary to observations of Li et al.
(2016) but consistent with findings of Mikulski et al. (2011). Free-range birds are exposed to variable environmental conditions which could increase the birds’ metabolic rates leading to extensive use of energy thus reduction in abdominal fat deposition. The higher muscularis externa
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thickness and aVSA in free-range birds are possible compensatory responses to hypo-caloric and hypo-protein diets, meant to increase the efficiency with which available nutrients are extracted from the lean diets. It is important to note that there are several key factors that have been inconsistent among studies on the effects of different raising systems on bird performance (Li et al., 2016) hence the need for more research in this area particularly focusing on the small intestine since changes in its fine morphology can alter absorption rate, weight gain, hence performance (Rezaian and Hamedi, 2012).