Genetic evaluation for persistency of lactation in Iranian Holstein dairy cattle using test-day records

In this research, a total number of 701212 (milk), 596331 (fat) and 505793 (protein) yields were used to estimate genetic parameters for persistency of lactation. The records were collected by the national animal breeding center of Karaj during 2006 and 2010. (Co) variance components were estimated using REML method and random regression models. Five persistency measures of lactation were evaluated using estimated breeding values. The estimated heritabilities of persistency for milk, fat, and protein yields ranged from 0.06 to 0.16, 0.03 to 0.17, and 0.06 to 0.20, respectively; the heritabilities for 305-d yield were 0.29, 0.28 and 0.28, respectively. Values of Spearman correlations between measures of persistency and EBVs of 305-d milk, fat, and protein yields ranged from 0.02 to 0.76, -0.24 to 0.58, and -0.50 to 0.80, respectively. Genetic correlations between persistency measures and EBVs of 305-d yield were varied; and were higher for milk and protein yields than for fat yield. For milk and protein yields, selection for EBVs of 305-d production identified sires or cows that are genetically superior in regards to persistency of lactation. But for fat yield, high breeding values for 305-d yield does not identify superior cows for yield persistency. Therefore, genetic evaluation for persistency of lactation is important for improving the milk production traits of Iranian Holstein cows and it is applicable in breeding programs for Iranian Holstein dairy cattle.