RESUMO
Persistency can be dened as the degree to which production is maintained after peak yield is reached. In Gyr cattle, persistency is an obstacle in milk production and profitability. So, persistency is an important trait that must be evaluated with total milk production in Brazilian Gyr cattle. The objective of this study was to calculate genetic tendency for persistency, accumulated 305-day milk yield and partial milk yields in first 100, 200 and 300 days in milk of Brazilian Gyr cows using breeding values predicted (EBV) by random regression models. Data was obtained from ABCZ and included 15052 first lactations from cows with calving year ranged from 1980 to 2006. Fixed effects was contemporary group (milking herd, year and season of milk control) and the days in milk were modeled using Legendre polynomials of order 4 for additive genetic and permanent environmental random effects and a heterogeneous residual variance structure using four classes. To estimate breeding values for 305-day milk yield, the daily EBV were added. To calculate the EBVs for partial periods was made the sum of EBVs daily in the first 100 days (P100), between 100 and 200 days (P200) and between 200 and 300 days (P300) in lactation. Persistency was obtained by the sum of the deviations of EBVs predicted between 30 and 270 days of production in relation to EBV predicted for peak production. The EBV for
O artigo não apresenta resumo em português.
RESUMO
Persistency can be dened as the degree to which production is maintained after peak yield is reached. In Gyr cattle, persistency is an obstacle in milk production and profitability. So, persistency is an important trait that must be evaluated with total milk production in Brazilian Gyr cattle. The objective of this study was to calculate genetic tendency for persistency, accumulated 305-day milk yield and partial milk yields in first 100, 200 and 300 days in milk of Brazilian Gyr cows using breeding values predicted (EBV) by random regression models. Data was obtained from ABCZ and included 15052 first lactations from cows with calving year ranged from 1980 to 2006. Fixed effects was contemporary group (milking herd, year and season of milk control) and the days in milk were modeled using Legendre polynomials of order 4 for additive genetic and permanent environmental random effects and a heterogeneous residual variance structure using four classes. To estimate breeding values for 305-day milk yield, the daily EBV were added. To calculate the EBVs for partial periods was made the sum of EBVs daily in the first 100 days (P100), between 100 and 200 days (P200) and between 200 and 300 days (P300) in lactation. Persistency was obtained by the sum of the deviations of EBVs predicted between 30 and 270 days of production in relation to EBV predicted for peak production. The EBV for
O artigo não apresenta resumo em português.