Effects of a mono-component protease in broiler diets with increasing levels of trypsin inhibitors

In nature, soybeans posses a series of secondary metabolites known as anti-nutritional factors. These compounds have rious effects on bird growth, feed efficiency, and nutrient digestibility, particularly protein digestibility. Anti-nutritional factors found most frequently in soybeans are condensed tannins, lectins, amylase inhibitors, phytate, alkaloids, saponin and, mainly, protease inhibitors, particularly Kunitz's and Bowman-Birk's factors (Hamza and Abu-Tarboush, 1998; Trugo et al., 2000; Oliva and Sampaio, 2009). This inhibitors are proteins and, as such, they are denaturalized when exposed to heat (Drew et al., 2007) during soy processing to produce roasted beans or soybean meal. The presence of these factors in soy is typically detected by the rapid urease test, which result is higher as these factors are less and less denatured (Ruiz et al., 2004). In poorly heat-treated soy products, such factors result in depressed animal growth due to inhibition of the action of pancreatic trypsin and chymotrypsin, causing pancreas hypertrophy (Carvalho et al., 2002). The addition of protease improves protein digestibility and it can complement the action of endogenous enzymes (Wiryawan and Dingle, 1999). This experiment was undertaken as an attempt to evaluate the supplementation of a mono-component protease in feeds based on soy subjected to different heat treatment times.

The experiment was carried in the metabolic room, Teaching and Research Aviary, Federal University of Rio Grande do Sul, Brazil. Four hundred (400) one-day-old Cobb male broilers were used (8 treatments x 5 repetitions x 10 birds per cage). On day one, the birds were weighed so that birds in each cage had similar weights. Sexing errors were culled at arrival. The birds were housed in metabolic cages (0.80 x 0.90 m²) and maintained under artificial lighting for 24 hours per day throughout the experimental period. On day one, the temperature was maintained at 32ºC to ben decreased at the rate of 0.5ºC per day, down to 22ºC, then remaining at this temperature until experiment completion. Baby-type drinkers and feeders were used from 1 to 14 days, then replaced to trough-type drinkers and feeders, from 15 to 28 days. Water and feed were offered ad libitum throughout the experimental period. Heat treatment (autoclave) was applied to soy in natura for 0, 105, 110 or 135 minutes (120ºC, 1.5 atmospheres), respectively, and urease activity was determined in duplicate (Table 1). Basal diets were prepared using all four autoclaved soy types, at the inclusion level of 20%, with two different enzyme supplementation levels (0 and 200 ppm). The feeds were formulated observing typical Brazilian standards, and offered in a mash form (Table 2). The overall health status and bird mortality were recorded daily, as well as maximum/minimum temperatures. Animal performance parameters (weight gain [WG], feed conversion rate [FCR] and feed intake [FI]) were recorded weekly. At 28 days of age, 320 birds (eight per cage) were selected and individually identified. These birds were euthanized in order to determine carcass yield and abdominal fat contents. In addition, pancreas and duodenum weights were also recorded. The experimental design was completely at random, with a 2 x 4 factorial arrangement (two protease levels x four urease levels).

Table 1. Urease activity values in heat-treated soybeans¹

Table 2. Composition of the experimental feeds and estimated nutrient levels

Results are shown in Table 3. The protease-supplemented birds had the best performance parameters and the highest carcass yields, with the lowest levels of abdominal fat deposition. Several authors have reported beneficial effects with the heat treatment of various grains and seeds, particularly soybeans, on poultry performance, particularly during their early life stages, due to the denaturalization of these proteins prior to digestion in the gastrointestinal tract (Gracia et al., 2003; Clarke and Wiseman, 2007). No protease x urease interaction existed, and the adverse effects on animal performance and carcass yields were gradually increased, together with urease levels. The absence of a more evident expression of enzyme supplementation on animal performance could have been due to the fact that protease inhibitors present in crude soybeans, could be only responsible for 40% bird grow depression, and pancreatic hypertrophy. This way, other substances with noxious effects on animal physiology should be considered, and their inactivation should be assured by using the appropriate heat treatments (Leeson and Atteh, 1996). It is thought that the increasing urease levels caused an increase the relative weights of both pancreas and duodenum, but protease supplementation reduced the size of these organs regardless of the urease levels (Table 3).

Table 3. Animal performance, carcass yield, and relative organ weights of birds fed growing urease levels¹, supplemented or not with protease, from 1 to 28 days of age

Feed supplementation with protease showed to be effective in improving bird performance and yield. Increasing urease levels had rious effects on performance and resulted in increased relative weights of both pancreas and duodenum.

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