Relationships between failure of passive transfer and subsequent mortality, bodyweights and lactation performance in 12-36 month old heifers on pasture-based, seasonal calving dairy farms in New Zealand.

This study examined the long-term effects of failure of passive transfer of immunity (FPT; diagnosed at 1-8 days of age) on subsequent milk production, growth, reproduction, and lactation performance in dairy heifers from 12 to 36 months of age. A total of 34 farms from the Waikato and Canterbury regions of New Zealand were enrolled in 2015. Each farm was visited on three occasions during the seasonal calving period (early, middle, and late). Blood samples were collected at each visit from 20 replacement heifer calves aged between 1 and 7 days to test for FPT. These heifers (n=1879) were monitored from birth until the end of their first lactation. From 12 months of age onwards, animals were weighed at 15 and 22 months, pregnancy tested 100 days following their first mating, and milk was sampled between 3-4 times during their first lactation to determine milk volume and milk component yields. Farmers recorded any mortality events. FPT had no effect on the odds of mortality from 12 to 22 months (P=0.57) and 12 to 34 months of age (P=0.44). There was no difference in bodyweight at 15 months (P=0.17) and 22 months of age (P=0.95), no significant difference in the odds of being diagnosed pregnant (OR 1.44; 95% CI 0.82-2.69), and no effect on milk solids (fat plus protein) yields (P=0.67). No associations were observed between serum total protein (STP) concentration and milk solids yields (P=0.22) and any other milk parameters. The data from this study indicate that FPT did not adversely affect productivity, performance, or mortality beyond 12 months of age in heifers reared in pasture-based systems.