Supplemental effects of rumen-protected L-tryptophan at various levels on starch digestion, melatonin, and gastrointestinal hormones in Holstein steers

Sang-Bum Lee, Jalil Ghassemi Nejad, Hong-Gu Lee*
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1Department of Animal Science and Technology, Sanghuh College of Life Sciences, Konkuk University, Seoul 05029, Korea, Seoul 05029, Korea.
*Corresponding Author: Hong-Gu Lee, Phone: +82-2-457-4387. E-mail:

© Copyright 2024 Korean Society of Animal Science and Technology. This is an Open-Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License ( which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.


<strong>Objective: </strong>The effects of different level of rumen-protected L-tryptophan (RPL-T) supplementation on starch digestion, melatonin and gastrointestinal (GI) hormones secretion in Holstein steers were evaluated. <strong>Methods:</strong> Four Holstein steers (201 ± 24 kg) were employed in a 4×4 Latin square design. The dietary treatments were the control (basal diet) and RPL-T groups of basal diet + 191.1 mg/kg BW, basal diet + 95.6 mg/kg BW, and basal diet + 19.1 mg/kg BW groups. Blood samples were collected to measure blood hormones on day 0, 1, 3, and 5 of the experiment to study serum melatonin (MEL) and gastrointestinal tract and duodenal starch degradability. The design was 4×4 Latin square and the data were analyzed using the ANOVA procedure by SPSS. <strong>Results: </strong>The D-glucose content in the RPL-T treatment groups was significantly reduced (p&lt;0.05) compared to the control group. The serum cholecystokinin (CCK) levels were increased in the RPL-T treatment group compared to the control group. However, there was no significant difference between all RPL-T treatment groups. The results of serum MEL were also similar to CCK results. The serum secretin levels were not significantly different (p&gt;0.05) between all groups. The apparent starch disappearance rates in GI track were lower (p&lt;0.05) in treatment groups compared with the control, and there was no significant difference between all RPL-T treatment groups. Digestion was increased (p&lt;0.05) in all treatment groups compared to the control. <strong>Conclusion: </strong>Overall, there were significant differences in starch digestibility, CCK, and MEL compared to the control group, but there were no significant differences in concentration of RPL-T. Therefore, considering the economic purpose, 19.1mg/kg BW is recommended as an appropriate level of addition to increase the productivity of beef cattle.

Keywords: Cholecystokinin; rumen protected tryptophan; ruminants; starch degradability; melatonin