I. Losses between the conception of heifer replacements and their calving

Abstract

Statistics derived from herds in the New Zealand Dairy Board's Sire Proving Scheme have been used to estimate sources of wastage of artificially bred (A.B.) replacement heifers from their conception to their calving and coming into production. A group of 100 cows all reported. in calf to A.B. sires at the conclusion of mating will produce 29 heifer replacements in milk in the herd three seasons later. Sillce the average herd wastage rate is 21% at least 72% of a herd must be in calf to A.B. sires if all replacement heifers are to be A.B: progeny. The calving rate (percent of cows reported in calf to A.B. which calve to A.B.) is lower for cows msemmated With Friesian semen than With Jersey semen, but this is offset by the lower death rate of heifer calves sired by Friesian bulls. Significant breed differences were found in sources of wastage in heifer calves from birth to identification but seasonal differences were more important from heifer identification to their calving at 2 years of age.