Abstract

New reproductive technologies may provide the possibility to derive the egg component of embryos for dairy cow replacements from only an elite proportion of the dairy herd, or from heifers or calves, thereby creating opportunities for greater genetic gain. Using modelling, four possible selection strategies in two case study herds were compared over 10 years at annual replacement rates of 15, 20 and 25%. In the different strategies, replacement calves were bred from: (1) AB bull + all herd cows, `status quo` vs (2) AB bull + only the top 25% of herd cows vs (3) AB bull + the top 100% of maiden heifers vs (4) AB bull + the top 100% of heifer calves. Assumptions used included (1) an average bull breeding worth (BW) of $135, gaining by $5 per year. (2) A `high BW herd` average BW of $62 (almost in the top 10% of all New Zealand herds). (3) A `low BW herd` average BW of $32 (nearly in the bottom 10%). The model predicted that rate of herd genetic gain increased with replacement rate. The low BW herd gained more than the high BW herd using any of the strategies because it started from a lower base. The BW of the 2 herds converged over 10 years and this effect increased with selection pressure. After 10 years, BWs of 150 and 149 were predicted for the high and low BW herds respectively by using eggs from calves as in Strategy 4 above. This compared with Strategy 1 where herd BWs at year 10 were 130 and 120 respectively.

DC, Smeaton, and HW Vivanco

Proceedings of the New Zealand Society of Animal Production, Volume 61, Christchurch, 199-202, 2001