Residual metabolizable feed consumption as a method of comparing feed efficiency in steers fed silage and silage–grain diets

Sixty crossbred steers averaging 237±18 kg were randomly allocated in a 2×3 factorial arrangement to one of six dietary treatments and were individually fed over 105 days. Dietary treatments were 100% alfalfa (cv. Beaver) silage; 85% alfalfa silage, and 15% barley grain; 70% alfalfa silage and 30% barley grain; 100% fenugreek (Trigonella foenum graecum L.) silage; 85% fenugreek silage and 15% barley grain; 70% fenugreek silage and 30% barley grain; all on a dry matter (DM) basis. Individual animal DM intake, weight gain, and feed-to-gain ratios were measured or calculated. Residual metabolizable feed consumption (RMFC), calculated as the difference between metabolizable energy (ME) intake and the predicted ME needed for maintenance and gain, was compared to feed:gain ratios to determine their relative ability to differentiate between efficiency of utilization of the feeds. In spite of an 8% higher ME intake by steers fed alfalfa, there were no differences in DM intake, weight gain, and feed:gain ratios. However, the RMFC of steers fed fenugreek was negative compared to those fed alfalfa silage which was positive (P<0.01), indicating that the efficiency of use of ME differed between steers fed fenugreek and alfalfa. We conclude the RMFC concept can be used to detect differences in efficiency of utilization of energy in feeds.