Estimate of multiplicative and additive age adjustment factors for milk yield for the Mediterranean Italian Buffalo Breed

Abstract

This study was carried out to estimate multiplicative and additive age adjustment factors for milk yield for the Mediterranean Italian Buffalo Breed for a mature age base (55 months, parity 2) with a mixed model procedure. Fixed effects of age-parity classes and random effects of contemporary group, genetic and permanent environmental effects of cows were included in the model. Two data sets were formed: Data Set I consisted of 83,810 lactation records and was used to develop the age factors. Data Set II consisted of 115,242 lactation records and was used for validation of the age factors estimated from Data Set I. Additive and multiplicative factors followed the same trend within the three parities (parity 1, parity 2, and parity ≥ 3). Averages of milk yield by age class for Data Set II pre-adjusted with multiplicative and additive factors from Data Set I were similar and tended to follow a straight line as expected. However, multiplicative factors might be preferred to additive factors because they would be affected less by an increase of the average milk yield due to selection or management. Age factors were smoothed to reduce differences among factors for similar age-parity classes. The smoothed age factors showed only slight differences among averages for similar age-parity classes, which suggests that the smoothed age factors may be more appropriate than the original factors.