| Summary: | Feed intake, methane and feed efficiency have important genetic correlations with growth, carcase weights and mature size that need to be considered when breeding for production whilst reducing feed requirements and methane production in the Australian sheep industry. Live weight, growth, fat and muscle have significant antagonistic relationships with feed intake, which may make simultaneous selection for efficiency traits and, growth and meat quality slower. For example, selecting animals that grow faster is known to reduce meat-eating quality. Therefore, we estimated the genetic and phenotypic correlations between feed intake, residual feed intake, methane, carbon dioxide, oxygen, live weight, growth, fat and muscle depth traits. Fat and muscle depth were corrected for live weight. Traits were recorded on Merino sheep (n = 2 717) in Western Australia between 2010 and 2016. Sheep were measured at postweaning (range 753–2 717 records across traits), hogget (∼18 months old; range 602–1 046) and adult ages (>2 years old; range 269–443). Live weight and growth rate had significant moderate to high positive genetic correlations with feed intake, residual feed intake, methane and carbon dioxide at postweaning, hogget and adult ages. Fat and muscle depth measured at the start and finish of the feed intake measurement period generally had negative genetic correlations with residual feed intake, feed intake and methane. These genetic correlations with feed intake and residual feed intake were more negative with fat and muscle measured at the start of the measurement period than at the end. Furthermore, in young sheep, selecting for lower feed intake and residual feed intake will mean a lower change in fat between the start and finish of the intake period. Fat and muscle had significant correlations with feed efficiency and greenhouse gas traits and should therefore be considered when estimating residual feed intake, particularly in young animals.
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