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

Abstract

The effects of different level of rumen-protected L-tryptophan (RPL-T) supplementation on starch digestion, melatonin (MEL) and gastrointestinal (GI) hormones secretion in Holstein steers were evaluated. 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 body weight (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 MEL and GI tract and duodenal starch degradability. The design was 4 × 4 Latin square and the data were analyzed using the ANOVA procedure by SPSS. The D-glucose content in the RPL-T treatment groups was significantly reduced (p < 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 > 0.05) between all groups. The apparent starch disappearance rates in GI track were lower (p < 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 < 0.05) in all treatment groups compared to the control. 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.