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The aim of this study was to determine the shortest test duration necessary for the evaluation of feed efficiency traits, i.e., dry matter intake (DMI), average daily gain (ADG), mid-test metabolic body weight, residual feed intake (RFI), feed conversion ratio (DMI/ADG), and feed conversion efficiency (ADG/DMI). A total of 313 Nellore bulls with an initial age of 283 ± 23.6 days were evaluated by automated feed intake measurement. The tests were divided into six durations (15, 29, 43, 57, 71, and 84 days), with at least one body weight recording at the beginning and one at the end of each period. Residual variances were estimated per period and correlation coefficients (Pearson and Spearman) were calculated between the 5 test durations and the 84-day test. The results indicated a minimum test duration of 15 days (two weight recordings without fasting) for the measurement of mid-test metabolic body weight, a minimum of 43 days for automated DMI measurement, and a minimum of 71 days for the determination of ADG, RFI, feed conversion ratio, and feed conversion efficiency. Individual analysis of the DMI records obtained with the GrowSafe® and Intergado® automated feeders showed that a test duration of 57 days is sufficient for measurement of this trait. We therefore recommend a test duration of 71 days after a minimum adaptation period of 21 days for the determination of feed efficiency in growing Nellore cattle, with weight recordings without fasting every 15 days.

Enteric methane (CH4) emissions are a natural process in ruminants and can result in up to 12% of energy losses. Hence, decreasing enteric CH4 production constitutes an important step towards improving the feed efficiency of Brazilian cattle herds. The aim of this study was to evaluate the relationship between performance, residual feed intake (RFI), and enteric CH4 emission in growing Nellore cattle (Bos indicus). Performance, RFI and CH4 emission data were obtained from 489 animals participating in selection programs (mid-test age and body weight: 414±159 days and 356±135 kg, respectively) that were evaluated in 12 performance tests carried out in individual pens (n = 95) or collective paddocks (n = 394) equipped with electronic feed bunks. The sulfur hexafluoride tracer gas technique was used to measure daily CH4 emissions. The following variables were estimated: CH4 emission rate (g/day), residual methane emission and emission expressed per mid-test body weight, metabolic body weight, dry matter intake (CH4/DMI), average daily gain, and ingested gross energy (CH4/GE). Animals classified as negative RFI (RFI<0), i.e., more efficient animals, consumed less dry matter (P <0.0001) and emitted less g CH4/day (P = 0.0022) than positive RFI animals (RFI>0). Nonetheless, more efficient animals emitted more CH4/DMI and CH4/GE (P < 0.0001), suggesting that the difference in daily intake between animals is a determinant factor for the difference in daily enteric CH4 emissions. In addition, animals classified as negative RFI emitted less CH4 per kg mid-test weight and metabolic weight (P = 0.0096 and P = 0.0033, respectively), i.e., most efficient animals could emit less CH4 per kg of carcass. In conclusion, more efficient animals produced less methane when expressed as g/day and per kg mid-test weight than less efficient animals, suggesting lower emissions per kg of carcass produced. However, it is not possible to state that feed efficiency has a direct effect on enteric CH4 emissions since emissions per kg of consumed dry matter and the percentage of gross energy lost as CH4 are higher for more efficient animals. [ABSTRACT FROM AUTHOR]