Genetic analysis of body condition and growth traits in beef females within and across ages and physiological states

This study estimated variance components of body condition and growth traits and the genetic relationships across time and traits for approximately 2,200 females from three tropically adapted northern Australian beef breeds. Body condition score, measured in yearling heifers and subsequently at the commencement of their annual mating seasons (1^st and 2^nd), was estimated to be heritable (h²: 0.32to 0.36) and with high genetic correlations (r_g) over time, ranging from 0.76 to 0.85. Hip height was also estimated to be strongly heritable at the three time points (h²: 0.59 to 0.67) and was genetically the same trait across the time points(r_g: 0.94 to 0.99). Similar results were found for live weight, with heritability estimates ranging between 0.61 and 0.65 and weight being strongly correlated across the different time points (r_g: 0.81 to 0.95). Genetic correlations between traits within the same time point showed that when cows were undergoing the fastest growth (commencement of mating 1) the genetic relationships varied compared to times points with slower growth. As yearling heifers and into mating 2 the genetic relationship between hip height and body condition score was small to moderately negative. However, at commencement of mating 1, a strong negative genetic correlation was observed. Likewise, the genetic correlation between live weight and body condition score was moderately positive, except for the commencement of mating 1, when it was not significantly different from zero. Body composition is moderately heritable but the physiological state impacts on the genetic relationships between traits, so having a clearly defined time of measurement will be essential in the trait definition.