Effects of Feed Acidification and Conditioning Temperature on Nutrient Digestibility and Performance of Broiler Starters Fed Wheat-Based Pelleted Diets

Currently, the majority of feed used in the production of broilers is fed in pelleted or crumbled form. One of the major issues in the manufacture of pellets is the application of high conditioning temperatures. The need to reduce potential levels of feed-borne pathogens such as salmonella and campylobacter for feed safety and to achieve high pellet quality has led to the application of relatively high (between 80 and 90 ºC) conditioning temperatures during conventional pelleting processes, a practice which may not favour optimal nutrient availability. However, the true impact of conditioning temperature on nutrient availability of pelleted diets has not been clearly delineated due to the confounding effects of conditioning temperature and feed form or has been ignored due to a focus on physical pellet quality and feed safety. Abdollahi et al. (2011), by differentiating the effects of conditioning temperature from feed form, showed that application of high conditioning temperatures per se adversely influenced nutrient digestibility and energy utilisation in wheat-based diets. In a recent study with maize-soybean meal diet, Loar II et al. (2014) found that, as conditioning temperature increased from 74 to 85 and 96 ºC, digestibility of some amino acids decreased by 3 to 5%, and feed per gain was impaired by 3 points (1.96 vs 1.99) and 8 points (1.96 vs 2.04), respectively.

There are several strategies which can be employed to improve the physical quality of the pellets, instead of applying high conditioning temperatures. Pre-conditioning moisture addition in the form of water and, to a lesser extent, pellet binder addition, as well as using small diameter die holes and longer pellet lengths can create high quality pellets under low conditioning temperature (Abdollahi et al., 2013). The only remaining concern is the need to eliminate salmonella in feed, which is thought to require high-temperature heat treatment.

Heat treatment is currently thought to be the most practical method to achieve satisfactory feed safety. But, considering the rising cost of feed ingredients and the negative impact of high conditioning temperature on nutrient availability and feed efficiency, there is a need to find new approaches to improving feed hygiene which are not detrimental to feed nutrients. The objective of the present study was to elucidate the influence of feed acidification and conditioning temperature on broiler growth performance and ileal nutrient digestibility.

The experimental design was a 3 × 2 factorial arrangement of treatments evaluating three inclusion levels of a feed acidifier containing formic acid and sodium formate (Amasil® NA; 0, 7.0 and 10.0 g/kg) and two conditioning temperatures (60 and 90 °C). Three basal wheat-soybean meal-based diets with the three different inclusions of acidifier were formulated to contain similar levels of apparent metabolisable energy (AME), amino acids and other nutrients. All the diets had a background of 100g BASF Natuphos E 10,000 G (hybrid 6- phytase) and 100g BASF Natugrain TS (endo-xylanase and β-glucanase). Following mixing, each diet was divided into two equal batches. One batch was steam-conditioned at 60 ºC and the other at 90 ºC, and then pelleted using a pellet mill capable of manufacturing 180 kg of feed/h and equipped with a die ring with 3-mm holes and 35-mm thickness. Conditioning time of the mash was 30 seconds and the conditioning temperature was measured at the outlet of the conditioner. All diets contained 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 were recorded at weekly intervals throughout the 21-day trial. On d 21, ileal digesta were collected for determination of apparent ileal digestibility (CAID) of dry matter (DM), nitrogen (N), starch, fat, calcium (Ca) and phosphorus (P).

The influence of acidifier inclusion and conditioning temperature on pellet durability index (PDI), growth performance and CAID of nutrients in broilers is shown in Table 1. Neither the main effects nor the interaction between acidifier addition and conditioning temperature was significant for weight gain and feed per gain (P > 0.05). There was a tendency (P = 0.066) for the acidifier to increase feed intake at 10.0 g/kg compared to diets with no acidifier. Whilst acidifier inclusion and increasing conditioning temperature both increased PDI, a significant (P < 0.001) interaction between acidifier addition and conditioning temperature also existed. While increasing conditioning temperature from 60 to 90 o C improved the PDI, regardless of the acidifier inclusion, its effect was attenuated in diets containing acidifier.

Significant main effect of acidifier was observed for DM (P < 0.001), N, starch and P (P < 0.05), and fat (P < 0.01) digestibility. The acidifier increased the CAID of DM, N, fat and P at both inclusion levels, and of starch at 10 g/kg inclusion. There was a significant (P < 0.05) effect of conditioning temperature on the CAID of DM, starch, fat and Ca, with diets conditioned at 90 o C having lower digestibility coefficients.

The use of acidifiers in poultry diets has increased over the years and their benefits on gut health (Adil et al., 2011), nutrient digestibility (Ao et al., 2009) and growth performance (Palamidi et al., 2017) have been documented. The current work also confirms the benefits in terms of enhanced nutrient digestibility, with acidification increasing the CAID of DM, N, starch, fat and P by an average of 6.6, 3.0, 1.1, 2.7 and 14.5%, respectively. It has been suggested that pH reduction in the digestive tract as a consequence of organic acid inclusion may increase the digestibility of protein, by enhancing pepsin activity and mineral absorption (Lückstädt and Mellor, 2011). It may also be speculated that, due to the shorter digesta retention time and an elevated gizzard pH, because of an under-developed gizzard in pelletfed birds (Abdollahi et al., 2013), the beneficial effects of feed acidification might be more pronounced in pellets than mash.

Feeding diets conditioned at 90 °C was associated with digestibility reduction by 3.6, 1.3, 1.9 and 36.5% for DM, starch, fat and Ca, respectively, compared to those conditioned at 60 °C. Several studies have shown the negative impacts of high conditioning temperatures on nutrient digestibility and bird performance (Raastad and Skrede, 2003; Abdollahi et al, 2010; Loar II et al., 2014). Abdollahi et al. (2011) reported decreases in starch digestibility from 0.98 in wheat-based diets conditioned at 60 ºC to 0.94 and 0.91 in diets conditioned at 75 and 90 ºC, respectively. Increasing conditioning temperatures above 60 ºC also reduced the AME of diets from 14.2 MJ/kg in diets conditioned at 60 ºC to 13.9 MJ/kg in those conditioned at 75 and 90 ºC. Overall, the current findings demonstrate that feed acidification, through inclusion of organic acids, is beneficial to nutrient digestibility in broilers fed pelleted diets, and also confirms the previously reported detrimental effects of application of high conditioning temperatures for poultry feed manufacture.