4.1. Corn-DDG composition and nutritional value for growing pigs (Exp. I)
For Brazilian corn-DDG/DDGS, Corassa et al. (2017; 2019) and Risolia et al. (2019) published contents of 28.60, 31.53, and 25.41% CP and 46.84, 50.04, and 60.53% NDF, respectively. Within these ranges were the crude protein (27.3%) and NDF concentrations (55.1%) observed for the Brazilian corn-DDG used in this study. In the international literature, high variation in the chemical composition of corn-DDG has also been observed, with crude protein ranging from 23.3 to 31.2%; ether extract ranging from 1.43 to 15.08%; NDF ranging from 40.24 to 61.30%; ADF ranging from 10.32 to 26.04%; and ash ranging from 1.98 to 7.72% (Lee et al. 2013; Jacela et al. 2011; Jie et al. 2013). The chemical composition of corn-DDG has shown high variation worldwide due to factors such as grain selection and variability, industrial characteristics and technologies applied to ethanol fermentation and production, ratio of soluble added before the drying process, drying temperature and duration (Spiehs et al., 2002; Martinez-Amezcua et al., 2007a,b). The gross energy (4248 kcal/kg) determined for the corn-DDG used in this study was lower than values reported in the literature from 4780 to 5550 kcal/kg (Corassa et al., 2017; Anderson et al. 2012). The technology applied by the ethanol industry might be mainly responsible for gross energy variations due to fiber separation and/or oil extraction procedures.
Compared to Corassa et al. (2019), lower digestibility coefficients of energy were determined in this study (~ 72% versus 86%) for Brazilian corn-DDG for growing pigs. Apparent digestible, metabolizable and N-corrected metabolizable energy values of the corn-DDG determined in the current study for growing pigs were 3760 ± 152, 3772 ± 183, and 3657 ± 189 kcal/kg, respectively (Table 3). In addition to grain and industrial variations, the amount in which the test ingredient replaced the reference diet has been a variation source associated with the total-tract digestibility method applied in this study. In fact, in this study, the ingredient test represented 25% of the diet test, while Corassa et al. (2019) used ingredient test inclusion varying from 20 to 40% DDGS, showing variation associated with the ingredient test inclusion rate. Nevertheless, variations in the fiber and oil contents in DDG/DDGS might be the main source of variations affecting the use of energy and nutrients and fecal production and excreted nutrients (Urriola et al. 2010; NRC, 2012).
4.2. Effects of dietary xylanase plus β-glucanase levels on total-tract nutrient and energy digestibility and N retention in growing pigs fed diets containing corn-DDG (Exp. II)
Feeding dietary fiber to growing-finishing pigs has been linked to reduced digestibility of feed energy-providing fractions, mainly by reducing substrate-digestive enzyme interactions and increasing endogenous energy and nutrient losses (Adeola and Cowieson, 2011). Positive effects of dietary xylanase and β-glucanase addition on nutrient utilization have been reported previously (Nortey et al., 2008; Feoli et al., 2008; Emiola et al., 2009). Tsai et al. (2017) reported that a mixture of xylanase and β-glucanase in diets containing 30% DDGS increased NDF, ADF and hemicellulose degradability in pigs. In contrast, dietary xylanase plus β-glucanase addition increased NDF without affecting ADF total-tract digestibility in the current study, indicating an effect on the hemicellulose fraction of diets. The fiber in DDG/DDGS is mainly insoluble fiber containing high levels of arabinose (5.5%), xylan (8%), mannose (1.5%) and cellulose (8.5%) (Song et al., 2010; Jones et al., 2010; de Vries et al., 2013, Jakobsen et al., 2015; Urriola et al., 2010; Xu et al., 2010). In this way, reducing luminal content viscosity might be the main effect responsible for increased total-tract crude protein digestibility.
The increased total-tract digestibility coefficient of crude protein with dietary xylanase plus β-glucanase addition might be an indication of the action of xylanase and β-glucanase on hemicellulose and cellulose fractions in diets. After the degradation of cell wall structures by xylanase and β-glucanase, the protein fraction that makes up the fibrous fraction structure may become available for the action of proteases secreted by the pancreas (e.g., trypsin, chymotrypsin). In fact, urinary N excretion (Table 5) increased with dietary xylanase plus β-glucanase addition, resulting in a reduced total-tract metabolizability coefficient of crude protein (Table 4). Together, these results indicate increased digestion and absorption of dietary protein without increasing pig protein retention, resulting in increased metabolic amino acid deamination.
Increased total N excretion through higher levels of urinary N excretion indicates an improvement in protein digestibility and metabolic availability. Considering animals in a constant feeding level per kg of metabolic body weight in this study due to standard protocols, dietary standard protein and amino acid levels were probably enough to meet the protein deposition needs (NRC, 2012), exceeding amino acids to energy metabolism with urinary N excretion.
In conclusion, the values of the apparent digestible energy, apparent metabolizable energy and N-corrected apparent metabolizable energy of corn-DDG for growing pigs were 3,760 ± 152, 3,772 ± 183, and 3,657 ± 189 kcal/kg, respectively. Dietary xylanase plus β-glucanase addition in growing-pig diets containing corn-DDG increased the total-tract apparent digestibility of crude protein and neutral detergent fiber, allowing additional apparent dietary energy availability.