Heterogeneity of variance for milk, fat and protein yield in small cattle populations: The Rendena breed as a case study

The aim of this work was to study the possible heterogeneity of variance for productive traits in the small Rendena cattle population. The herds were divided into two productive levels (medium and high) based on average milk yield recorded in each farm. A total of 171,104 test-day records of milk, fat and protein yields belonging to 10,430 cows were used to estimate genetic parameters among groups in separated analysis that accounted for primiparous cows only or for up to the third lactation animals (whole dataset). The (co)variance components were greater in high than in medium productive levels for both milk, fat and protein yields, both in the primiparous dataset and in the whole dataset. Heritability for all yields traits in the medium productive level was lower (0.160, 0.134 and 0.137, resp.) than in the high productive level (0.292, 0.230 and 0.234) and on average greater when primiparous cows were analysed alone (from 0.025 to 0.045 in medium and high productive group, resp.). However, the genetic correlations between productive groups resulted greater than 0.965 for all productive traits and in both datasets analysed. The rank correlation between EBVs of bulls that had daughters in both groups was 0.99, but a significant deviation from the theoretical frequency expected in medium and high productive groups was observed in the number of top cows. This may be related to the heterogeneity of variance. This study suggests the need for a correction method for the heterogeneous variance in the small cattle breed used as a case study, particularly in the selection of best cows that are more susceptible to biases in EBVs.