Genetic trends for 305-day milk yield and persistency in Iranian Holsteins

The objectives of this study were to estimate the genetic parameters and genetic trends for milk yield persistency and 305-day milk yield in Holstein dairy cows of Iran. Data set consisted of 432,152 test-day records from cows calving during 1991 to 2007. (Co) variance components and breeding values of 305-day milk yield (305MY) and three different measures of milk persistency were estimated using a random regression test day model and the Average Information Restricted Maximum Likelihood method. The Legendre polynomial functions of orders 4 and 5 were chosen to fit the additive genetic and permanent environmental effects and homogeneous residual variance was assumed throughout lactation. Three different persistency measures were included: 1) P1, the average of estimated breeding values (EBVs) for test day milk yields from day 255 to day 305 as a deviation from the average of EBVs from day 50 to day 70, 2) P2, the sum of contribution for each day in the period from day 60 to day 279 as a deviation from day 280 and 3) P3, comprising the difference between EBVs for day 290 and day 90. Low absolute values of these measures indicated the higher values of persistency. The highest and the lowest heritabilities were observed for P1 and P2, respectively. Heritability estimate for 305MY was 0.31. Highest genetic correlation was observed between P2 and P3. In general, genetic correlations with 305MY were low and P2 had the lowest correlation. Average EBVs of sires for persistency of lactation and 305MY demonstrated a positive trend for milk yield, but no significant trend for persistency. These results implied that selection for higher 305MY might not result in selection of sires that are genetically higher in lactation persistency and therefore, simultaneous evaluation for milk yield and lactation persistency would be necessary.