Possible effects of 25 years of selection and crossbreeding on the geneticmerit and productivity of New Zealand dairy cattle

A deterministic model was developed to evaluate the concurrent effects of s election and crossbreeding on the rate of genetic gain and productivity of New Zealand dairy cattle over 25 yr. Selection was based on an index, which included estimated breeding values for mature cow live weight and lactatio n yields of milk, fat, and protein. Mating strategies involving Holstein-Fr iesian, Jersey, and Ayrshire breeds were evaluated. Effects of heterosis an d age were included to calculate live weight and yields of milk, fat, and p rotein per cow. Feed requirements were estimated for maintenance, lactation , and pregnancy and for replacement heifers. Stocking rate was calculated b y assuming 12,000 kg of dry matter annually utilized per hectare. Upgrading to either Jersey or Holstein-Friesian increased the number of potential bu ll mothers and resulted in genetic gains of 0.27 genetic standard deviation s/yr for both options. Rotational crossbreeding of Holstein-Friesian x Jers ey decreased the number of potential bull mothers and resulted in a genetic gain of 0.24 genetic standard deviations/yr. Upgrading to Jersey resulted in the least increase in milk (5%) and the greatest increase in fat (16%) a nd protein (27%) per hectare with a small decrease in stocking rate (0.4%). Upgrading to Holstein-Friesian reduced the stocking rate by 11% and increa sed production of milk, fat, and protein per hectare by 10, 8, and 21%, res pectively. Rotational crossbreeding of Holstein-Friesian x Jersey resulted in greater production per hectare than the intermediate production between upgrading to Jersey and upgrading to Holstein-Friesian. Crossbreeding can b e used in combination with selection to exploit the effects of heterosis wh ile maintaining genetic diversity to cover changes in market conditions.