Quality of South African soybean meal as a factor affecting the efficacy of exogenous protease in broiler diets

Abstract

The poultry feed industry relies heavily upon soybean meal (SBM) as a relatively affordable, high quality source of protein. Soybean meal composition and quality may vary between batches, mainly contributed to variation in production practice, cultivar and processing technique. Dietary supplementation with a mono-component, exogenous protease, has the potential to reduce feed cost, improve the health and welfare of poultry and reduce the already substantial carbon footprint of poultry production. The effects and interaction of exogenous protease in soybean-maize based diets, however, is unpredictable and cost- effective inclusion is not always realised. Matrices have to be adjusted continually to accommodate the differences in nutritional composition. Studies on the effects of exogenous protease on ileal amino acid digestibilities have yielded contradicting results. Given that the South Africa crushing industry is relatively young, having an understanding of the relationship of protease on apparent ileal amino acid digestibility (AIAAD) and SBM quality is important. However, insufficient data is available in order to more fully understand this relationship. To improve our understanding of the nutritional composition and quality of local SBM, two surveys were conducted in which SBM samples were collected from various soybean processors in South Africa. Survey 1 comprised of SBM samples which were collected from seven of the largest processors in South Africa from June to October 2020. Survey 2 comprised of SBM samples from five of the largest processors from September 2019 to March 2020. Both sets of samples were analysed for nutritional composition, including amino acid (AA) concentration and quality parameters. From samples collected for Survey 2, one low quality (LQ), one high quality (HQ) SBM sample and one over-processed sample (OP) were selected. An under processed (UP) sample was also obtained which served as a negative control. These four samples were incorporated into maize-soybean based diets, either with or without an exogenous protease and fed to broilers in a digestibility study aimed at investigating the interaction between SBM quality and protease efficacy.

In the two surveys conducted, significant differences were observed among at least one of the seven sources for Survey 1 and one of the five sources for Survey 2 in all nutritional components and quality parameters except for zinc, iron or copper. The findings highlighted substantial variability in South African SBM indicating challenges in achieving consistency in SBM composition due to various factors including cultivar, planting and harvesting techniques and origin. The significant variability observed between processors indicates that ensuring consistency in SBM composition and quality is challenging. The efficacy of protease varied across treatments containing different SBM qualities. Protease supplementation significantly increased the apparent ileal digestibility of crude protein (CP). The impact of protease on AIAAD was most pronounced in the treatment. with UP, followed by HQ, LQ, and then OP. This suggests that protease supplementation may be effective in increasing the AIAAD of some AAs in SBMs that have not undergone sufficient thermal exposure. This may also suggest that it may not always be justified to use a protease to increase the AIAAD of nutritionally important AAs, such as essential AAs, in diets containing SBM that contains insufficiently processed SBM.

With protease supplementation it may not be accurate to assume improvement in CP digestibility based on fixed matrix values recommended by the manufacturer. Nutritionists have to regularly adjust matrices according to batch and supplier. Protease supplementation led to higher ileal CP digestibility across all SBM samples, but this did not consistently translate to increased digestibility of all AAs. The varying effects of protease supplementation on AIAAD indicates an interaction between processing level and protease efficacy on AA digestibility. Protease supplementation had a greater effect on SBM with lower CP content.

Therefore, it is, incorrect to assume a linear relationship between CP digestibility and AA digestibility. The use of protease in soybean-maize diets could potentially enhance the utilization of dietary protein. However, its effectiveness varies. Further research into how protease functions on SBM would be beneficial, given that its impact on SBM of different qualities is unpredictable and challenging to quantify.