Application of the slope‐ratio growth assay technique to estimate tryptophan availability in soybean meal fed to young rats

Abstract

The slope‐ratio assay for rat was used to determine whether tryptophan (Trp) availability in soybean meal (SBM) is affected by the presence of other amino acids (AAs). In a preliminary study, rats were fed graded levels of Trp‐supplemented diets to establish the Trp concentration range over which the weight gain response was linear. This range was found to be from 0.04% to 0.12% Trp. Subsequently, rats were fed basal (0.045% Trp) or Trp‐supplemented diets from three different sources: l‐Trp alone, SBM, or l‐Trp mixed with other AAs to reflect AA levels in the test SBM (AA‐mix). Weight gain in rats increased linearly with supplemental Trp intake (p < .05) for all Trp sources. Compared to the slope achieved with l‐Trp alone, the estimated availability of Trp in SBM was 84.4%, while for the AA‐mix it was 93.4%. It is evident that the 6.6% reduction in l‐Trp availability in AA‐mix is due to metabolic costs derived from excess levels of other AAs beside Trp, given that the absorption of crystalline l‐Trp in the small intestine is 100%. In conclusion, the Trp availability of SBM was estimated to be around 90.4% (i.e., 84.4/93.4 × 100) after correcting for the effects of the other AAs in SBM.

Variation in lysine, threonine, and tryptophan availability in meat and bone meal as estimated by the slope‐ratio growth assay technique in growing rats

“…While no previous report has estimated Thr availability in MBM using the growth assay, many digestibility studies have been conducted. Parsons (1986) reported similar results of Lys and methionine (Met) availability in a chick growth assay and digestibility determined in cecectomized cockerels. Moreover, using a chick growth assay and a precision‐fed cecectomized rooster assay, Wang and Parsons (1998b) reported all digestible Lys and Met in MBM were available for protein synthesis and metabolism.…”

“…Trp availabilities in MBMs were estimated using the slope-ratio technique (Kosaka & Yoshida, 1969;Rider & Perez-Maldonado, 2004;Sato et al, 1987Sato et al, , 2019. Supplemented Lys, Thr, and Trp intake (mg/ day) was used as an independent variable (X) and weight gain (g/day) assay.…”

“…were determined using the one-way ANOVA and the Tukey-Kramer HSD (honest significant difference) method. Growth rate data were subjected to regression analysis with JMP software, and Lys, Thr, and Trp availabilities in MBMs were estimated using the slope-ratio technique (Kosaka & Yoshida, 1969;Rider & Perez-Maldonado, 2004;Sato et al, 1987Sato et al, , 2019. Supplemented Lys, Thr, and Trp intake (mg/ day) was used as an independent variable (X) and weight gain (g/day)…”

“…The slope‐ratio growth assay conducted on rats has been used traditionally to estimate AA availability as a pilot indicator for pigs. To the best our knowledge, studies using the slope‐ratio assay to evaluate the nutritional value of MBM have focused mainly on the availability of Lys (Batterham & Darnell, 1986a; Batterham et al., 1979, 1986b) and methionine (Parsons, 1986; Wang & Parsons, 1998b), meaning that an evaluation of threonine (Thr) and Trp availability is highly warranted for a more‐precise nutritional evaluation of MBMs. However, there are few reports on Thr and Trp availability of MBM, and most of previous work for lysine (Lys) availability did not show the effects of the other AAs introduced by test MBMs on availability.…”

“…However, there are few reports on Thr and Trp availability of MBM, and most of previous work for lysine (Lys) availability did not show the effects of the other AAs introduced by test MBMs on availability. To this end, we analyzed here the AA compositions and variation in Lys, Thr, and Trp contents of 19 differently processed MBMs, and compared our findings with those obtained for soybean meal (SBM) and fish meal (FM) Furthermore, we estimated the availability of Lys, Thr, and Trp in six MBMs using the slope-ratio growth assay with rats, as the experimental model for pigs as previously shown by Sato, Miura, Fujieda, and Toyomizu (2019), to evaluate precise available AA contents and their variation in these MBMs.…”