Crossbred cows tend to be more profitable than straightbred cows because they fit the present milk payment system and show favourable heterosis for milk traits. This may encourage greater use of crossbred cows eroding the number of potential bull mothers (active cows) thereby reducing genetic progress. The objective of this research was to evaluate the effects of crossbreeding on active cow numbers and genetic gain over 25 years. A deterministic model was used to calculate the number of cows and the breeding worth (BW) of Holstein-Friesian (F) and Jersey (J) bulls each year. Three strategies were considered: upgrading to F (UPGF); upgrading to J (UPGJ); and some two-breed rotational mating plans. In 1996, sizes of F and J active populations were taken to be 250,000 and 76,400 cows and mean bull BWs were $72 (F) and $82 (J). After 25 years of UPGF the number of active F and J cows were 1,520,000 and 13,000, the BWs of F and J bulls were $245 and $213 and genetic gains of F and J bull teams were 8.5 and 4.25 $BW/year. UPGJ had the opposite effect. Mating 10% of •2/3F cows to F bulls and 10% of 1/3F cows to J bulls (ROTATE10) for 25 years led to active F and J populations of 70,200 and 75,800 cows, BWs of F and J bulls of $227 and $230 and genetic gains for F and J bull teams equal to 7.0 $BW/year. Results suggest that widespread adoption of a rotational crossbreeding scheme could be achieved without penalising annual genetic gain in bulls. However, annual recruitment of new active cows should be routinely monitored to ensure long term gain will be maintained.
Proceedings of the New Zealand Society of Animal Production, Volume 57, , 22-25, 1997
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