Persistency in lactation – a review
Persistency in lactation can be defined as the capacity of a cow to maintain her milk yield after reaching the maximum level early lactation. Several studies document the existence of genetic differences among animals for persistency in lactation. Four different approaches to measuring this trait are found in the literature, these being based in: 1)ratios between of partial, total or other yields during the lactation, 2)variation based on test-day milk yields, 3)mathematical lactation curve models and 4)breeding values obtained through random coefficients of the random regression models. However, at present the procedure most used involves random regression models in analyses that consider the test-day milk yield. Use of those models improves the accuracy of the genetic evaluations and allows to predicting the breeding value of the animals in different periods of lactation. Persistency in lactation is directly related with economical aspects of daring, since its improvement con contribute to reduction of production costs. The economical importance of lactation persistency in lactation relates to four components: 1) health cost; 2) feed costs; 3) reproductive performance; and 4) milk production per lactation. For these reasons various studies have indicates that selection for this trait would be advantageous.