Genetics of heat tolerance for days open in US Holsteins
Oseni, Saidu Oyarekhua
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The objectives of the first study were to present statistics on the seasonality of calving and regional trends for days open (DO) across the US. Data included 8,676,915 records on DO for Holsteins from 1997 to 2002 covering all regions of the U.S. Fixed effects in the model included herd, parity, milk-class, state´month of calving (MOC), year of calving´ MOC and parity´MOC. Least squares means of DO were highest for calvings in March and lowest for calvings in September. Distributions of DO were bimodal for some months of calving; the second mode at > 200 d was highest in the Southeast but also could be observed in TX, WI and CA. High level of heat stress for DO exists in the Southeast and in selected states of the Midwest and the Southwest. A second study investigated the effect of different editing of DO records on the genetic parameters of DO and pregnancy rates (PR). Data included first parity 305-d milk yield and DO records in eight states of the US. Days open upper limits were set to 150, 200, 250, 300 and 365 d. A bivariate animal model for DO (or PR) and 305-d milk yield included fixed effects of herdyear, month of calving and age of cow, and random animal and residual effects. Genetic and residual variances for DO changed up to 8 times as DO upper bound increased from 150 d to 365 d. Estimates of heritability for DO varied between 3 and 6 %. For most states, estimates of heritability for 150 and 200 d bounds were similar, and lower than estimates for bounds >250 d. Heritabilities for PR varied by the length of the VWP. Records of DO > 250 d carry little genetic information whereas records < 120 d carry more or less information depending on management. In a third study, a reaction norm approach was used to estimate the genetic parameters of DO with a model that accounted for heat stress. The reaction norm model included the effect of animal with random regression on a heat stress index (HI). Results indicated that all the estimated parameters varied by month of calving. The results of a four-trait model based on calving seasons validated the reaction norm model.