It should be noted that the SBM used in the present study comes from the same batch with that used by Malebana et al. (2018). Although ANC has lower CP compared to SBM (Malebana et al., 2018), the CP of ANC (Table 1) is still advantageous to other conventional feed resources such as maize and sunflower meal that contain almost half or even lower CP content than the ANC. The increased fat and energy content with ANC inclusion in the diet (Table 4) is due to the fact that ANC contains higher residual oil and thus energy, but lower CP content compared to SBM (Malebana et al., 2018). Lysine is the first limiting amino acid in pigs and the ANC contains 6.8 g lysine/kg (Table 2), which is lower than 31 g lysine/kg reported in SBM by Malebana et al. (2018).
Under adequate feeding and management systems, growing pigs are expected to have an average daily gain of 640 g/d (Payne, 1990). Pigs in the present study had ADG that meets this benchmark except for those that were fed diets that contained > 15 % ANC, which were lower than this benchmark. In a study by Smit and Beltranena (2017), dietary addition of 15 % of Camelina cake (a by-product with an oil content of between 10 and 20 % and CP of > 30 %) (Pekel et al., 2015) decreased the ADFI, ADG and body weights (BW) of growing pigs. Results by Smit and Beltranena (2017) concurred with the findings of the current study when more than 15 % of ANC is added to the pig diets. Pig diets containing 15.5 % of moringa oleifera as a replacement to SBM resulted in a lower ADFI due to increased dietary fibre (Ruckli and Bee, 2016). Results in this study show that feed intake was not affected by the dietary treatments although the inclusion of ANC increased the dietary fibre in the treatments.
The lower ADG in pigs fed diets containing > 15 % ANC might be attributed to the increased dietary fibre in the treatments, which increased energy requirements for body maintenance at the expense of growth (Agyekum et al., 2015). Mthiyane and Mhlanga (2017) reported the negative effects of ANC on growth performance of broiler chickens to be related to extensive lipid peroxidation of the ANC. Feeding of oxidised lipids has been reported to reduce daily gains, which leads to poor feed efficiency. Unfortunately, lipid peroxidation of ANC was not determined in the present study. It is well documented that pigs fed high fibre diets are heavier in relation to those fed low fibre diets (e.g. Thacker and Campbell, 1999). This could be because of the increased weight of the pigs’ visceral organs and the gastro-intestinal tract (Len et al., 2008). In contrast, the final body weights of pigs fed increased dietary inclusion levels of ANC was reduced (Table 6), which might be related to the reduced CP digestibility and ADG by the pigs. According to the National Research Council (NRC, 1998), an intake of 2320 g/d is recommended for finishing pigs. Pigs in the present study had a daily feed intake (DFI) that ranged from 1500 to 1700 g/d, which is lower than the reported value. The low DFI by pigs in the present study could be attributed to the pigs’ lower initial body weights than to those reported by the NRC (1998) for finishing pigs. Reduced ADFI by pigs fed the ANC diets was expected due to increased dietary fat compared to the control. This is because pigs often reduce intake as the dietary energy concentration increases (Liu et al., 2019). Surprisingly, the ADFI was not affected by the dietary treatments.
The digestibility of nutrients in a diet is mostly affected by the composition of the nutrients as well as ANFs in the nutrients. In most cases, dietary fibre has been reported to reduce nutrient digestibility in monogastric animals (e.g. pigs). Galassi et al. (2010) reported a trend towards a reduction in nutrient digestibility by pigs with increasing dietary fibre content. In addition, Landero et al. (2011) reported a linear reduction in diet nutrient digestibility values with increasing inclusion of canola meal, which was likely attributed to increased fibre content. Reduction of nutrient digestibility in diets with increased dietary fiber in this study was only apparent with the digestibility of CP, whereby diets containing ANC had reduced CP digestibility compared to the control. Woyengo et al. (2017) reported that plant-derived dietary protein sources contain anti-nutritional factors (ANFs) such as tannins, saponins, chelating agents, protease inhibitors, and phytohaemagglutintins. These ANFs interfere with nutrient digestion, absorption, and utilization (Akande et al., 2010). Since Amarula nut cake is a plant-derived protein source, it is likely to contain ANFs, which might have interfered with the digestibility of CP by the pigs in the present study. Poor CP digestibility could affect the efficiency with which feed are converted to weight gain. Thus, the poor feed conversion ratio (FCR) in pigs fed diets with increased levels of ANC could be related to ANFs that might be present in the ANC. Wang et al. (2016) demonstrated that high residual oil in diets negatively impacts nutrient digestion and absorption, which was apparent in this study with pigs fed diets that contained high levels of ANC. The higher fibre digestibility observed for pigs fed diets containing higher levels of ANC (Table 7) could have resulted from fermentation of the fibre in the hind gut with the volatile fatty acids (VFAs) produced, contributing to the net energy requirements of the pigs (Mwesigwa et al. 2013).
High fibre diets have been reported to increase total empty weight of the gastrointestinal tract and the volume of digesta in the gut, resulting in lower carcass DP (Smit and Beltranena, 2017). Since the ANC diets contained high fibre compared with the control, it was expected that pigs fed these diets would have carcasses with a low DP. However, the DP was not affected by the dietary treatments (Table 8). Feeding diets that contain high fibre might result in reduced carcass yield in pigs compared to those fed diets containing SBM, due to reduced dietary fibre (Ruckli and Bee, 2016). Consistently, the warm and cold carcasses of the pigs in the present study were reduced with diets that contained increased inclusion levels of ANC.
Back-fat thickness is usually affected by composition of the diet (Hernandez-Lopez et al., 2016). Apple et al. (2004) reported that higher dietary energy leads to increased back-fat thickness and reduced back fat depth reflects a reduction in lipid deposition brought by decreased energy intake. Smit and Beltranena (2017) reported a decreased back fat thickness in pig carcass when Camelina cake was increased in the diet of growing pigs, likely because of feed aversion that caused reduced fat deposition. In contrast, the back-fat thickness of the carcass in the present study was not affected by the dietary treatments.
According to Kanengoni et al. (2014) drip loss in pork is affected by various and complex factors, which include, among other factors, the rate of pH decline and the ultimate pH. The ultimate pH of the pig carcasses in the present study was similar across treatments; hence, the drip loss percentage was not affected.
Data on the effects of dietary ANC inclusion on meat colour is shown in Table 9. The redness (a*) value is related to the concentration of pigments and to the pH value, while the lightness (L*) value is related to the moisture and fat contents of the carcass, and is also affected by the pH of the carcass. Increasing ANC in the diet improved both the redness and lightness of the meat, while the yellowness of the meat was not affected. The average values of the colour of the meat are consistent with the report of Temperan et al. (2014) who reported colour components (L*, a* and b*) in meat from pigs of different breeds to be in the ranges of 44–58, 5–10 and 4–9, respectively.