INTRODUCTION
Soybean meal´s good amino acids (AA) balance makes it an excellent protein source in poultry diets, although it does have the disadvantage of containing antinutritional factors such as trypsin inhibitors and lectins. These can negatively affect animal growth if the SBM is not adequately processed(1) using a proper cooking time and temperature to deactivate these factors(2). Excessive heat, however, can cause Maillard reactions, destroying thermolabile AA such as methionine and lysine, reducing nutritional value and the bioavailability of some AA (e.g. lysine)(3- 5). To control quality during soybean meal (SBM) processing, indicators such as urease activity (UA) are used. Probably the most widely used in vitro test for SBM under-processing, UA is based on pH, with a 0.05 to 0.20 unit increase in pH indicating adequate processing. Values above this range indicate under-processing and those below it overheating (4).
Wider UA value ranges (0.03 to 0.30) have been reported as indicating an adequate processing temperature(1,6,7). Nonetheless, use of SBM with a UA value higher than 0.20 units in diets for chickens is reported to decrease AA digestibility (including lysine), with higher UA values lowering AA digestibility by 91 to 92 %(4,5). This coincides with a study of UA in chicken diets containing SBM with 0.06, 0.11 or 0.19 UA units, which found lower essential and nonessential AA levels at higher UA values(8). However, in another study of SBM in chicken diets, UA values between 0.11 and 0.22 had no effect on growth(7).
Amino acid bioavailability is measured through growth tests and the slope technique. A single synthetic AA is added to a control diet deficient in this AA. The tested ingredient is then added to the control diet at one or various levels, the resulting growth response documented and compared to the response in treatments using only the crystalline AA(9-11).
The present study objective was to evaluate lysine bioavailability in SBM containing raw or thermally processed soybean hulls in diets for broiler chickens using slope comparison with a multiple linear regression.
MATERIALS AND METHODS
The experiment was done at the Center for Poultry Production Teaching, Research and Extension (Centro de Enseñanza, Investigación y Extensión en Producción Avícola - (CEIEPAv) of the Faculty of Veterinary Medicine of the National Autonomous University of Mexico (Universidad Nacional Autónoma de México - UNAM). The installations are located at 2,250 m asl in the Valley of Mexico (28°13’ N; 98°36’ W). Climate is humid temperate (Cw) with the lowest temperatures in January and the highest in May. Average annual temperature is 16 °C and average annual rainfall is 747 mm.
Experimental animals were a mixed batch of Ross strain broiler chicks (n= 210; 1 to 21 d of age) obtained from a commercial incubator, and housed in Petersime Batteries cage floors with thermostatically controlled temperature. They were distributed in 21 pens with ten animals each, half of each pen being male and the other half female. Pens were distributed randomly into seven treatments with three replicates each and ten chicks per replicate. 1) Control base diet; 2) Control + 0.05% L-lysine; 3) Control + 0.10% L-lysine; 4) Control + 0.05% lysine from SBM A (+ raw hulls); 5) Control + 0.10% lysine from SBM A (+ raw hulls); 6) Control + 0.05% lysine from SBM B (+ processed hulls); and 7) Control + 0.10% lysine from SBM B (+processed hulls).
A lysine-deficient control base diet containing sorghum, SBM and sesame paste was used as the control and all experimental diets were formulated using this diet (Table 1). Protein, amino acids(12), lysine and UA(13) analyses were done of all ingredients before base and experimental diet formulation (Table 2). The experimental diets were formulated by supplementing the base diet with crystal L-lysine (HCl), SBM A (containing raw hulls) or SBM B (containing thermally-processed hulls) in replacement of sugar. The calculations for SBM A and SBM B inclusion levels were made based on their lysine contents. Using the same source SBM (48.85 % protein, 2.98 % lysine, 0.12 UA), both the SBM A and SBM B were
formulated with 87.3 % SBM and 12.7% hulls. Protein content was 44 %, and lysine content 2.98 % in the SBM A and SBM B, but UA was determined by hull condition. The SBM A was formulated using raw hulls (10.76 % protein, 0.68 % lysine, 0.5 UA), resulting in a UA of 0.17. The SBM B was formulated using cooked hulls (10.76 % protein, 0.68 % lysine, 0.12 UA), resulting in a UA of 0.12 (Table 2).
Weight gain, feed intake and feed conversion data were collected for the experimental animals and initially analyzed using an analysis of variance (ANOVA) for completely random data (SPSS statistics package)(14). Lysine intake (mg/ chick) was calculated based on inclusion levels for the synthetic lysine and SBM A and SBM B. Weight gain and lysine intake data were analyzed with a multiple linear regression. Lysine bioavailability was estimated by comparing slopes to the multiple linear regression(9), using chick weight gain data (Y= dependent variable), the source ordinate with synthetic lysine intake (β1X1= standard curve), and a comparison with lysine intake (independent variable) in SBM A or SBM B.
RESULTS
After the 21-d feeding trial, weight gain and feed efficiency improved to an equivalent degree in the L-lysine, SBM A and SBM B treatments (Table 3). Weight gain in relation to lysine intake was explained (P< 0.049) by the regression equation Y= 375.419 + 0.0378 X1 + 0.0366 X2 + 0.0376 X3; where X1 corresponds to L-lysine supplementation, X2 to SBM A (i.e. raw hulls), and X3 to SBM B (i.e. processed hulls). Compared to the L-lysine slope (100 %), the SBM A slope indicated 97 % lysine digestibility and the SBM B slope 99 %. Clearly, under processing SBM affects lysine digestibility since values are lower as UA increases.
DISCUSSION
When soybean is processed under appropriate conditions (temperature and time) its antinutritional factors are inactivated, allowing improved growth performance in chickens(15,16). Bioassays are the best in vitro method for evaluating SBM nutritional value(8). High protease inhibitor content, particularly trypsin inhibitors, negatively affects protein and AA digestibility(3). Urease activity decreases when SBM receives proper thermal treatment. Values from 0.05 to 0.20 units increase in pH are recommended so as not to lower AA bioavailability, especially the essential AA lysine and methionine, and not to affect production yields(3-5).
Urease is an enzyme found in SBM that can cause ammonia poisoning in ruminants fed diets containing urea. Although it has no physiological function in monogastric animals, it is usually best to maintain very low UA levels in SBM used in poultry feed. Soybean processing plants in the United States use a 0.10 UA as satisfactory and safe for poultry diets. Values for UA between 0.031 and 0.088 are indicative of adequate soybean processing and that lectin levels are sufficiently low(1).
Lysine bioavailability in SBM A (0.17 UA) and SBM B (0.12 UA) treatments was lower than in the L-lysine treatment. This coincides with a study indicating that a 0.10 UA is adequate for optimum quality soybean and whole SBM, as shown in improved growth performance in broilers and AA digestibility(17).
Based on the present results, UA levels in SBM must be equal to or less than 0.12 units increase in pH to ensure greater lysine bioavailability
CONCLUSIONS AND IMPLICATIONS
The bioassay data generated here using broiler chicks and comparison of slopes to a synthetic lysine standard curve identified differences in nutritional quality for SBM with different urease activity. The SBM with the higher (0.17) urease activity had lower lysine availability which negatively impacted growth and feed efficiency in growing chickens.
