Longevity is an important economic trait in dairy cattle. Including this trait in a breeding scheme, increases profit. The aim of this study was to evaluate the relationship between length of productive life (LPL), genetic trend of milk production and profitability of herds. LPL has been defined as time from first calving to culling.A Dynamic stochastic model was used to simulate dairy herd system. This model consisted of biological characteristics such as reproduction, genetic and economic components. Both discrete (time-oriented) events such as freshening and breeding as well as continuous processes such as milk production and feed consumption were simulated individually for each animal. The basic characteristics of the animal component included pedigree, genetics, age at calving, number of service per conception, number of lactations and LPL. Other characteristics included time-oriented characteristics such as weight, age, physiological status, lactation stage, open days, pregnancy days, estrus cycle, service date and feed requirements. The herd was described as several animal groups: young stock (<1 year old), heifers (>1 year old) and several groups of lactating and dry cows. Increasing mean LPL of herd from 35 to 65 months over 20 years resulted in decreased herd genetic merit of milk from 2025 to 1751 kg and mean of herd genetic trend per year was decreased from 101.24 to 87.56 kg, because of increased generation interval. Increasing LPL resulted in increased profit. Increasing LPL was associated with decreased costs for raising replacement heifers and sale of surplus heifers increased. The ratio of cumulative discounted profit (CDP) for herds with 40, 45, 50, 55, 60 and 65 months of LPL to the lowest level of LPL (35 month), were 1.22, 1.43, 1.55, 1.68, 1.79 and 1.90 respectively across time.
Key words: Length of productive life (LPL), profitability, genetic trend, herds simulation.
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