Insulin Sensitivity in Tropically Adapted Cattle With Divergent Residual Feed Intake

Residual feed intake (RFI) is one method to identify feed efficient animals; however, this method is costly and time consuming therefore, identifying an indirect measure of RFI is important. Evaluating the glucoregulatory mechanisms in cattle selected for divergent RFI may provide insight into metabolic processes involved in feed efficiency.

This study evaluated the effect of a glucose (GLUC) challenge on efficient (LRFI) and inefficient (HRFI) tropically adapted bulls and heifers. Insulin (INS) secretion was determined by radioimmunoassay (RIA) and GLUC was determined by colorimetry. Insulinogenic index (IIND) was calculated as the ratio of INS to GLUC (I/G).

Bonsmara heifers were evaluated in two experiments. Similar results were observed in both experiments. RFI affected (P < 0.05) INS response; with LRFI heifers having a greater INS response than HRFI heifers. Similarly, RFI affected (P < 0.05) IIND; with LRFI heifers having a greater IIND than HRFI heifers.

In Santa Gertrudis bulls, RFI did not affect (P > 0.05) GLUC conc. or Ins. response; however, numerically HRFI bulls had a greater INS response than LRFI bulls. RFI affected (P < 0.05) IIND; with LRFI bulls having a lower IIND than HRFI bulls.

In Brahman heifers (Exp 1), RFI did not affect (P > 0.05) GLUC concentration or INS. response; however, numerically HRFI heifers had a greater INS response than LRFI heifers. RFI affected (P < 0.05) IIND; with LRFI heifers having a lower IIND than HRFI heifers. In Brahman bulls (Exp 2), RFI affected (P > 0.05) INS response; with HRFI bulls having a greater INS response than LRFI bulls. RFI affected (P < 0.05) IIND; with LRFI bulls having a lower IIND than HRFI bulls.

Bonsmara cattle evaluated for RFI had a response to an influx of exongenous glucose that was opposite to that observed in the Brahman and Santa Gertrudis cattle evaluated for RFI. Insulinogenic index was significantly different between RFI groups in each experiment. The lower amount of INS required for clearance of the GLUC from the circulation of the Brahman and Santa Gertrudis cattle fits with our hypothesis that more efficient cattle would require less INS than the less efficient cattle. Further research and studies need to establish glucoregulatory differences between breeds and sexes of cattle evaluated for RFI.