Milk yield  persistency in Brazilian Gyr cattle based on a random regression model.
With  the objective of evaluating measures of milk yield persistency, 27,000 test-day  milk yield records from 3362 first lactations of Brazilian Gyr cows that calved  between 1990 and 2007 were analyzed with a random regression model. Random,  additive genetic and permanent environmental effects were modeled using  Legendre polynomials of order 4 and 5, respectively. Residual variance was  modeled using five classes. The average lactation curve was modeled using a  fourth-order Legendre polynomial. Heritability estimates for measures of  persistency ranged from 0.10 to 0.25. Genetic correlations between measures of  persistency and 305-day milk yield (Y305) ranged from -0.52 to 0.03. At high  selection intensities for persistency measures and Y305, few animals were  selected in common. As the selection intensity for the two traits decreased, a  higher percentage of animals were selected in common. The average predicted  breeding values for Y305 according to year of birth of the cows had a  substantial annual genetic gain. In contrast, no improvement in the average  persistency breeding value was observed. We conclude that selection for total  milk yield during lactation does not identify bulls or cows that are  genetically superior in terms of milk yield persistency. A measure of  persistency represented by the sum of deviations of estimated breeding value  for days 31 to 280 in relation to estimated breeding value for day 30 should be  preferred in genetic evaluations of this trait in the Gyr breed, since this  measure showed a medium heritability and a genetic correlation with 305-day  milk yield close to zero. In addition, this measure is more adequate at the  time of peak lactation, which occurs between days 25 and 30 after calving in  this breed.
Departamento de Zootecnia, Faculdade de Ciências Agrárias e Veterinárias, Universidade Estadual Paulista "Júlio de Mesquita Filho", Jaboticabal, SP, Brasil. rodjunper@yahoo.com.br
R J
RJ
Pereira
R S
RS
Verneque
P S
PS
Lopes
M L
ML
Santana
M R
MR
Lagrotta
R A
RA
Torres
A E
AE
Vercesi Filho
M A
MA
Machado
D000818:Animals; D001947:Breeding; D002417:Cattle; D005260:Female; D007774:Lactation / Q000502:physiology*; D008297:Male; D008892:Milk