On the other hand, a strong correlation ( r = 0.73) was obtained between the CPR to gain ratio ( CGR) and FCR. The correlation ( r = 0.27) between RCP MA and RFI, though significantly different from zero, was not strong enough for its use as proxy for RFI.
One of the RCP traits (RCP MA) was computed by maintaining metabolic BW ( M) and average daily gain ( A) in the formula for RFI, but substituting the DMI with CPR.
Most of the carbon dioxide traits were significantly ( P < 0.05) correlated with one or both feed efficiency traits. The correlation between CPR and DMI ( r = 0.84 for heifers r = 0.83 for steers) was similar to that between CPR and BW ( r = 0.84 for heifers r = 0.87 for steers). For the combined heifer and steer datasets, the R 2 for the relationship between CPR and BW, and between CPR and DMI was 0.82 and 0.78, respectively. The relationship between CPR and DMI, and between CPR and BW was both positive and linear, for the heifers and also for the steers.
Feed efficiency traits studied included feed conversion ratio ( FCR) and residual feed intake ( RFI). Other traits studied included carbon dioxide yield ( CY CPR/DMI) and intensity ( CI CPR/BW) and 5 forms of residual carbon dioxide production ( RCP), which is a measure of actual minus predicted CPR. The corresponding mean CPR was 5760 ± 644 g/d for heifers and 8939 ± 1212 g/d for steers. The data were from 119 Angus heifers over 15 d on a roughage diet and 326 Angus steers over 70 d on a feedlot diet. In both projects, the GreenFeed Emission Monitoring system, which provides multiple short-term breath measures of carbon dioxide production, was used as a tool to measure CPR. Angus cattle from 2 beef cattle projects on which carbon dioxide production rate ( CPR) was measured were used in this study to examine the relationships among BW, DMI, and carbon dioxide traits of beef cattle fed ad libitum on a roughage diet or a grain-based feedlot diet, and to evaluate potential proxies for DMI and feed efficiency.
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