Get reliable rangeland science

In Montana, most beef cattle Bos taurus are fed hay for 3-5 months each winter, which is typically a rancher's highest variable cost. Without feeding, beef cattle may lower their metabolic rates (MRs) while grazing winter range, similar to wildlife, without impacting reproductive performance. Our objective was to determine if MRs of beef cattle track short-term step-wise decreases, then increases in feeding level. Over a 30 day period, we measured MRs (based on O2 consumed) and respiratory quotients (CO2 respired/O2 consumed) of six non-pregnant, non-lactating Black Angus cows. Initially, cows were fed chopped hay (7.5% CP) ad libitum for 16 days to determine full feed (100%). Beginning on d 0, three cows were exposed to a Moderately Restricted (M) feeding level (110%>110%>55%>110%>110%), three cows were exposed to a Restricted (R) feeding level (110%>70%>35%>70%>110%) in four-day increments. Metabolic rates of all cows were measured the morning after each four-day, step-wise decrease and increase in feeding level. Metabolic rates tracked feeding level. As expected, respiratory quotients declined as feeding levels declined indicating that cattle were catabolizing fat. Overall, these results suggest that cattle entering winter in good body condition which are not fed hay during winter, but just graze, may conserve energy by lowering their metabolic rates. Alternatively, ranchers who feed hay to their cattle during winter may be unnecessarily maintaining elevated metabolic rates at great expense.