Effect of transgenerational environmental condition on genetics parameters of Italian Brown Swiss

: The aim of this study was to infer the effects of heat stress (HS) of dams during late gestation on direct and maternal genetic parameters for traits related to milk production and milk quality parameters (90,558 records) in Italian browns Swiss cattle (12,072 cows in 617 herds). Daily Average Temperature-humidity indices (THI) during the last 56 d of pregnancy were calculated, using the climate data from the nearest public weather station for each herd. Heat load effects were considered as the average across the entire periods considering a thermoneutrality condition for data below the THI 60. For parameter estimation a random regression model using the second order Legendre polynomial regression coefficient for THI considering both animal and maternal effect for heat load. Direct heritability increased sharply from THI 60 to 65, then decreased gradually up to THI ∼72, and sharply thereafter. Maternal heritability showed a different trend, with values close to 0 up until to THI 65 and slightly increasing toward extreme THI values. The study suggests a lower threshold of THI 60 for the onset of heat stress. Higher heritability values indicate greater selective efficiency in the THI range of 65-70, even if an higher standard deviation value have been detected. The effects of high THI during intrauterine life varied among traits with different heritability levels. Genetic correlations for milk, fat and protein content at 60 THI with increasing value of environmental variable, remained constant (∼0.90) until THI >75, where they slightly decreased (∼0.85). Fat and protein yields, as well as milk and ECM, showed correlations dropping to 0.80 around THI 67-68 and stabilizing between 0.75 and 0.85 at extreme THI values. Maternal component correlations dropped close to zero, with negative values for protein content at THI 65-70. Antagonism between direct and maternal components was stronger for intermediate THI values but less divergent for extremes. G x E interaction was observed, indicating the selection of resilient animals would be theoretically possible. The application of climate variables in selection schemes should in the future take into account above all the dimensions of the genetic correlations to be able to decide between the simple inclusion of the environmental effect in the statistical models rather than a real parallel genetic evaluation.