Italian Simmental cattle is a dual-purpose breed whose breeding program included both production traits as well as body conformation (e.g., body size or muscularity) and functional characteristics. Selection for body conformation traits, in particular muscularity, must be seen not only for productive reason (associated to the dual-purpose attitude), but also as a functional trait. Several studies have reported a positive relationship between muscle coverage and fertility. The aim of the present study was to model the variation of muscularity and BCS curves across lactation of Italian Simmental dairy cows in Emilia Romagna dairy herds. Data were collected from 2656 Italian Simmental dairy cows from 324 dairy herds located in Emilia Romagna region, Northern Italy. Cows involved in the study were linear classified between the 2002/2003 to 2019/2020 dairy seasons. Muscularity and BCS curves were generated for each animal using random regression model by fitting different orders of Legendre polynomial on the fixed effect of days-in-milk and on the cow random effect. Moreover, age at linear scoring and herd-year of classification were fitted as fixed and random effects, respectively. The absolute muscularity growth rate (AMGR) was calculated as (Mux-Muy)/(tx-ty), where Mux and Mux are the corresponding predicted Muscularity at DIM x and y, and tx ty are the respective cow age. The relative muscularity growth rate was calculated as ln(Mux)-ln(Muy)/(tx-ty). Models with the lowest Akaike information criterion were those fitting a fixed cubic Legendre polynomial and a random linear polynomial. Results of absolute muscularity rate depicts a period of lowest muscularity across lactation corresponding to lactation peak, when animals have strong
nutrients requirements; after this period, on average, cow’s muscularity recovered up to pre-lactation levels. The results regarding the relative muscularity rate variations of muscularity reported periods of lower muscularity between 82 to 271 days. This moment correspond with the period when foetal growth increase. Result reported in the current research indicate that random regression using Legendre polynomials can be effectively employed to predict muscularity and body condition score of dairy cows
