Genetic association of net feed intake measured at two stages with insulin-like Growth Factor-I, growth and ultrasound scanned traits in Angus cattle

Net feed intake (NFI) is a measure of feed efficiency in beef cattle, calculated as the amount of feed eaten by an animal after adjusting for its growth rate and body weight. NFI was measured in weaned bulls and heifers of less than one year of age (NFI-P) and also in feedlot finishing steers mainly 18 months and older (NFI-F). Genetic parameters and their genetic relationship with insulin–like growth factor-I (IGF-I) around weaning age, growth traits and scanned traits were estimated for NFI-P and NFI-F. Estimated heritabilities for NFI-P and NFI-F were 0.41±0.05 and 0.34±0.09, respectively and the genetic correlation between the two was 0.65±0.14. Genetic correlations between NFI-P and IGF-I, ultrasound scanned subcutaneous fat depth at the rump (P8) in males and females and intramuscular fat percentage (IMF) in male and females were 0.18±0.11, 0.50±0.14, 0.49±0.10, 0.48±0.18 and 0.27±0.13, respectively. Genetic correlations between NFI-F and IGF-I, P8 in males and females and IMF in males and females were -0.14±0.18, 0.43±0.17, -0.13±0.14, 0.36±0.23 and -0.22±0.18 respectively. Both NFI-P and NFI-F had negative genetic correlation with weights at birth, 200, 400 and 600 days of age. This study showed that although the NFI measured at an early age and in the feedlot were moderately heritable, they were two different traits with varying genetic associations with growth and ultrasound scanned traits. Selecting for lower NFI at either stage will genetically decrease the ultrasound scanned fat traits in males. Selection for lower NFI-P can decrease fatness in heifers but this association was not evident with NFI-F. IGF-I, as recorded in this study, would have limited use as a genetic indicator trait for NFI in beef cattle due to genetic correlation differing in direction and of low magnitude with NFI-P and NFI-F.