The Effect of Alternative Economic and Genetic Covariation Structures on the Relative Economic Gain from Selection Using Stayability Traits

Three alternative cost ratios and the ratio of annual fixed cost for cows to net replacement cost for three populations that differed in genetic (co)variances were investigated. We studied their effect on the value of longevity records to supplement production records to achieve genetic gains for reducing the nonfeed cost of producing a unit of milk. Data files were constructed from stochastically generated production records and culling variant records; the later records represented the relative value for cow survival of all nonproduction traits. The use of one stayability trait instead of none increased gain by 1 to 9% across the populations and cost ratios. Only 2% additional gain occurred with two stayability traits instead of one. Populations that differed in genetic correlation between production and the survival value of nonproduction traits and in survival heritabilities contributed 2 to 5% toward the greater gains. The different cost ratios contributed 1 to 3%. The effect of increasing the length of herd life independent of production is inherently secondary to the effect of increasing the production during lactation. If genetic antagonism between production and the survival of nonproduction traits is found and if the net replacement costs are high, the use of production and longevity records for selection can enhance gain by up to 10%.