A model for four pathways of selection accounting for overlapping generations and crossbreeding strategies was developed to evaluate the effects of selection and crossbreeding on advancing the progress in genetic merit of bulls, total production of milk, fat and protein and total feed requirements of the New Zealand daily industry over the next 25 years. Rotational crossbreeding strategies reduced the sizes of active populations (bull mothers) with only minor changes in the annual genetic gain of bulls. Assuming a fixed area (985,000 ha) for grazing the national herd and calculating feed intake per head based on current productivity and assumed genetic gains, 25 years of upgrading to Jersey (J) caused the smallest decrease in cow number (15,000), the largest decrease in milk volume (33 million /) and the largest increases in fat (62 million kg) and protein (75 million kg) compared to the values for the season 1995/96. Upgrading to Holstein-Friesian (F) had opposite effects causing the largest decrease in cow number (240,000), the largest increase in milk volume (702 million /) and the smallest increase in fat (27 million kg) and protein (59 million kg). Effects of rotational crossbreeding strategies were intermediate between the effects caused by upgrading to J and upgrading to F. The overall net financial outcome of all these changes must be assessed carefully.

Nicolas Lopez-Villalobos, Dorian J Garrick, Hugh T Blair, C. W Holmes

Proceedings of the World Congress on Genetics Applied to Livestock Production, Volume 25: Lactation; growth and efficiency; meat quality; role of exotic breeds in the tropics; design of village breeding programs; n, , 363–366, 1998
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