I. INTRODUCTION
With the recognition that pelleting can enhance the feeding value of barley in poultry diets, mostly through the break-down of cell wall matrix resulting a greater accessibility of encapsulated nutrients to digestive enzymes (Abdollahi and Ravindran, 2019), barley has been successfully used in pelleted broiler diets (Perera et al., 2019b; Perera et al., 2020). While the optimum inclusion level (Perera et al., 2019b) and particle size (Perera et al., 2020) for barley in pelleted broiler diets have been evaluated, the optimum conditioning temperature (CT) for pelleting barley-based diets remains unexplored. High CT (> 80°C) are commonly employed by feed manufacturers to obtain superior pellet quality and feed hygiene. However, the resultant nutritional losses (Papadopoulos, 1989) and viscosity-induced interference to nutrient absorption (Almirall et al., 1995) due to high CT lead to impaired nutrient utilisation by birds. On the other hand, lower CT can hinder the inactivation of anti-nutritive factors and result in insufficient starch gelatinisation, protein denaturation and poor pellet quality (Abdollahi et al., 2013). As effects of CT also vary depending on the grain type (Abdollahi et al., 2010), the importance of determining the optimum CT for each grain type used is highlighted. Studies evaluating the influence of CT on barley-based diets are limited (Samarasinghe et al., 2000).
Supplementation of non-starch polysaccharide (NSP)-degrading enzymes is a routine practice in poultry diets based on viscous grains such as wheat and barley to overcome the adverse effects of NSP, mainly the high digesta viscosity in birds fed barley-based diets. As high CT during the pelleting process may exacerbate the adverse effects of viscosity, through solubilising the insoluble NSP (Cowieson et al., 2005), the use of exogenous enzymes becomes even more critical in pelleted diets. A better understanding of possible interactions between NSP-degrading enzyme and CT would allow the poultry industry to optimise the use and potential of barley in poultry diets. Accordingly, the present study was conducted to evaluate the effect of a supplemental carbohydrase (Carb), and CT on growth performance and nutrient utilisation in broilers fed barley-based starter diets.
II. MATERIALS AND METHODS
Normal-starch hulled barley (cultivar, Fortitude) was ground in a hammer mill to pass through the screen size of 8.0 mm. Nutrient composition, nitrogen (N)-corrected apparent metabolisable energy and standardised digestible amino acid contents of barley determined in a previous study (Perera et al., 2019a) were used in formulating a basal diet that was then used to develop two feed batches, without and with a supplemental Carb (Ronozyme® Multigrain; 0 and 0.15 g/kg of feed). Each diet was then divided into three equal batches and conditioned at three different temperatures (60, 74 and 88°C) by adjusting the steam flow rate. Mash diets were steam-conditioned for 30 s and the CT was measured continuously at the conditioner outlet (close to the exit point). Following conditioning, all diets were pelleted using a pellet mill equipped with a die ring with 3.0 mm holes and 35 mm thickness. The diets contained 5.0 g/kg of titanium dioxide as an indigestible marker. Each of the six dietary treatments was offered ad libitum to six replicate cages (eight birds per cage). Body weights and feed intake (FI) were recorded at weekly intervals throughout the 21-d trial. On d 21, ileal digesta were collected for determination of the coefficient of apparent ileal digestibility (CAID) of N and starch. Jejunal digesta were collected for determination of intestinal digesta viscosity.
III. RESULTS AND DISCUSSION
As there was no significant (P > 0.05) interaction between Carb and CT for any tested parameter, only the main effects of enzyme addition and CT on growth performance and nutrient utilisation are summarised in Table 1. Addition of Carb increased the weight gain (WG; P < 0.05) and reduced the feed conversion ratio (FCR; P < 0.001) by 30 g/bird and 6.5 points, respectively. Owing to the absence of Carb effect on jejunal digesta viscosity, the improvement in WG and FCR due to supplemental Carb might be attributed to the degradation of endosperm cell walls by added Carb, and possible generation of prebiotic oligosaccharides (Bedford, 2018). Birds fed diets conditioned at 60 and 74°C had similar (P > 0.05) WG, but greater (P < 0.05) than those fed the diets conditioned at 88°C. Compared to diets conditioned at 60 and 74°C, those conditioned at 88°C impaired the WG by 62 and 85 g/bird, respectively. Conditioning the diets at 88°C tended (P = 0.054) to lower the FI by 29 g/bird compared to CT at 60°C, due possibly to the slower feed passage associated with greater digesta viscosity (Almirall et al., 1995). Conditioning at 88°C increased (P < 0.05) the FCR compared to those conditioned at 60 and 74°C. Supporting the fact that elevated digesta viscosity is primarily responsible for the poorer performance of birds fed high-temperature conditioned diets (Cowieson et al., 2005), FCR of birds in the current study was impaired by 2.4 points per 0.1 cP increase in jejunal digesta viscosity in response to the increasing CT from 60 to 88°C.
It has been suggested that WG and FI responses of broilers fed diets pelleted at different CT represent the balance between the negative effect of high CT on nutrient availability and the positive effect of high CT on pellet quality (Abdollahi et al., 2013). In the current study, the superior durability of pellets conditioned at 88°C (P < 0.05) compared to those conditioned at 60°C (66.4 vs. 62.2%), however, was insufficient to overcome the adverse effects of high CT on nutrient utilisation and could not support the growth performance of birds.
No interaction between supplemental Carb and CT was observed for CAID of N or starch (Table 1). Regardless of the CT, supplemental Carb enhanced starch digestibility by 1.2%. The enhanced starch digestibility, and the lack of Carb effect on jejunal digesta viscosity, implies the action of Carb on hydrolysing the cell wall matrix (Bedford, 2018) to release encapsulated starch granules, leading to better interactions with digestive enzymes.
Diets conditioned at 88°C resulted in 1.4% lower (P < 0.05) starch digestibility than those conditioned at 60°C, due probably to the interference caused by elevated intestinal digesta viscosity and the formation of resistant starch. Digestibility of N was influenced (P < 0.001) by the CT, where diets conditioned at 88°C had 5.3% lower N digestibility compared to those conditioned at 60°C. Increasing the CT to a certain extent benefits the protein digestibility by inactivating proteinaceous enzyme inhibitors and denaturing proteins to expose more sites for enzyme attack (Abdollahi et al., 2013). However, extreme CT can reduce N digestibility by degradation of heat-labile amino acids (Papadopoulos, 1989).
Supplemental Carb and CT did not interact (P > 0.05) to influence the viscosity of jejunal digesta. Jejunal digesta viscosity was significantly (P < 0.05) influenced by the CT, as the diet conditioned at 88°C resulted in 10.1% (0.32 cP) greater digesta viscosity compared to those conditioned at 60 and 74°C. The viscosity of feed and intestinal digesta can be elevated by an increased release of encapsulated NSP, increasing solubilisation of NSP (Cowieson et al., 2005), presence of NSP with greater molecular weights due to less depolymerisation of carbohydrates (Abdollahi et al., 2013) or destruction of enzymes (Silversides and Bedford, 1999), during the application of high CT.
In conclusion, steam-conditioning barley-based diets at 88°C negatively influenced the WG, feed efficiency, and ileal digestibility of N and starch. Despite the more durable pellets obtained in diets conditioned at 88°C, feed efficiency and nutrient utilisation was severely compromised, most likely due to the increased digesta viscosity. The lack of interactions between supplemental Carb and CT indicated that the exogenous enzyme had similar efficacy at each CT in improving the WG, feed efficiency and starch digestibility in broiler starters.
Table 1 - Influence of carbohydrase enzyme addition and conditioning temperature (CT) on weight gain (WG; g/bird)1, feed intake (FI; g/bird)1 and feed conversion ratio (FCR)1, coefficient of apparent ileal digestibility (CAID)2 of nitrogen (N), starch and jejunal digesta
Presented at the 32th Annual Australian Poultry Science Symposium 2021. For information on the next edition, click here. 
