ECONOMIC EVALUATION OF REPLACEMENT RATES IN DAIRY HERDS .2. SELECTION OF COWS DURING THE 1ST LACTATION

The economic optimal rate of culling for milk production during the first lactation was studied with an extended version of a model presented by Renkema and Stelwagen (1979). Four culling policies were compared. The usual moderate culling procedure was the most profitable, the policy with no culling for milk yield being a close second. Heavy culling policies did not appear to be attractive. For a situation without genetic progress in milk production, production levels were calculated, below which, cows in first lactation with average performance in other traits should be removed from the herd. For this purpose future milk yields were estimated. These were based upon the actual milk yield of the cow, and included the probability of disposal due to ill-health at different ages, as well as correlations between partial and total milk yield of first lactation and correlations between milk yield in the completed first lactation and successive lactations. The results showed that the strongest selection should be made 2 or 3 months after calving. At that time cows producing less than about 88% of the herd level at the same age can be culled. At the end of the lactation this so-called critical milk yield decreased to about 86%. The results implied that about 20% of cows in first lactation could be culled for production. Seasonal effects may influence the critical milk yield considerably. The article was restricted to short-term aspects and did not consider any effects of the culling policy on the genetic capacity of future generations of cows. Additional calculations showed that the rate of genetic progress in a herd is affected very little by the intensity of culling for milk production. © 1979.