Estimates of genetic parameters and genetic trend for Wood's lactation curve traits of Tunisian Holstein-Friesian cows.

This study was conducted to (1) estimate Wood's incomplete gamma function parameters (a, b and c) for individual registered Tunisian Holsteins in the 3 first lactations; (2) estimate persistency (Per), peak time (PT), peak yield (PY) and 305-day lactation milk yields (Y305); (3) estimate the genetic parameters of the predicted traits and (4) evaluate the genetic trend of the most heritable traits. Pedigree data and 380,709 test-day milk records of 20,758 Holstein cattle collected from 2008 to 2018 in 33 herds were analyzed. Initially, Wood's incomplete gamma function parameters were determined and subsequently along with some production characteristics (Per, PT, PY and Y305). Each estimated trait was subjected to a single trait animal model to estimate genetic parameters using MTDFREML program (Boldman et al. 1995). Multivariate animal model was used to estimate genetic and phenotypic correlations between the most heritable traits and their breeding values (EBVs). Genetic trends were defined using BREEDPLAN multi-trait EBVs by averaging the EBV of all animals born by year of birth. The resulting heritability estimates varied from 0.04 to 0.06 for Per, 0.01 to 0.02 for PT, 0.08 to 0.16 for PY and 0.09 to 0.16 for Y305. The greatest heritabilities were within the first lactation for PY and Y305 traits. Genetic and phenotypic correlations were very high between the most heritable two traits (0.91 and 0.83, respectively). Heritability estimates of PT and Per were close to zero indicating that these traits are more influenced by the management and the environment. The regression coefficients, relative to the genetic trend graph, mark an overall rate of increase of 13.18 kg/year and 0.056 kg/year in Y305 and PY, respectively, over 16-year period. A number of policies should be developed by the Tunisian government to attend better genetic progress.