Assessment of Standardised Ileal Crude Protein and Amino Acid Digestibilities in Protein Supplements for Piglets

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A cautious assessment of standardised ileal digestibilities (SID) of crude protein (CP) and amino acids (AA) as a measure to describe AA bioavailabilities is critical for evaluating the protein value of feed ingredients for piglets. Estimates of SID have been generated for most feed ingredients for grower-finisher pigs, whereas corresponding values for piglets hardly exist. Until now, SID values determined in grower-finisher pigs are used in diet formulation for piglets as well. However, there is some concern that these values may not be valid in piglets due to a limited digestive capacity for feed protein in piglets.
A guideline for the standardisation of the experimental procedure in digestibility experiments had already been proposed for grower-finisher pigs. These guidelines firstly include threshold levels for CP and AA in the assay diets to obtain so-called plateau apparent ileal digestibility values, which are independent of dietary CP and AA levels, and secondly values for IAALB. For piglets, similar guidelines, including dietary threshold levels and values for IAALB, have currently been established.
Therefore, the objective of the thesis was, based on these guidelines, to determine SID of CP and AA in 24 assay feed ingredients, including 11 products from soybean processing, 7 by-products from starch processing, 2 fish meals and 4 whey proteins. For this purpose, a total of 4 digestibility experiments with piglets fitted with simple T-cannulas at the distal ileum was conducted. For each experiment, 14 barrows were weaned at 18 days of age. Each experiment consisted of 3 periods for digesta collection. A total of 24 semisynthetic cornstarch-based diets with various inclusion levels of casein were formulated to which 1 of the 24 assay feed ingredients each was added. The daily feed allowance was restricted to 30 g/kg of individual body weight throughout all experimental periods. The SID values were determined by difference to SID values in casein.
The results of the present studies show large variations in SID values among differently processed soybean products (p<0.05). The SID values in extruded soybeans (SBe) were lower compared to high-protein soybean meal (SBMhp), soy
protein concentrate (SPC) or partially hydrolysed soy protein isolate (SPIh) (p<0.05). Aqueous alcohol or water extraction in SPC or SPIh improved SID values compared to SBMhp (p<0.05). The SID values among the 4 batches of SPC were similar (p>0.05) and consistently high. Moreover, the SID values were similar in SPC and SPIh (p>0.05). The SID values of in alternatively processed soy proteins including enzymatically fermented SPA and microbially fermented SPB, were inconsistent. For the 3 SPA products, SID increased numerically from values similar to SBMhp to values similar to SPC with increasing duration of enzymatic treatment. For SPB SID values were low and similar to SBe, which may be attributed to protein denaturation during microbial fermentation.
The results of present study show for most AA similar SID values for all by-products of starch processing including pea protein (PeaP), wheat gluten (WG), corn gluten (CG), and potato protein (PotP) compared to SPC (p>0.05) except for SID of Lys in WG and CG, and SID of Trp in CG were considerably lower compared to other products (p<0.05). The particular low SID of Lys and Trp in WG and CG might be simply a reflection of an experimental error, because SID values by means of the difference method rely on sufficient high AA contribution levels to the assay diets, which was not achieved for the aforementioned AA.
The results of the present study show that SID values in fish meal (FM1) and extracted fish meal (FM2e) vary considerably (p<0.05). The SID values in FM1 were similar to those in SPC, whereas SID of FM2e were significantly lower (p<0.05) compared to SPC. These differences may have to be attributed to different heat treatments or catching seasons of the fish.
In the present study, SID of most indispensable AA originating from sweet whey powder (SWP), whey protein concentrate (WPC1, WPC3) (SID > 90%) were consistently higher compared to SPC (p<0.05), except for SID of Phe and Thr. In contrast, consistently lower SID values for most indispensable AA were observed in the hydrolysed whey protein concentrate (WPC2h) (p<0.05). The lower SID values in WPC2h may be attributed to a relatively high amount of ether extracts in these products, which, in turn, may have coated the peptides, thus preventing them from further digestion and absorption.
The present study provides for the first time a comprehensive database on SID values for protein ingredients often used in piglet nutrition. These data allow firstly to evaluate feed ingredients for piglets in terms of their capacity to supply standardised ileal digestible AA for maintenance and growth. Secondly, the data may aid to formulate complex diets for piglets on the basis of standardised ileal digestible AA. The present study indicates that SID values in feed ingredients for piglets may be lower than tabulated values for grower-finisher pigs. However, for some batches of potato protein, fish meal, and whey protein SID values may also be higher than those tabulated values for grower-finisher pigs. Thus there may be a need for a separate digestibility table for piglets. The present database includes, however, mainly highly digestible feed ingredients with relatively high protein content. Thus, there is a need to include low-protein feed ingredients with presumed lower AA digestibilities. Moreover, the established SID values have been validated at a restricted feed intake level of 30 g/kg of body weight and should be confirmed under different feeding conditions. Finally, in terms of practical diet formulation, the additivity of the established SID values in complex diets for piglets warrants further investigations.