| dc.description.abstract | Cattle classified as having low residual feed intake (RFI) phenotypes are those that
consume less feed than expected based on body weight and growth performance. Mechanisms
contributing to the variation in RFI are not fully understood. Previous studies have shown that
cattle of divergent RFI phenotypes have different levels of energy expenditures, which are
associated with heat increment, basal metabolism, thermoregulation responses, and physical
activity. The objectives of this experiment were to characterize residual feed intake (RFI) in
growing heifers and to determine if variation in whole-animal energy expenditure contributes to
differences in RFI. Brangus heifers (n =120) were individually fed a roughage-based diet (1.93
Mcal ME/kg DM) diet twice daily and feed refusals measured weekly. Heifers were weighed
once weekly for 70 d and RFI calculated as the difference between actual and expected DMI from
linear regression of DMI on ADG and mid-test BW0.75. Immediately following the 70 d study,
oxygen pulse rate (mL O2/heart beat) and 48-h heart rates were measured on 8 high and 8 low
RFI heifers to estimate energy expenditure. Daily heart rates and oxygen pulse rates were higher
(P < 0.05) in heifers with high RFI compared to those with low RFI. As a result, energy
expenditure (kcal/BW0.75) was estimated to be 17.4 percent greater (P < 0.05) in high-RFI heifers then
low-RFI heifers.
Mortality and morbidity losses caused by bovine respiratory disease (BRD) continue to
negatively impact the net revenues of the beef cattle industry. Stress can predispose calves
arriving at feedlots to BRD by impairing their immune system with calves having more excitable
temperaments possibly having a greater risk. The objectives of the second study was to examine the effects of metaphylactic treatment and temperament on performance, feed intake, feed
efficiency, and feeding behavior traits in steers. Santa Gertrudis steers (n =119) were weighed
and randomly to control (CON; no antimicrobial treatment) or metaphylactic (MET; 1.5 mL/45
kg BW of ceftiofur crystalline free acid) treatments. Steers were weighed at 14-d intervals and
individual intakes and feeding behavior traits measured using a GrowSafe sytems while fed a
roughage-based diet (2.21 Mcal ME/kg DM). Objective (relative exit velocity; REV) and
subjective (chute score; CS) measurements of temperament were measured on arrival and on day
28 of the study. Steers with higher REV weighed less, grew slower, consumed less feed, spent
less time consuming feeding, had more feeding bouts per meal, had less backfat, smaller
longissimus muscle area, and higher cortisol levels. Steers treated with MET had higher ADG
than those receiving CON. Cattle with higher REV that received MET had less of a decrease in
ADG, DMI, time spend consuming feed, and less of an increase in feeding bouts compared to
high REV steers receiving CON. Results from this study suggest that process-control strategies,
which quantify and manage inter-animal variation in calf temperament may facilitate more
judicious use of antimicrobial products and provide more consistent and predictable responses to
metaphylactic strategies. | en |
