The effects of different replacement rates on the genetic gain for lactation yields of milk and milksolids (fat + protein) per cow and farm profit per hectare were evaluated using a simulation model. Three levels of replacement rate were simulated 15, 19 and 27%, by varying the levels of involuntary culling, such as with low fertility, and voluntary culling based on production worth. The effect of selection of replacements based on breeding worth was also evaluated. In the base year, farm profit was higher for lower replacement rates. Annual genetic gain in average herd breeding values for milksolids after 20 years of using bulls of high breeding worth was higher at higher replacement rates. However, farm profit at Year 20 was still higher for a herd with a low replacement rate than for a herd with a high replacement rate. A herd with a 15% replacement rate in combination with selection of replacement heifers based on breeding worth, achieved the highest gain in breeding values for milksolids and achieved the highest farm profit at Year 20. These results demonstrate that low replacement rate in combination with selection of high breeding worth heifers can achieve high genetic gain in both milksolids and farm profit.

N, Lopez-Villalobos, and CW Holmes

Proceedings of the New Zealand Society of Animal Production, Volume 70, Palmerston North, 46-50, 2010
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